United States
Environmental Protection
Agency
Office of Air Quality
Planning and Standards
Research Triangle Park, NC 27711
EPA-452/R-9S-005
September 1995
Air
c/EPA
REVIEW OF THE NATIONAL AMBIENT AIR QUALITY STANDARDS
FOR NITROGEN DIOXIDE
ASSESSMENT OF SCIENTIFIC AND TECHNICAL INFORMATION
OAQPS STAFF PAPER
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TABLE OF CONTENTS
Acknowledgements i
List of Tables iv
List of Figures v
Executive Summary vi
I. Purpose 1
II. Background 1
A. Legislative Requirements 1
B. Establishment of Nitrogen Dioxide NAAQS .... 3
C. First Review of Nitrogen Dioxide NAAQS .... 4
D. .Current Review of Nitrogen Dioxide NAAQS ... 5
III. Approach 5
IV. Air Quality Trends 6
A. Long-Term Trends 7
B. Recent Trends in Urban Areas of the U.S. ... 9
C. Nitrogen Deposition Trends in the U.S. 10
V. Health Effects of Nitrogen Dioxide . < 13
A. Introduction 13
B. Mechanisms of Toxicity, Transport, and Fate . . 13
C. Health Effects Evidence 15
1. Susceptibility to Respiratory Illness ... 15
a. Epidemiological Studies 16
1. Indoor Studies 17
2. Outdoor Studies 22
b. Meta-analysis of Gas Stove Studies ... 24
c. Biological Plausibility 28
2. Lung Function, Symptoms and
Airway Resistance 33
3. Increased Airway Responsiveness 36
4. Emphysema 38
D. Populations Potentially at Risk 39
E. Effects of Concern 43
VI. Averaging Time and Form of the Primary NAAQS ... 46
A. Averaging Time of the Primary NAAQS 46
B. Form of the Primary NAAQS 48
VII. Summary of Staff Conclusions and Recommendations
for the Nitrogen Dioxide Primary NAAQS 49
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VIII. Environmental Effects of Nitrogen Deposition
52
A. Introduction 52
1. Context of Current Standard Review 52
2. Atmospheric Nitrogen Inputs 53
3. Nitrogen Cycle 55
B. Ecosystem Response 57
1. Terrestrial Systems 58
a. Vegetation 58
b. Plant/Soil Interactions 60
c. Soils 62
2. Wetlands 65
a. Vegetation 66
b. Soils 68
3. Aquatic Systems 68
a. Acidification 69
1. Chronic Acidification 69
2. Episodic Acidification 71
b. Eutrophication 74
1. Freshwater Eutrophication 74
2. Estuaries and Coastal Waters 76
c. Direct Toxicity 77
C. Critical Loads and Other Potential
Regulatory Actions 78
1. Critical Loads Concept 78
2. Target Loads •. . 85
3. Applicability to U.S. Situation 85
D. Visibility 86
1. Major Categories 86
2. Contributors to Visibility Reduction 87
E. Non-biological Materials 90
IX. Summary of Staff Conclusions and Recommendations
for Secondary Nitrogen Dioxide NAAQS 91
Appendix A:
Appendix B:
Appendix C:
Appendix D:
CASAC Correspondence
Analysis of Current NO2 NAAQS Attainment
Definitions
References
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IV
List of Tables
V-l Health outcome and nitrogen dioxide exposure measures
used in selected indoor nitrogen dioxide epidemiology
studies 20
V-2 Summary of odds ratios from indoor studies of the
effects of nitrogen dioxide 25
V-3 Key epidemiological studies of exposure to nitrogen
dioxide - Summary of meta-analysis 27
V-4 Key animal toxicological effects of exposure to
nitrogen dioxide 32
V-5 Summary of potentially sensitive groups 42
VIII-1 Measurements of various forms of annual nitrogen
deposition to North American and European
ecosystems 81
VIII-2 Nitrogen input/output relationships for several
ecosystems 83
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Acknowledgments
This Staff Paper is the product of the Environmental
Protection Agency (EPA), Office of Air and Radiation (OAR),
Office of Air Quality Planning and Standards (OAQPS). The
principal authors are Ms. Victoria V. Atwell, Ms. Chebryll C.
Edwards, and Dr. David J. McKee. The authors express special
thanks to other staff within EPA for their comments,
recommendations, and technical and clerical support during the
development of.this paper.
The Clean Air Scientific Advisory Committee (CASAC) formally
reviewed this document in December 1994. CASAC comments and
recommendations have been carefully considered in revising this
draft. A copy of the CASAC closure letter for this Staff Paper
is included in Appendix A of this document.
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V
List of Figures
IV-1 National trend in the composite annual average nitrogen
dioxide concentration at both NAMS and all sites with
95 percent confidence intervals, 1983-1992 8
IV-2 Regional comparisons of 1990, 1991, 1992 composite
averages of the annual mean nitrogen dioxide
concentrations .... 9
IV-3 Mean annual wet nitrate and ammonium deposition to
various states located throughout the United States. 11
VIII-l Schematic representation of the nitrogen cycle,
emphasizing human activities that affect fluxes of
nitrogen. .... ...... 56
VIII-2 Schematic diagram'of cation exchange for base cations,
aluminum ions, and hydrogen ions in circumneutral and
acid soils 63
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VI
Executive Summary
In 1971, the Environmental Protection Agency (EPA)
promulgated identical primary and secondary national ambient air
quality standards (NAAQS) for nitrogen dioxide (NC^) at 0.053 ppm
annual average (36 FR 8186). Section 109 of the Clean Air Act
requires EPA to periodically review the NAAQS to ensure the
scientific adequacy of these air quality standards. The last
review of the NC>2 criteria and NAAQS was completed in 1985. At
that time, EPA published a rule retaining the NC^ NAAQS at 0.053
ppm annual average (50 FR 25532).
As part of the current review, "the Office of Air Quality
Planning and Standards (OAQPS) developed this Staff Paper to
summarize and integrate the key studies and scientific evidence
contained in the revised document, "Air Quality Criteria for
Oxides of Nitrogen" (EPA, 1993), and to identify the critical
elements the staff believes should be considered in the review of
the NAAQS. Summarized below are the staff's key findings and
recommendations from the current review of the N©2 NAAQS.
Air Quality _Tre,pd_s
Nitrogen dioxide (NO2) is a brownish, highly reactive gas
which is formed in the ambient air through the oxidation of
nitric oxide (NO). Nitrogen oxides (NOX), the term used to
describe the sum of NO and N02, play a major role in the
formation of ozone in the atmosphere through a complex series of
reactions with volatile organic compounds. Anthropogenic (i.e.,
man-made) sources of NOX emissions account for a large majority
of all nitrogen inputs to the environment. The major sources of
anthropogenic NOX emissions are mobile sources and electric
utilities. Ammonia and other nitrogen compounds produced
naturally do play a role in the cycling of nitrogen through the
ecosystem.
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Typical peak annual average ambient concentrations of
range from 0.007 to 0.061 ppm (CD, pg, 7-10). The highest hourly
NC>2 average concentrations range from 0,04 to 0.54 ppm (CD, pg.
7-10) . Currently, all areas of the United States, including Los
Angeles {which is the only area to record violations in the last
decade), are in attainment of the annual NC>2 NAAQS of 0.053 ppm.
Analysis of air quality data indicates that if the annual
standard continues to be attained, risk of exposure to high
short-term peaks is low (McCurdy, 1994).
Health Effects
The staff concludes that exposure to NC^ is associated with
a variety of acute and chronic health effects. Clearly adverse
health effects (e.g., pulmonary edema, death) have been reported
following accidental short-term (e.g., 6 to 7 hours) exposures of
humans to 150'to 200 ppm of N02 or greater (CD, pg. 14-55) .
Furthermore, animal studies have provided evidence of emphysema
caused by long-term exposures to greater than 8 ppm NC^. It is
clear that these health endpoints are caused by exposures which
are much higher than those found in the ambient air and only
serve as indicators of the most extreme effects of a potentially
very noxious oxidant.
Other acute health endpoints that have been associated with
much lower N02 concentrations include (1) changes in pulmonary
function, (2) increases in airway responsiveness, and (3-)
increased risk for developing respiratory diseases and illnesses.
Based on the assessment of available clinical and epidemiological
information regarding the health effects associated with short-
term exposure to NO2, the staff concludes that the two
potentially sensitive population groups at risk are (1)
individuals with pre-existing respiratory diseases and (2)
children five to twelve years old.
The available clinical data indicate that short-term
exposure (e.g., less than 3 hours) to N02 may cause an increase
in airway responsiveness in asthmatic individuals at rest.
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This response has been reported only at relatively low
concentrations (mostly within the range of 0.2 to 0,3 ppm
which are of concern in the ambient environment. Similarly, NO2
induced pulmonary function changes in asthmatic individuals have
been reported at low, but not high, NQ2 concentrations. For the
most part, the small changes in pulmonary function that have been
observed in asthmatic individuals have occurred at concentrations
between 0,2 and 0.5 ppm, but not at much higher concentrations
(i.e., up to 4 ppm)(CD, pg. 16-3). These findings contrast with
the findings for healthy individuals exposed to low N02
concentrations. In healthy individuals, there is no evidence
of lung function decrements or changes in airway responsiveness
at concentrations below i.o ppm N02.
The epidemiological evidence includes a meta-analysis of
nine epidemiological studies of children (5-12 years old) living
in homes with gas stoves. The results of the meta-analysis show
that children (5-12 years old) living in homes with gas stoves
are at increased risk for developing respiratory diseases and
illnesses compared to children living in homes without gas
stoves. Typical mean weekly NC^ concentrations in
bedrooms in studies reporting NCU levels were predominately
between 0.008 and 0.065 ppm NO2• Most of the N02 measurements
reported in the studies are for indoor 1- to 2-week averages, but
very little information is provided regarding either peak hourly
exposures or measurements providing estimates of annual
exposures. Because of this, the studies and, therefore, the
meta-analysis cannot distinguish between relative contributions
of peak and longer term exposure and their relationship with the
observed health effects. Given the uncertainty associated with
determining actual exposure patterns in these homes and the
difficulty in extrapolating the data to ambient exposures,
results of the meta-analysis provide insufficient data to support
specific limits for either short-term or long-term standards for
nitrogen dioxide. Based on this current assessment of the
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available scientific and technical information (which remains
largely unchanged since the 1985 review), the staff again
recommends consideration be given to setting the level of the
annual primary standard within the range of 0,05 to 0.08 ppm NO2.
In the staff's judgement, selecting a standard within this range
would provide adequate protection against the health effects
associated with chronic or long-term N02 exposure. In reaching a
determination on the standard, the staff also recommends that
consideration be given to the degree of protection that would be
provided against repeated short-term peak exposures. Based on
air quality analyses, a standard selected from the lower portion
of this range .would effectively limit the frequency and magnitude
of 1-hour NQ2 concentrations. McCurdy (1994) estimated that if
the existing standard of 0.053 ppm N02 is attained, the
occurrence of l-hour N02 values greater than 0.2 ppm would be
unlikely in most areas of the country. At the upper end of the
range, the frequency of 1-hour N02 peaks of 0,2 or higher could
increase significantly. However, because all areas of the
country reporting NOj air quality data are attaining the existing
standard and because of the nonlinear relationship of 1-hour
peaks and annual averages, it is not possible to estimate with
any degree of confidence what the frequency and magnitude of 1-
hour peaks would be if the standard was selected from the upper
end of the suggested range. Given this uncertainty, the staff
recommends consideration be given to selecting the standard from
the lower portion of the range in order to provide a reasonable
measure of protection against repeated 1-hour peaks of potential
health concern.
Environmental Effects
Nitrogen oxides have been associated with a wide range of
effects on vegetation, natural ecosystems, visibility, and
materials. Studies show that nitrogen oxides are contributing
to the observed effects either through direct deposition or by
indirectly altering processes within the system.
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After reviewing the key criteria concerning the effects of
•-exposure to nitrogen oxides on the environment, the staff
•"concludes that the impact on terrestrial vegetation from short-
term exposures to NCu under existing ambient levels is
insignificant and does not warrant a change in the secondary
-N&AQS at this time. Studies of short-term, acute effects of
nitrogen additions to vegetation have demonstrated that NC>2 in
mixtures with other pollutants or at higher than ambient
concentrations can produce adverse effects on plant growth or
reproduction. Though the sensitivity of vegetation to NC>2 varies
both within and between species and can be modified by season,
developmental -stage, or other climate or environmental factors,
taking all of these variables into account, the' ambient levels of
N02 in the United States are considered below those that evoke a
short-term, acute response. The staff also concludes that there
is little potential for concentrations of nitrogen deposition to
be sufficient to acidify soils to the extent that it would cause
direct phytotoxic effects in plants.
Based on existing European studies and information in the
United States, it seems that addition of nitrogen to ecosystems
in the U.S. (specifically in mature forests and wetlands which
host a number of endangered species adapted to nitrogen-poor
habitats) may lead to shifts in species competition and
composition and may represent a significant change in the
environment. However, there is not sufficient evidence to
conclude that changes in plant communities have occurred in the
past or are now occurring in the United States due to nitrogen
inputs. Furthermore, there are only limited long-term data
available on plant community composition. Additional research is
needed to more accurately characterize any potential threat to
mature forests and wetland species from atmospheric nitrogen.
Once this information is available, the need for additional
ecosystem protection should be reassessed by the Administrator.
Based on the review of the available scientific information
in the CD, the staff concludes that nitrogen deposition is a
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• XI'
contributor to the episodic acidification of some streams and
lakes in the United States. Thus, it can also be concluded that
some freshwater ecosystems will benefit from reductions in
nitrogen inputs from man-made sources. However, at this time,
the staff finds insufficient evidence to quantify how much of a
contribution nitrogen deposition is making to the acidification
problem and what levels of reduction are necessary to remedy the
situation. Therefore, staff recommends that further research be
conducted to provide the information needed to evaluate this
issue in its next standard review, when additional scientific
information is available that adequately quantifies the
relationship between nitrogen deposition and acidification, the
staff recommends that the Administrator consider appropriate
regulatory approaches to address the problem. Key considerations
will be the major differences evident in the occurrence, nature,
location, timing of episodes at different sites, and the
subsequent effects produced. Such differences will make setting
a single national standard which would adequately protect all
areas of the country difficult and complex. Therefore, staff
believes regional approaches that take into account such
variations should be considered for providing the level of
control needed to address the problem of acidification.
It is very unlikely that eutrophication of freshwater
systems is due to atmospheric nitrogen additions since
phosphorus, not nitrogen, is generally the limiting factor for
algae growth. However, both upland and direct atmospheric
nitrogen deposition can significantly affect the trophic status
of estuarine and coastal waters. As mentioned before, once a
relationship can be reasonably quantified, staff recommends that
the Administrator consider appropriate regulatory approaches,
including a uniform national standard or regional standards, to
protect estuarine and coastal waters.
The concept of critical loads is addressed in the Staff
Paper as a possible means for assessing and controlling
emissions in the future. Critical loads are defined as
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".quantitative estimates of an exposure to one or more pollutants
below which significant harmful effects on specified sensitive
elements of the environment do not occur according to present
knowledge." Staff concluded that it would be premature to
consider establishment of a long-term national critical loads
secondary ambient air quality standard for NO2 given the state of
the science of critical loads estimation that exists at the
current time. However, this methodology has the potential to
provide a useful tool for making regulatory decisions at the
regional level in the future.
With regard to visibility impairment, the scientific
evidence indicates that light scattering by particles is
generally the primary cause of degraded visual air quality. The
evidence also suggests that aerosol optic effects alone can
impart a reddish-brown color to a haze layer. While it is clear
that particles and N02 contribute to brown haze, in the staff's
judgement reducing ground level NOo concentrations would result
, in little if any improvement in visual air quality. Therefore,
the staff concludes that an ambient secondary standard for NO2 to
,protect visibility is not warranted at this time,
Based on the available data, the staff concludes that it is
unlikely that N02 is playing a significant role in the damage to
non-biological materials and therefore does not recommend
consideration be given to setting a secondary standard to protect
against material damage.
If the Administrator determines that a separate secondary
standard is appropriate, the staff recommends that it be set
identical to the primary standard for NO2. A secondary standard
set within the range recommended for the primary standard would,
in the staff's judgement, provide adequate protection against the
direct effects of N02 on the environment.
It is clear that additional studies need to be conducted to
reduce the uncertainty in the relationship between nitrogen
deposition and forest health and in determining the adverse
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effects on surface waters and estuaries arising from the long-
term accumulation of nitrogen. The staff recommends substantial
efforts be undertaken to illuminate the quantitative
relationships between nitrogen deposition and changes in forest
nitrogen cycling, episodic and chronic acidification of surface
waters, and estuarine eutrophication, either through ongoing
research or new initiatives. Such efforts will provide an
improved scientific basis for the next standard review.
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REVIEW OF THE NATIONAL AMBIENT AIR QUALITY STANDARDS
FOR NITROGEN DIOXIDE •
ASSESSMENT OF SCIENTIFIC AND TECHNICAL INFORMATION
I, PURPOSE
The purpose of this Office-of Air Quality Planning and
Standards (OAQPS) Staff Paper is to summarize and integrate the
key studies and scientific information contained in the revised
EPA document, "Air Quality Criteria for Oxides of Nitrogen" (U.S.
EPA, 1993a; henceforth referred to as CD), and to identify the
critical elements EPA staff believes should be considered in the
review of the national ambient- air quality standards (NAAQS) for
nitrogen dioxide (N02). Factors relevant to the evaluation of
current primary (health) and secondary (welfare) NAAQS, as well
as staff conclusions and recommendations, are provided in this
Staff Paper.
II. BACKGROUND
A. Legislative Requirements
Two sections of the Act govern the establishment and
revision of NAAQS. Section 108 (42 U.S.C. 7408) directs the
Administrator to identify pollutants which "may reasonably be
anticipated to endanger public health and welfare" and to issue
air quality criteria for them. These air quality criteria are to
"accurately reflect the latest scientific knowledge useful in
indicating the kind and extent of all identifiable effects on
public health or welfare which may be expected from the presence
of [a] pollutant in the ambient air ..."
Section 109 (42 U.S.C. 7409) directs the Administrator to
propose and promulgate "primary" and "secondary" NAAQS for
pollutants identified under Section 108. Section 109(b)(1)
defines a primary standard as one "the attainment and maintenance
of which, in the judgment of the Administrator, based on the
criteria and allowing an adequate margin of safety, [is]
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requisite to protect the public health."1 A secondary standard,
as defined in Section 109(b)(2), must "specify a level of air
quality the attainment and maintenance of which, in the judgment
of the Administrator, based on [the] criteria, is requisite to
protect the public welfare from any known or anticipated adverse
effects associated with the presence of [the] pollutant in the
ambient air." Welfare effects as defined in Section 302(h) [42
U.S.C. 7602(h)] include, but are not limited to, "effects on
soils, water, crops, vegetation, manmade materials, animals,
wildlife, weather, visibility and climate, damage to and
deterioration of property, and hazards to transportation, as well
as effects on economic values and on personal comfort and well-
being.11
The U.S, Court of Appeals for the District of Columbia
Circuit has held that the requirement for an adequate margin of
safety for primary standards was intended to address
uncertainties associated with inconclusive scientific and
technical information available at the time "of standard setting.
It was also intended to provide a reasonable degree of protection
against hazards that research has not yet identified. Lead
Industries Association v. EPA. 647 F.2d 1130, 1154 (D.C. Cir.
19805, cert, denied, 101 S. Ct. 621 (1980); American Petroleum
Institute v. Costle. 665 P.2d 1176, 1177 (D.C. Cir. 1981), cert.
denied, 102 S. Ct. 1737 (1982). Both kinds of uncertainties are
components of the risk associated with pollution.at levels below
those at which human health effects can be said to occur with
reasonable scientific certainty. Thus, by selecting primary
1The legislative history of Section 109 indicates that a
primary standard is to be set at "the maximum permissible ambient
air level . . . which will protect the health of any [sensitive]
group of the population," and that for this purpose "reference
should be made to a representative sample of persons comprising
the sensitive group rather than to a single person in such a
group." S. Rep. No. 91-1196, 91st Cong., 2d Sess. 10 (1970).
The legislative history specifically identifies bronchial
asthmatics as a sensitive group to be protected. .Id,.
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standards that provide an adequate margin of safety, the
-Administrator is seeking not only to prevent pollution levels'
-that have been demonstrated to be harmful but also to prevent
lower pollutant levels that may pose an unacceptable risk of
•harm, even if the risk is not precisely identified as to nature
or degree,
In selecting a margin of safety, the EPA considers such
factors as the nature and severity of the health effects
involved, the size of the sensitive population(s) at risk, and
the kind and degree of the uncertainties that must be addressed.
Given that the "margin of safety" requirement by definition only
comes into play where no conclusive showing of adverse effects
exists, such factors, which involve unknown or only partially
quantified risks, have their inherent limits as guides to action.
The selection of any numerical value to provide an adequate
margin of safety is a policy choice left specifically to the
Administrator's judgment. Lead industries Association v_. EPA,
supra, 647 F.2d at 1161-62,
Section 109(d)(1) of the Act requires that "not later than
December 31, 1980, and at 5-year intervals thereafter, the
Administrator shall complete a thorough review of the criteria
published under Section 108 and the national ambient air quality
standards . . . and shall make such revisions in such criteria
and standards ... as may be appropriate . . . ." Section
109(d)(2)(A) and (B) require that a scientific review committee
be appointed and provide that the committee "shall complete a
review of the criteria . . . and the national primary and
secondary ambient air quality standards . . . and shall recommend
to the Administrator any . . . revisions of existing criteria and
standards as may be appropriate ... . ."
B- Establishment of NitrogenDioxide NAAQS
On April 30, 1971, the EPA promulgated NAAQS for N02 under
Section 109 of the Act (36 PR 8186}. Identical primary and
secondary NAAQS were set at 100 micrograms per cubic meter
(/Kj/m3) [0.053 parts per million (ppm) ] as an annual arithmetic
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mean. Scientific and technical bases for these NAAQS are
provided in the document, "Air Quality Criteria for Nitrogen
Oxides" (U.S. EPA, 1971). The primary standard was based largely
on several epidemiology studies (Shy et al., 197Qa,- 1970b;
Pearlman et al., 1971} conducted in Chattanooga. These studies
reported respiratory effects in children exposed to low-level
ambient N02 concentrations over a prolonged period, Reevaluation
of these studies, particularly the method used to monitor NC>2/
indicates that the studies provide only qualitative evidence for
an association between health effects and exposure to NC>2 •
C. First Review of Nitrogen Dioxide NAAQS
The fixst review of NO2 criteria and NAAQS was initiated in
1977, The Clean Air Scientific Advisory Committee (CASAC} held
meetings in 1979 and 1980 before providing written closure on the
revised criteria document (U.S. EPA, 1982} in June 1981. A staff
paper, which-identified critical issues and summarized staff's
interpretation of key studies received verbal closure at a CASAC
meeting in November 1981 and formal written closure in July 1982.
In the paper, staff recommended that the Administrator select an
annual standard "at some level between 0.05 ppm and 0.08 ppm,"
Based on the analysis of the criteria, staff concluded that
choosing an annual standard within this range would "provide a
reasonable level of protection against potential short-term
peaks."
On February 23, 1984, the EPA proposed to retain the
existing annual primary and secondary standards and defer action
on the possible need for a separate short-term primary standard
until further research on health effects of acute exposures to
N02 could be conducted. CASAC met to consider the Agency's
proposal July 19-20, 1984. In an October 18, 1984 closure
letter, based on weight of evidence, CASAC concurred with the
Agency's recommendation to retain the annual average primary and
secondary standards at 0.053 ppm. The Committee further
concluded that, "while short-term effects from nitrogen dioxide
are documented in the scientific literature, the available
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information was insufficient to provide an adequate scientific
basis for establishing any specific short-term standard,,.."
Following a period of public review and comment, the final
rule to retain the NAAQS for NO2 was published in the Federal
Register on June 19, 1985 (50 FR 25532).
' D. Current jReview _ of Nitr.pge.n,_ 13ioxide, NAAQS
EPA's Environmental Criteria and Assessment Office (ECAO)
initiated action on a revised CD, which was released for public
review in August 1991. The revised CD includes a meta-analysis
of several studies, conducted in homes with gas stoves, which
reported increased rates of respiratory symptoms and illness for
children living in those homes. The CD also discussed
information relevant to nitrogen deposition and potential impacts
on -natural ecosystems. The CASAC reviewed the CD at a meeting
held on July 1, 1993 and concluded in a letter to the
Administrator that the CD ". . . provides a scientifically
balanced and defensible summary of current knowledge of the
effects of this pollutant and provides an adequate basis for
EPA to make a decision as to the appropriate NAAQS for NC^" (see
Appendix A).
The current review of the primary NAAQS will focus primarily
on the health effects associated with exposure to nitrogen
dioxide since it is the NOX compound measured in most
epidemiological studies and is currently of greatest concern from
a public health perspective. On the other hand, staff will
consider the total class of nitrogen compounds and their effects
on the environment when evaluating the secondary standard.
XXX. APPROACH
The approach taken in this Staff Paper is to assess and
integrate the scientific and technical information contained in
the revised CD in order to evaluate the adequacy of the current
NAAQS for N02. Consideration of a qualitative exposure analysis
is used in assessing the primary NAAQS, and information on
nitrogen deposition is used in determining the possible need for
revising the secondary NAAQS for N02- Critical elements have
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been identified which staff believes should be considered in
review of the NAAQS for N02. Attention is drawn to those
judgments that must be based on careful interpretation of
incomplete or uncertain evidence. In such instances, the Staff
Paper provides staff's evaluation, sets forth alternatives that
staff believes should be considered, and recommends a course of
action.
Following a short presentation of air quality information in
Section IV, the Staff Paper provides in Section V a discussion of
mechanisms of N02 toxicity, an evaluation of effects of concern
and effect levels, and a description of most sensitive population
groups and .size estimates. Staff judgments are made concerning
which effects are important for the Administrator to consider in
selecting appropriate primary standard(s). Section VI identifies
and discusses factors important in selecting a primary standard
including possible averaging times and forms of the standard.
Drawing on these factors and information contained in Section V,
staff conclusions/recommendations are presented in Section VII
for the Administrator to consider in selecting primary N02 NAAQS,
In a similar approach for selecting a secondary standard or
standards for NO,, Section VIII provides information on ecosystem
response (i.e., impact on terrestrial systems, wetlands and bogs,
and aquatic systems), critical loads and other policy options,
visibility effects, human welfare effects, and materials damage.
Using this information, Section VIII offers an analysis of the
key information the Administrator needs to consider in selecting
an appropriate secondary standard for N02• Staff conclusions and
recommendations concerning the secondary standard for NO2 are
summarized in Section IX.
IV. AIR QUALITY TRENDS
Nitrogen dioxide (N*^) is a brownish, highly reactive gas
which generally exists at higher concentrations in urban than in
rural atmospheres, It is formed in the ambient air through the
oxidation of nitric oxide (NO), a primary pollutant emitted
directly from stationary (e.g., electric utility and industrial
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•'boilers) and mobile (e.g., automobiles) sources. Nitrogen oxides
;:;{NOX) , the term used to describe the sum of NO and N02, play a
major role in the formation of ozone in the atmosphere through a
complex .series of reactions with volatile organic compounds.
;About 5 to 10 percent by volume of the total emissions of NOX
•:from combustion sources is in the form of NC>2< although
substantial variations from one source to another have been
observed (CD, pg. 3-10).
A, Long-Term Trends
Long-term trends for NO2 are based on monitoring data
collected over a ten-year period using instruments capable of
recording as many as 8760 hourly ..observations per year. For the
period 1983 to 1992, the long-term trends were based on data from
183 sites, which were selected because each had annual mean NC^
concentrations based on a minimum of 4380 hourly observations.
Although the sites had to meet only the minimum data requirements
'to be included in the trends database,.90 percent of the sites
•. had greater than 75 percent data completeness. 43 National Air
Monitoring Station (NAMS) sites had data for at least 8 of the
•f'last 10 years and thus met the selection criteria for the 10-year
database.
Figure IV-1 represents: the composite long-term trend for the
N02 mean concentrations at 183 trend sites and 43 NAMS sites.
The 95 percent confidence intervals shown represent the
confidence range of the annual averages for the years reviewed.
For the period 1983 to 1989, the 95 percent confidence intervals
about the composite means indicates that NO2 levels are
statistically indistinguishable. However, the 1992 composite '
• average N02 level is 8 percent lower than the 1983 level, and the
difference in the two levels is statistically significant.
At the NO2 NAMS sites, which are located only in large .urban
•areas with populations of one million or greater, a similar trend
is apparent, although the composite averages of the NAMS data are
higher than those of all 183 sites. Compared to the composite
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8
average in 1983, the composite average of the annual NO, mean
concentration of data collected at the 43 NAMS sites was n
CONCENTRATION, PPM
4\ f\f- .. __.. . ___...
0.05-
0.04-
0.03-
0.02 H
0.01 H
0.00
NAAOS
AULSdES (183)
NAMS SITES (43)
i i i i(i(i i r
1983 1984 1985 1986 1987 1988 1989 1990 1991 1992
figure IV^l .National trend in the composite annual overage nitrogen dioxMe concentration at both
NAMS and all sites with 95 percent confidence intervals, 1983-1992,
Source: Figure 3-13. p- 3-17 of the Rational Air Quality and Emissions Trends
Report (U.S. EPA, 1993b).
percent lower. This is also a statistically significant
difference.
It is important to note here the relationship between short-
term exceedances of N02 concentrations and the annual N02 mean.
In 1994, McCurdy analyzed air quality data from the period 1988-
1992 to determine the estimated number of exceedances of various
NOj short-term air quality indicators would occur given
attainment of a range of annual averages. The annual averages
McCurdy analyzed ranged from 0.02 to 0.06 ppm and included the
current NO2 NAAQS of 0.053 ppm. The 1-hour and daily
concentration levels chosen for analyses were 0.15, 0,20, 0.25,
and 0.30 ppm. The results of this analysis are reported in
"Analysis of High 1 Hr NO2 Values and Associated Annual Averages
Using 1988-1992 Data" (McCurdy, 1994). In his report, McCurdy
-------
concluded that areas attaining the current annual NO2 NAAQS
reported few, if any, one hour or daily exceedances above 0.15
ppm. Excerpts from the conclusion section of this report can be
found in Appendix B of this Staff Paper.
B." Recent Trends in Urban Areasof the United States
Los Angeles (LA) is the only city in the U.S. to record
violations of the annual average NO2 NAAQS during the past
decade. However, in 1992, Los Angeles reported air quality
measurements which meet the NO2 NAAQS for the first time. Thus,
currently, the entire U.S. is in attainment of the current N02
NAAQS.
Regional trends in the composite average N02 concentrations
for the years 1990 to 1992 are presented as bar graphs in Figure
IV-2 with the exception of Region X, which had inadequate data.
0.040 -
CONCENTRATION, PPM
0.035 T
COMPOSITE AVERAGE
•i 1990 mm 1991 a 1992
EPA REGION I II Hi JV V VI VII VIII IX
NO, OF SITES 17 12 35 22 23 26 11 10 79
taavgen dioxide conomtrations.
source:
1931, im aaipasiie amps
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10
Although there was no change between 1990 and 1991 in the
composite annual mean N02 levels at 235 sites, there was a 3
percent decrease between 1991 and 1992.
Different selection criteria are used to determine which
sites will be included in the long-term national trends database
described in the preceding section than is used to determine
which sites will be used in the short-term regional trends
analysis. 50 NAMS sites had data in each of the last three years
and thus met the criteria to be included in the 3-year database.
For these 50 NAMS sites, the composite mean concentration
decreased 4 percent between 1991 and 1992.
Bar graphs for five of the nine regions (I, II, III, V, and
IX) depicted in Figure IV-2 show 1992 composite average NC>2
annual mean concentrations that are lower than the composite
average NOj annual mean concentrations for 1990 and 1991. The
other four regions (IV, VI, VII, and VIII) recorded increases
between 1991 and 1992, although the 1992 composite average N02
annual mean concentrations for Region IV are less than the 1990
composite mean. (These graphs are intended to depict relative
changes in regional concentrations only and should not be used to
indicate differences in absolute concentrations). For a more
thorough discussion and description of national and international
air quality trends, the reader is referred to the National Air
Quality and Emissions Trends Report, 1992 (U.S. EPA, 1993b).
C- Nitrogen'Deposition Trends in the United States
Atmospheric nitrogen inputs occur as both wet and dry
deposition. Most of the data on nitrogen deposition are limited
to information on nitrate deposition in rainfall (i.e., wet
deposition). Figure IV-3 summarizes mean annual wet deposition
rates for nitrate for states that were part of the National Acid
Deposition Program (NADP). The figure shows that the highest
deposition rates are concentrated in the northeastern United
States. The highest rates noted were in Pennsylvania and New
York which had nitrate deposition rates of 12 and 11 kg/ha/yr,
-------
Kg/ha
Total Nttrogen
!- Nitrate
- Ammonium
14
« IV-3 Mean «MU8l wet nitrate and ammonium deposition to various states located throughout the United States.
D* fen to Nriknrf Atnwipheric Dq^i.ion Pwgrtffl (I9W) « for . .
1988.
«l to.
Bohm (1991) .« for th« period
-------
12
respectively. Illinois, Tennessee, and Georgia showed rates of
approximately 8 kg/ha/year.
Accurate measurements of wet deposition are carried out by
analyzing nitrogen in precipitation immediately following a
precipitation event. Frequently, however-, the rainfall is
accumulated over some period of time before it is analyzed. The
resulting measurement of deposition rate is usually referred to
as bulk deposition because it combines wet deposition with some
component of dry deposition.
Because of the difficulty associated with accurately
measuring dry deposition, researchers have assumed that dry'
deposition .rates are some fraction of wet deposition rates.
Researchers have attempted to model the fate of NOX emissions to
the atmosphere. The models have estimated that dry deposition
accounts for one-half to two-thirds of the total NOX deposition
in North America (Levy and Moxim, 1987; Hicks et al., 1991). The
correlation between wet and dry deposition is discussed further
in Section VIII.A.2 of this Staff Paper and in Chapters 5 and 10
of the Criteria Document.
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13
V. HEALTH EFFECTS OF NITROGEN DIOXIDE
A. introduction
This section presents those elements which are critical to
the review of the primary NAAQS for N02• In addition to NO2,
there is a variety of NOX and related nitrogen compounds which
occur in the ambient air both naturally and as a result of human
activities, including NO, nitrous oxide (N20), gaseous nitric
(HN03) and nitrous (HNQ2) acids, dinitrogen pentoxide (N205),
dinitrogen trioxide (N2Q3), dinitrogen tetroxide (N204), and
ammonia (NH3). However, based on currently available
information, only N02 is sufficiently widespread and commonly
found in ambient air at high enough concentrations to be a matter
of public health concern. Therefore, this section will focus on
mechanisms of toxicity, health effects of concern, and
susceptible subpopulation groups for NO2, the control of which
serves as a surrogate for -the control of other nitrogen compounds
in the troposphere,
B. Mechanisms of Toxicity, Transport.,. and Fate
Nitrogen dioxide has been shown to produce lung cell injury
and/or death and other respiratory effects in humans and animals
exposed both acutely and chronically. The principle mechanisms
of toxicity associated with N02 inhalation involve oxidation of
unsaturated fatty acids in cell membranes and of functional
groups in soluble (e.g., enzymes) and structural (e.g., cell
membranes) proteins (Menzel, 1976; Freeman and Mudd, 1981) .
Support for this mechanism of toxicity is provided by studies
which have shown an increase in both lipid peroxidation products
and in lung antioxidant enzymes immediately following exposure to
N02 (Sagai et al., 1984). Further supporting this oxidative
mechanism are reports that animals deficient in vitamins C and E
(i.e., antioxidant vitamins) tend to be more susceptible to N02
exposures (Belgrade et al., 1981; Sevanian et al., 1982). The
fundamental mechanism of pulmonary edema (i.e., excess fluids in
the lungs) resulting from exposure to N02 may be the cytotoxic
effects of N02 directly on epithelial cell membranes, while the
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14
mechanism responsible for increased susceptibility to viral and
bacterial infection may be the cytotoxicity of NO2 on membranes
of alveolar macrophages (AM's) (CD, p. 13-197). The above
mechanisms of toxicity are generally considered to be related to
those health effects of N02 which are of greatest public health
concern and will be discussed further in Section V.C below.
Because N02 is not very soluble in the fluids lining the
respiratory tract, it is capable of penetrating to the distal
airways, particularly during heavy exercise. The reactive nature
of NQ2, however, tends to result in scrubbing of a large fraction
in the upper respiratory tract under normal conditions. Total
respiratory tract uptake in humans has been reported between 72
and 92%, depending on investigator and breathing state (Wagner,
1970; Bauer et al,, 1986). Similarly, total respiratory tract
uptake in dogs at rest•is 78% versus 94% in exercising dogs
(Kleinman and Mautz, 1991). Generally, as ventilation rate
increases, the percentage uptake of NO2 in the lower respiratory
tract, as well as total uptake, increases (CD, p. 13-196),
therefore resulting in an increase in effects associated with NQ2
exposure.
Mathematical modeling of the lower respiratory tracts of
humans, rats, guinea pigs, and rabbits by Miller et al. (1982)
and Overton et al. (1984) has indicated that the greatest dose of
N02 is delivered to the centriacinar region (CAR) (i.e., the
junction between conducting airways and the gas-exchange region).
This is consistent with findings in animal studies reporting that
the CAR is the site where NQ2~induced lesions are observed
morphologically.
The ultimate fate of N02 following reaction with respiratory
fluids and tissues appears to be formation of other chemicals,
such as nitrous and nitric acids,, which then can be systemically
transported (Goldstein et al., 1973). Nitrite produced in the
lungs may enter the bloodstream and react with hemoglobin to
increase methemoglobin levels (Postlethwait and Mustafa, 1981,
1989; Saul and Archer, 1983). For a more thorough discussion of
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15
mechanisms of toxicity, transport, and fate of inhaled N02» the
-reader is referred to Chapter 13 of the CD.
C. Health Effects Evidence
Health effects information which is pertinent to review of
the NAAQS for N02 has been thoroughly reviewed in Chapters 13
through 16 of the CD. Many of the health studies, particularly
animal toxicology studies, reviewed in the CD were conducted at
levels of NQ2 much higher than those typically found in the
ambient air. Although reference is made in this section to some
studies conducted at very high NO2 levels, the focus of this
Staff Paper is primarily on those key studies conducted at levels
of NO2 which are relevant to regulatory decision making.
Key health effects which have been associated with exposure
to NQ2 include: (1) increased susceptibility to respiratory
symptoms and disease in children; (2) pulmonary function
decrements [e.g., forced expiratory volume in 1 second (FEV-jJ ,
forced vital capacity (FVC)], symptoms (e.g., cough, odor
detection, nasopharyngeal irritation), and increased airway
resistance (Raw) in asthmatic subjects and in patients with
chronic obstructive pulmonary disease (COPD); (3) increased
airway responsiveness in asthmatics; and (4) emphysema observed
only in animals after exposure to very high N02 levels for
extended periods. Although all of these effects have been
reported in numerous studies, there continue to be great
uncertainties regarding the concentration-response relationships
between each category of health effect and exposure to N02.
1. Susceptibility toRespiratory Illness
Respiratory illness and those factors which affect either
susceptibility or severity are important public health concerns.
Any increase in susceptibility to respiratory illness in children
which might be caused by N02 is of particular concern due to the
potential for human exposure to NQ2, the common occurrence of
respiratory illness in children, and the fact that recurrent
childhood respiratory illness may be a risk factor for later
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16
increased susceptibility to lung disease (Samet et al., 1983;
Samet and Utell, 1990; Glezen, 1989),
Support for the hypothesis that exposure of children to N02
increases susceptibility to childhood respiratory illness comes
primarily from epidemiological and animal toxicological studies
and to a lesser extent from clinical studies. Epidemiological
evidence is largely from studies which investigated respiratory
effects on children living in homes with gas stoves. Although
there continues to be great uncertainty in quantitatively
extrapolating results of animal studies to human health effect
levels, the animal studies do provide a biologically plausible
hypotheses for relating NC^ exposure to the types of respiratory
morbidity observed in humans. Although the studies are few,
controlled human exposures suggest a link between NO2 inhalation
and alterations in host defenses.
a. Epidemiological Studies
A thorough discussion of relevant epidemiological studies
reporting evidence of an association between N(>2 exposure and
respiratory illness can be found in Chapter 14 of the CD. The
most pertinent of these studies reported effects of indoor NC^ on
children (ages 5 to 12) and infants (ages < 2) living in homes
with gas stoves as the major source of NO2. Although far fewer,
there also have been several studies published which investigated
the impact of ambient N02 on prevalence of respiratory disease in
children and young adults,
One important consideration in analyzing these studies for
the purpose of standard setting is that human exposure patterns
may differ significantly for the indoor environment in a home
with a gas stove versus a typical outdoor environment.
Typically, indoor exposures can be characterized by higher peak
exposures. Because of the marked differences in exposure
patterns, extrapolation of the results of the meta analysis
described below to ambient conditions is difficult. Further
discussion concerning the uncertainties associated with exposure
patterns can be found in section V.C.l.b below.
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17
1) Indoor Studies
During the past 15 years, most of the key epideraiological
studies of NQ2 have been conducted in homes with gas stoves.
Among the earliest of the gas-stove studies were those conducted
by Melia et al. (1977, 1979, 1980), Goldstein et al. (1979,
1981), and Florey et al. (1979, 1982), who reported that children
from randomly selected areas of Scotland and England had an
increased risk of respiratory disease if they lived in homes with
gas stoves. In a later study conducted by Melia et al. (1982),
results of a questionnaire completed by parents showed that
respiratory symptoms of girls living in urban areas were
positively related to bedroom levels of NO2. Subsequent
reanalysis by Hasselblad et al. (1992) suggested that an increase
of 0.015 ppm in bedroom NC>2 levels yields an 11% increase in the
odds of respiratory illness, where mean weekly concentrations in
bedrooms in studies reporting NOj levels were predominantly
between 0.0008 and 0.065 ppm NO2.
Odds ratios offer a useful measure for health assessment and
have been used to report results for many of the gas-stove
studies. For example, when Hasselblad et al. (1992) reanalyzed
the Melia et al. (1977) data using a multiple logistic model,
they found the combined odds ratio for boys and girls to be 1.31
(95% confidence limits of 1.16 and 1.48). This suggests that gas
stove use in this study is associated with an estimated 31%
increase in the odds for children having respiratory illness
symptoms. Similar reanalysis of Melia et al. (1979) data by
Hasselblad et al. (1992) produced a combined odds ratio for both
genders of 1.24 (95% confidence limits of 1.09 and 1.42),
suggesting that gas-stove use is associated with an estimated 24%
increase in the odds of having respiratory symptoms in children.
In another series of studies conducted in six U.S. cities
(often referred to as the Six City Study), respiratory illness of
children was investigated in a large number of homes with gas
stoves. In an early analysis of Six City Study data, Speizer et
al. (1980) calculated an odds ratio of 1.12, suggesting that N02
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18
emissions from gas stoves increase the rate of serious
respiratory illness before age 2 by 12%. Later analysis of 1974-
1979 data from the Six City Study by Ware et al, (1984) yielded
an unadjusted odds ratio of 1.08 (95% confidence limits of 0.97
and 1.19) for a lower respiratory illness index associated with
gas stove use. Other indicators such as bronchitis, cough, and
wheeze did not show any increased incidence.
Dockery et al. (1989a) analyzed 1983-1986 data on 5,338
white children (ages 7 to 11); they reported only a marginally
significant association between gas stove usage and doctor-
diagnosed respiratory illness but found no association with
chronic cough,- bronchitis, persistent wheeze, and restriction of
activity due to chest illness.
A different Six City Study cohort was investigated by Neas
et al. (1990, 1991), who studied a stratified one-third random
sample of the children that were part of the Dockery et al.
(1989a) analysis. The sample was restricted to 1286 white
children (ages 7 to 11) living in homes with at least one valid
indoor measurement of N02 and respirable particles. Parents
completed a questionnaire regarding symptoms during the previous
year, which included attacks of shortness of breath with wheeze,
persistent wheeze, chronic cough, chronic phlegm, and bronchitis.
Neas et al. (1990, 1991) defined a combined symptom measure as
the presence of any of the above symptoms, and in a multiple
regression of this combined lower respiratory symptom measure
estimated the odds ratio to be 1.40 (95% confidence limits of
1.14 and 1.72) for an additional exposure to NC>2 of 0.015 ppm.
This estimated effect was consistent across seasons and sampling
locations.
Several other studies provided somewhat more equivocal
evidence. In a survey of 1,355 children (ages 6 to 12) conducted
in Iowa City, Ikwo et al, (1983) reported a statistically
significant association between gas stove use in the home and
hospitalization for chest illness before age 2, but the
association between gas-stove use and chest congestion/phlegm
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19
with colds was not significant. A study of 775 children (ages 6
to 12) conducted in the Netherlands by Brunekreef et al. (1989)
and Dijkstra et al, (1990) showed no evidence of an increase in
respiratory disease with increasing NC?2 exposure, but the range
of uncertainty is large and rates were not adjusted for
covariates such as parental smoking and child's age. Keller et
al. (1979a,b) reported no statistically significant changes in
respiratory disease associated with living in homes with gas
stove use (odds ratio of 0.72, 95% confidence limits of 0.30 and
0.74). Many of the studies of children (ages 5 to 12) discussed
above are summarized in Table V-l and form the basis for a meta-
analysis to be- discussed in Section V.l.b.
Individual studies of the effects of NC^,. exposure on infants
(ages <, 2 years) provide no consistent relationship between
estimates of NO2 exposure and the prevalence of respiratory
symptoms and disease. In Albuquerque, NM, Satnet et al. (1993)
conducted a prospective cohort study of infants, during their
first 18 months of life, using 2-week average NC>2 concentrations
in bedrooms as estimates of exposure to NC>2. In this carefully
conducted study, Samet et al. (1993) defined illness events of at
least two consecutive days of either runny nose, wet cough,
wheezing, or trouble with breathing. The analysis was limited to
1205 subjects completing at least one month of observation.
Findings indicate that in a population of healthy infants there
was no significant association between NO^ exposure estimates (in
the range 0 to 0.04 ppm NO2) and respiratory illness when
precaution was taken to make an accurate assessment of exposures,
'to validate measurements of respiratory illness, to eliminate
potentially confounding variables, and to adjust for key
variables (CD, p. 14-33).
Several other epidemiology studies attempting to associate
N02 exposure with increased risk of respiratory effects in
infants offer mixed results. Presence of a gas stove was
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TABLE V-I. HEALTH OUTCOME AND NITROGEN DIOXIDE EXPOSURE MEASURES USED IN
SELECTED INDOOR NITROGEN DIOXIDE EPIDEMIOLOGY STUDIES
Vfr. rente
HeaMh Outcome Us«! in Me! A- Ana lysis
Melhod3
NC>2 Exposure
Measure Used in
Analysis" •
Age Sample Where/When
(years) Site
Melifl ef ai.,
0977)
Nfelta et al.,
0970)
Meliaet at., (1980)
Rorcy et al., (1979)
Goldstein et al.,
(1979)
Colds going to chest showed a prevalence
of26.8-!9.R%.
Responses to respiratory questions grouped
into (a) none or (b) one or more symptoms
or disease types. Colds going to chest
(26.4-19.6%) showed the highest
prevalence, followed by wheeze (10.1-
6.291), cough, and episodes of asthma or
bronchitis in last year
Group response to respiratory questions as
above.
Symptoms during
past 12 mo
recalled by child's
parent In
completing
respiratory
symptoms
questionnaire.
As above.
As above.
Gas stove vs.
electric stove.
Gas stove vs.
electric stove.
NC»2 measured
wiih Palmes
tubes. Oas stove
homes only.
6-11 5,658 28 Areas of
England and
Scotland (1973)
5-10 4,827 27 areas of
England and
Scolland(1977)
6-7 103 Mtddlesborough,
England (1978)
NJ
O
ia et al.,
As above.
As above.
NOj measured
with Palmes
tubes. Gas stove
homes only.
5-6 188 Middlesborough,
England (1978)
-------
Reference
Ware et at.,
(1984)
Neas et al,,
(1990, 1991)
Ekwo ef al,,
(1983)
Dtjkstra «t nlM
(1990)
Brunekreef et at.,
(198?)
Ketler et at.,
(I9?9a,b)
Health Outcome Used in Mela-Analysis
Lower respiratory illness index (index of
respiratory health) indicating daring past
year the presence of (a) bronchitis, (b)
respiratory illness that kept the child home
3 days or more, or (e) persistent cough for
3 mo of lh« year.
Combined indicator of one or mow lower
respiratory symptoms as defined. The
highest prevalences were for chronic
phlegm and wheeze. The other symptoms
In tha index are shortness of breath,
chronic cough, and bronchitis. Chest
illness reflects a restriction' of the child's
activities for 3 or more days.
Chest congestion and phlegm with cotds.
Respiratory Illness combination variable of
presence of one or more of cough, wheeze,
or asthma.
Respiratory HIness,
Method*
Questionnaire
(Ferris, 1978)
completed by
parent for
symptoms during
previous 12 mo.
Symptom
questionnaire
completed by
parent for ihe year
during which
measurements of
NC>2 were taken.
Questionnaire
(ATS) completed
by parent.
Questionnaire
' (WHO) completed
by parent.
Telephone
Interview by nurse
epidemiologist.
N02 Exposure
Me»sum Used in
Analysis''
Gas vs. electric,
NC>2 measured
with Palmes
tubes. G»s and
electric stoves.
Gas stove vs.
eleclrie stove.
NC>2 measured
with Palmes
tubes. Gas and
electric
appliances,
Gas Hove vs.
eleclrie stove.
Age Sample Where/When
(years) Size
6-10 8,240 Six U.S. cities
(1974- 1979)
7-11 1,486 Six U.S. cities
(1983-1916)
6-12 1,138 Iowa Cily, lows
6-12 7?S Netherlands
(1986)
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22
associated with increased upper respiratory illness in 1-year
olds living in northern Scotland (Ogston, et al. 1985), while
Melia et al. (1983) found no relationship between use of gas
stoves and prevalence of respiratory symptoms in infants under
one year old, Margolis et al. (1992) studied the prevalence of
persistent respiratory symptoms in 393 infants of different
socioeconomic status (SES) living in North Carolina during their
first year of life and reported that infants in the low-SES were
more likely to have symptoms than high-SES infants, though the
difference was statistically significant only for those not in
day care. Of the infants studied by Margolis et al, (1992), only
41 lived in homes with gas stoves, and the relative risk of
persistent respiratory symptoms among those infants exposed to
NO-5 from gas stoves, unadjusted for any covariates, was 1.12 (95%
£*
confidence interval of 0.63 to 2.04). The studies of infants
discussed above formed the basis for a meta-analysis discussed in
Section V.C.l.b below.
2) Outdoor Studies
Several epidemiological studies have been conducted in the
U.S. and other countries to examine the relationship between
ambient N02 levels and respiratory health effects. In one part
of the Six Cities Study. Dockary et al. (I989b) used data. from
centrally located ambient monitors to investigate associations
between respiratory symptoms and air pollution and found the
strongest associations were between respiratory symptoms and
particular matter. Even though the odds ratios for respiratory
symptoms (e.g., bronchitis, chronic cough, chest illness) with
ambient N0? were not statistically significant, the direction was
consistent with results reported in the indoor studies.
Several studies conducted in various locations around the
U.S. have reported little or uo relationship between ambient N02
levels and respiratory illness In a study conducted in Chestnut
Ridge, PA,. Veda*1 et al, (1987) repotted no quantitative
relationship between dajly respiratory symptoms (e.g., wheeze,
pain on brf-athing, or phlegm) .and ambient N02 levels; however,
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23
the number of subjects included in the analysis was only 55, A
California Seventh Day Adventist study (Euler et al., 1988)
concluded that N02 exposure levels were not linked to chronic
respiratory disease symptoms, Although the Chattanooga studies
conducted by Shy et al. (1970a,b; 1973) offer qualitative
evidence of higher respiratory illness rates for families living
in higher-N02 compared to lower-N02 neighborhoods, Pearlman et
al, (1971) reported results in Chattanooga that were not
completely consistent with the exposure gradient because rates of
bronchitis were just as high in the intermediate-pollution area
as in the high-pollution area,
In contrast, several European studies have investigated and
found a relationship between respiratory disease and ambient NC^
levels, albeit with the contribution of other pollutants. In a
study conducted in Switzerland, Braun-Fahrlaender et al.; (1989,
1992) found that outdoor levels of N02 were predictive of
duration of respiratory disease episodes. In two studies
conducted in Germany, Schwartz et al. (1991) showed a
relationship between short-term fluctuations in'ambient
NQ2/particulate matter levels and'medical visits for croup
symptoms, while Rebmann et al. (1991)'reported a relationship
between croup with positive virologic testing and ambient NO2
levels. Finally, in Finland Jaakkola et al. (1991) report a
significant association between the occurrence of upper
respiratory infections and living in an air-polluted area for
both infants and 6-year old children. The authors did conclude
that other pollutants (e.g., particulate matter, S02, H2S, etc.)
may have contributed to the effects.
Outdoor epidemiological studies do appear to provide limited
evidence of an association between ambient exposures to NO2 and
increases in respiratory symptoms and illness. However,
uncertainties regarding actual exposures to NO2 and the extent to
which other factors (e.g., other pollutants, allergens, weather)
may have contributed tend to limit development of a quantitative
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24
relationship. Thus, staff concludes that this information should
be factored into developing an adequate margin of safety.
b, Meta-Analysis of Gas Stove Studies
Conclusions of the individual epidemiological studies cited
above and others discussed in Chapter 14 of the CD are somewhat
mixed with regard to the effect of NC^ on lower respiratory
symptoms and disease. However, most of the indoor studies used
in a synthesis of evidence, also referred to as a meta-analysis,
conducted by Hasselblad et al. (1992) showed significantly
increased respiratory disease rates associated with increased N02
exposure in children (ages 5 to 12) but not in infants. As
previously .mentioned, human exposure patterns differ
significantly for the indoor environment in a home with a gas
stove versus a typical outdoor environment. Therefore,
extrapolation of the results of the meta-analysis to ambient
conditions is difficult. A detailed discussion of the meta-
analysis can be found in the CD (pp. 14-55 to 14-78), and only a
summary will be provided here.
The standard health effect endpoint chosen for the analysis
was the presence of lower respiratory symptoms and illness.
Requirements for inclusion of a study in the analysis were:
(1) the health endpoint of the study must be reasonably, close to
the standard endpoint; (2) significant exposure differences
between subjects must exist and some estimate of exposure must be
available; and (3) an odds ratio for a specified exposure
estimate must have been calculated or data must be presented so
that an odds ratio can be calculated. Table V-2 is a summary of
studies selected for the meta-analysis and of estimated odds
ratios arid confidence intervals for each,
Meta-analysis of data from the selected gas-stove studies
gives an estimated odds ratio of 1.2 (95% confidence limits of
1.1. and 1.3) for increased lower respiratory symptoms and
illness in children ages 5 to 12 (Hasselblad et al., 1992). This
odds ratio corresponds to each increase of 0,015 ppm in estimated
2-week average NO, exposure, where mean weekly concentrations in
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25
TABLE V-2. SUMMARY OF ODDS RATIOS FROM INDOOR STUDIES
OF THE EFFECTS OF NITROGEN DIOXIDE
Estimated 2.5 and 97.5 Percentiles
Author Odds Ratio (Confidence Interval)
Melia et al. (1977) 1.28
Melia et al . (1979) 1.22
Melia et al . (1980) 1.49
Melia et al. (1982a) 1.11
Ware et al. (1984) 1.07
Neas et al. (1991) 1.40
Ekwo et al. (1983) 1.09
Dijkstra et al. (1990) 0.94
Keller et ml. (1979b) 0.75
1.14 to 1.43
1.08 to 1.37
1.04 to 2.14
0.84 to 1.46
0.98 to 1.17
1.14 to 1.72
0.62 to 1.45
0.70 to 1.27
0.35 to 1.62
Sources Table 14-19, p. 14-71 of the CD (U.S. EPA, 1993a).
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26
bedrooms in studies reporting N02 levels were predominately
between 0,008 and 0,065 ppm N02.
For studies of infants 2 years old and younger, a meta-
analysis yielded no consistent relationship between estimates of
NC>2 exposure and prevalence of respiratory symptoms and illness.
The combined odds ratio for increased respiratory disease per
increase of 0.015 ppm NQ2 (2-week average) was 1.09 (confidence
interval of 0.95 to 1.26). Based on these results which clearly
contain the no-effect value of 1.0 (i.e., not statistically
significant), the CD (p. 17-78) concluded that the evidence did
not suggest an effect in infants comparable to that seen in
children 5 to '12 years of age. Results of the both the 5-12 year
old and the infant meta-analysis are summarized in Table V-3.
In summary, two major conclusions which can be drawn from
the indocr-N09 epidemiology studies and ,the associated meta-
£t
analysis are: (1) for studies of children (ages 5 to 12 years),
an increased risk of about 20% for developing respiratory
symptoms and disease corresponds to each, increase of 0.015 ppm
N02 in estimated 2-week average N02 exposure, where mean weekly
concentrations in bedrooms reporting NO2 levels were
predominately between 0.008 and 0.065 ppm N02 (CD, p.'14-73); and
(2) for studies of infants (< 2 years old)', although the combined
odds ratio estimate is positive (1.09, CI 0,95 to 1.26) for the
increase in respiratory disease per increase of 0.015 ppm NC^, it
clearly contains the no-effect value of i.O (i.e., is not
statistically significant), and so the evidence does not suggest
an effect in infants comparable to that seen in older children
(CD, p. 14-75).
Assessing the potential value of the meta-analyses to
developing the basis for a NA/iQS for N02 is limited by several
considerations, There remains substantial uncertainty about the
actual exposures of subjects in the above studies. The NC>2
levels which were monitored j.n the gas-stove studies are only
estimates of exposure and do not represent actual exposures. The
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27
TABLE V-3. KEY EPIDEMIOLOGICAL STUDIES OF EXPOSURE TO
NITROGEN DIOXIDE - SUMMARY OF META-ANALYSIS
NO2 (ppm)
(Exposure Duration)
Observed Effects
References
0.015-ppm increase, where
mean weekly
concentrations in
bedrooms in studies
reporting levels were
predominately between
0.008 and 0.065 ppm NO2
(in 1- and 2-week
integrated average NO2
concentration estimating
an unspecified long-term
average)
A meta-analysis shows increased risk
of lower respiratory
symptoms/disease in children 5 to 12
years old associated with exposure
estimates of NO2 levels. The 95%
confidence interval of the odds ratio
estimated by Hasselblad et al. (1992)
was 1.1 to 13. Predominant source
of exposure contrast is homes with
gas stoves vs. homes with electric
stoves.
Melia et al.
(1977, 1979,
1980, 1982)
Ware et al.
(1984)
Neas et at.
(1991)
Ekwo et al.
(1983)
Dijkslra et al.
(1990)
Keller et al.
(1979)
0.015-ppm increase in
annual average of 2-week
NO2 levels, where mean
weekly concentrations in
bedrooms were
predominately between
0.005 and 0.050 ppm NO2
In individual indoor studies of
infants 2 years of age and younger,
no consistent relationship-was found
between estimates of NO2 exposure
and the prevalence of respiratory
symptoms and disease. Based on a
meta-analyses of these infant studies,
the combined odds ratio for the
increase in respiratory disease per
increase of 0.015 ppm NO2 was 1.09
with a 95% confidence interval of
0.95 to 1.26. Thus, although the
overall combined estimate is positive,
it dearly contains the no-effect value
of 1.0, (he., is not statistically
significant); and so we cannot
conclude that the evidence suggests
an effect in infants comparable to
that seen in older children (see
Chapter 14).
Samet et al.
(1993)
Margolis et al.
(1992)
Dockery et al.
(1989)
Ogston et ml.
(1985)
Ware et al.
(1984)
Ekwo et al.
(1983)
Melia et al.
(1983)
bource: lablc 16-2, p. 16-14 of the CD (U.S. EPA,
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28
studies collected 2-week average NC>2 measurements and, therefore,
cannot distinguish between relative contributions to respiratory
symptoms and illness of peak and average exposure to N02- To the
extent that the endpoints investigated are affected by a peak
exposure, repeated peak exposures, or a combination of peak and
longer-term average exposures, uncertainty is increased in the
results and conclusions of the meta-analysis. Another factor to
consider is the presence of indoor nitrous acid, which is a
byproduct of gas combustion; although nitrous acid might affect
the health endpoints observed, very little is known about either
health effects or indoor concentrations associated with this
substance. Finally, and perhaps most significant to this NAAQS
review, is the fact that indoor exposure patterns- to N00 are
AS*
quite different compared to outdoor exposure patterns. With much
higher peaks and average indoor exposures than would ,be found
outdoors, it is extremely difficult to extrapolate the results of
the meta-analysis in a manner which would provide quantitative
estimates of health impacts for outdoor exposures to NC>2.
c. Biological Plausibility
Animal toxicology studies, and to a lesser extent controlled
human exposure studies, provide evidence for possible underlying
mechanisms of NC^-induced respiratory illness. These studies
have shown that exposure to NC^ can impair components of the
respiratory host defense system and increase susceptibility to
respiratory infection. The increased respiratory symptoms and
illness in children reported in the epidemiology studies cited
above may be a reflection of the increased susceptibility to
respiratory infection caused by the impact of NO2 on pulmonary
defenses The studies discussed below, and in greater detail in
Chapters 13, 15, and 16 of the CD, provide a plausible biological
basis for making such a hypothesis.
Although the limgs ars commonly invaded by microorganisms
(e.g., viruses, bacteria) which have the potential to initiate
respiratory infection, respiratory defense mechanisms play a key
role in eliminating the infectious agent, thereby reducing the
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29
risk of illness. Animal studies support the contention that
inhalation of N02 damages respiratory defense systems, including
important components of the host defenses such as the humoral and
cell-mediated immune system and the alveolar macrophages (AMs).
Animal infectivity studies also have provided evidence that N02
exposure increases susceptibility to infection by affecting
overall function of the host defense mechanisms.
Humoral and cell-mediated immune systems are essential for
both antibody production and secretion of cellular products,
which regulate normal defense responses and/or destroy certain
invading organisms. Although the pulmonary immune system has not
been adequately studied to assess the impact of NO2 exposure,
there is some indication that NO2 suppresses some systemic immune
responses and that these responses may be both concentration and
time dependent. Although the cause of suppression was not clear
in either study, 4 weeks of continuous exposure to 0.4 ppm N02
resulted in significant suppression of antibody production by
spleen cells (Fujimaki et al., 1982), and 7 weeks of 7 hour/day
exposure to j> 0.25 ppm NO2 had a significant systemic effect on
cell-mediated immunity in mice (Richters and Damji, 1988, 1990) .
When particles deposit below the tnucociliary region in the
gaseous exchange region of the lungs, the AMs kill and remove
viable particles, remove nonviable particles, and process/present
antigens to lymphocytes for antibody production. In this area of
investigation, animal studies of N02 exposure have shown: (1) a
decrease in phagocytic ability of AMs at 0.3 pptn for 13 days for
2 hours per day (Schlesinger et al., 1987) but increased
phagocytosis at 1.0 ppm for 2 days (Schlesinger, 1987a,b); (2)
decreased pulmonary bactericidal activity, altered metabolism,
increased numbers of macrophages,.and morphological changes
(Rombout et al., 1986; Aranyi et al., 1976; Goldstein et al.,
1973; Suzuki et al., 1986; Chang et al., 1986; Mochitate et al,,
1986; Robison et al., 1990); and (3) morphological changes in the
ciliated epithelial cells involved in mucociliary transport
following exposures of 0.5 ppm for 7 months (Yamamoto and
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30
Takahashi, 1984), however, mucociliary clearance was not affected
for exposures up to 5.0 ppm (Schlesinger et al., 1987).
A series of animal infectivity studies has shown that
exposure to NC^ can increase susceptibility to respiratory
infection and result in tnicrobial-induced mortality. These
studies involve exposure of animals to varying concentrations and
durations of NC>2 followed by exposure to an aerosol laced with an
infectious agent (e.g., bacteria or virus). Although the lowest
acute (2-hour) exposure to affect bacteria-induced mortality was
2.0 ppm (Ehrlich et al., 1977), subchronic exposures to NO2
concentrations as low as 0.5 to 1.0 ppm have increased both
bacteria-induced and influenza-induced mortality (Ehrlich and
Henry, 1968; Ito, 1971; Ehrlich et al., 1977). These as well as
numerous other studies (Parker et al., 1989; Gardner et al.,
1977afb, 1979, 1980, 1982; Graham et al., 1987; Jakab, I987a,b;
Motomiya et al., 1973; Miller et al., 1987; Coffin et al.f 1977)
have provided support for the contention that N02 increases
microbial-induced mortality by impairing the host's ability to
defend the respiratory tract from infectious agents, thereby
increasing susceptibility to' viral, mycoplasma, and bacterial
infections. Using susceptibility to respiratory infection as an
index, Gardner et al, (1977a,b) and Coffin et al. (1977)
•concluded that incidence of mortality was significantly more
influenced by concentration of N02 than by duration of exposure.
These studies however, used a large range of exposure
concentrations (0,5 - 28 ppm), beginning above typical ambient
concentrations In the ambient range of exposures, time may be a
more important influence than concentration. However, there was
no data showing clearly the effect of time on effects of long-
term, low-level exposures representing ambient exposure levels.
In the urban air, the typical pattern of N02 is a low-level
baseline exposure on which peaks are superimposed. When the
relationship of the peak to baseline exposure and of enhanced
susceptibility to bacterial infection was investigated, the
results indicated that no simplistic concentration times time
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31
relationship was present, and that peaks had a major influence on
the outcome (Gardner, 1980; Gardner et al,, 1982; Graham et al.,
1987) .
Several other animal infectivity studies (Miller et al,
1987; Gardner et al., 1982; Graham et al., 1987) offered evidence
which indicated that mice exposed to baseline plus short-term
peaks were more susceptible to respiratory infection than either
those exposed to control or background levels of N(>2. This
research also indicated that the pattern of NQ2 exposure had a
major influence on the response. Several of the above-cited
animal studies, which have been identified in the CD (p. 16-16)
as representing key toxicological effects of exposure to NQ2, are
summarized in Table V-4.
Relatively few controlled-exposure studies have been
conducted using human subjects exposed to NC^ and infectious
agents. One such study (Goings et al., 1989) examined the
effects of N02 on pulmonary host defense systems using live
attenuated influenza virus and reported a non-statistically
significant trend toward elevated rate of infection. Similarly,
Frampton et al. (1989a) reported a trend for less effective
inactivation of virus by AMs taken from subjects exposed
continuously to 0,6 ppm NO, for 3 hours, although no effects were
£t
reported in those exposed continuously to 0.05 ppm with three 15
minute 2.0 ppm spikes with exercise. In a related investigation,
Frampton et al. (I989b) found that 3 hours of exposure to 0.60
ppm N02 may transiently increase levels of antiprotease alpha-2-
macroglobulin in lung lavage fluid and thereby may alter AM
defenses against infection. Although these studies are
suggestive, they do not provide clear evidence that N02 increases
susceptibility of humans to respiratory infection.
The weight of evidence provided by animal toxicology and
human exposure studies supports the contention that N02 impairs
the ability of host defense mechanisms to protect against
respiratory infection. Although some of the health endpoints may
not be valid for humans (e.g., increased mortality), there are
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32
TAB1£ V-4. KEY ANIMAL TOXICOLOGICAL EFTECTS OF EXPOSURE TO
NITROGEN DIOXIDE3
(Exposure Duration) Species Observed Effects
References
0.2 ppm (continuous Mouse
base for 1 year) plus
0.8 ppm (I«h peak,
2*/day,
5 days/week)
Q.2S ppm Mouse
(7 h/day),
S days/week,
7 weeks)
03 ppm Babbit
(2 h/day, 2 days)
€.4 ppm Mouse
(continuous,
4 weeks)
0.5 ppm Mouse
(continuous,
3 mo)
03-28 ppm (6 mtu Mouse
to I year)
Increased susceptibility to respiratory Miller et si. (1987)
infection and decreased vital capacity
and respiratory system compliance,
compared to control or baseline only
Systemic effect on cell-mediated
immunity
JUchters and Damji (1988,
1990)
Decreased phagocytosis of alveolar Schlesinger (1987s,b)
macrophages
Decreased systemic humoral
immunity
Increased susceptibility to respiratory
infection
linear increase in susceptibility to
respiratory infection with time;
increased slope of curve with
increased concentration; C more
important than T
Fujimaki et at., (19S2)
Ehrlidi and Henry (1968)
Gardner et aL (1977a,b)
CofTin et al. (1997)
*N02 « Nitrogen dioxide.
C = Concentration of exposure.
T = Duration
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33
many shared mechanisms between animals and humans which support
the hypothesis of association between N02 exposure and increases
in respiratory symptoms and illness reported in the
epidemiological studies. According to the CD (p. 16-9), however,
testing this hypothesis would require performing additional
animal toxicology studies, conducting more diagnostic tests for
infection in future epidemiological studies, and applying
additional approaches in controlled human exposure studies.
2. L.ung ..Function. Symptoms and Airway Resistance
At sufficiently high exposures (e.g., 2 to 8 ppm), acute
exposure to NCu can induce statistically significant pulmonary
function decrements, symptomatic effects, and increased airway
resistance (Raw) in both healthy and sensitive subjects. Airway
resistance is defined as the (frictional) resistance to airflow
afforded by the airways between the airway opening at the mouth
and the alveoli. Although many controlled human exposure studies
of healthy individuals conducted at NC^ concentrations even above
1.0 ppm report negative results, some studies of asthmatics and
patients with chronic obstructive pulmonary disease (COPD)
observe effects following exposures to N02 at concentrations less
than 1.0 ppm.
Human exposure studies summarized in Section 15,2 of the CD
(pp. 15-10 to 15-36} provide very limited evidence of functional
alterations, symptoms, or R&w changes in healthy subjects exposed
for short periods (5 minutes to 2 hours) to NC>2 concentrations
ranging from 2 to 8 ppm. Most of the studies cited report no
changes in lung function or symptoms; however, increased R_,, was
elW
observed after short-term exposures of healthy subjects to 5-6
ppm N02 (Von Nieding et al., 1979; Von Nieding and Wagner, 1977;
Von Nieding et al., 1980; Islam and Ulmer, 1979 a,b). When
healthy subjects were exposed to somewhat lower N02 levels (2 to
4 ppm), no changes were reported in either spirometry or R_
(Linn et al,, 1985b; Mohsenin, 1987b, 1988). None of the studies
investigating healthy subjects exposed to less than 1.0 ppm N02
demonstrated any clear responses to N02 (CD, p. 15-36).
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34
In. contrast to the lack of effects reported with healthy
subjects at lower NO2 levels, there is some evidence that
asthmatics experience symptoms, functional changes, and increased
Raw when exposed to N02 levels below 1,0 ppm (CD, p. 15-36 to 15-
65). In one early study of asthmatics, symptoms of respiratory
discomfort were experienced by 4 of 13 asthmatics exposed to 0.5
ppm for 2 hours; however, Kerr et al. (1979) concluded that the
symptoms were minimal and did not correlate well with functional
changes. In several other studies of asthmatics, very small
changes in spirometry or plethysmography were reported following
acute exposures in the range of 0.1 (Hazucha et al., 1982, 1983)
to 0.6 ppm NO2 (Avol et al., 1988). Hazucha et al. (1982, 1983)
found an eight percent increase in specific airway resistance
(SRaw) after mild asthmatics were exposed to 0.1 ppm NO2 at rest.
However, this finding is not considered statistically
significant. Bauer et al., (1986) reported statistically
significant changes in spirometric response in mild asthmatics
exposed for 20 minutes (with mouthpiece) to 0.3 pprn NO2 and cold
air. Avol at al, (1988) found significant changes in SRaw and
PEV.^ as a function of exposure concentration and duration for all
'exposure conditions (i.e. exposure of moderately exercising
asthmatics for 2 hours to 0.3 ppm and 0.6 ppm NO2) ; however, it
was concluded that there was .no significant effect of N02
exposure on these measures of pulmonary function (CD, p. 15-47).
Exercising adolescent asthmatics exposed (with mouthpiece) to
air, 0 12 ppm and 0,18 ppw NO2 exhibited small changes in FEV1(
but thare wer? no differences 'in symptoms between air and either
of the NOj exposures (Koeuig et al., T987a»b), The absence of
spirometry or plethysmography changes in studies (Avol et al.,
1986; Bylin et al., 1985; Linr et al., X985b; Linn et al,, 1986)
conducted at higher N02 concert-rations makes developing a
concentration-response relationship problematic (CD, p. 15-62) .
Patier.v.a with COPD .ilso hrjve been used as subjects in NO.;,
exposure studies. Due to the hyperresponsiveness of their
airways to physical and chemical stimuli, their already
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35
compromised lung function, and the poor distribution of
ventilation leading to greater N02 delivery to the segment of the
lung that is well ventilated (thus resulting in a greater local
dose), patients with COPD might be expected to experience a
heightened response to N02 exposures compared to healthy
individuals. Early studies (Von Nieding et al., 1970, 1971, -
1973; Von Nieding and Wagner, 1979) found increased Raw with
exposure of COPD patients to 1,6 ppm NQ2 or greater. However, in
a more recent comparative study of healthy and bronchitic
subjects, Von Nieding et al. (1980) reported that responses of
subjects with bronchitis were similar to.those seen in healthy
subjects. Linn et al. (1985a) investigated effects of a 1-hour
exposure to 0.5, 1.0., and 2.0 ppm N02 on intermittently
exercising patients with emphysema and chronic bronchitis; they
found no statistically significant changes in arterial
oxygenation, lung function, or symptoms. Finally, Morrow and
Utell (1989) exposed COPD patients for 3.75 hours to 0.3 ppm N02
during intermittent mild exercise and found progressive and
statistically significant decrements in FVC and'FEV1 during and
after exposure.
In summary, in healthy individuals, even very high acute N02
exposures do not appear to cause pulmonary function effects,
symptoms, or increases in Raw- The current database does,
however, show that small pulmonary function changes have occurred
in asthmatics at low, but not high (i.e., up to 4 ppm), NO2
concentrations. Although the observed effects were noted in
different studies, no plausible explanation is offered to account
for this lack of a concentration-response relationship. The CD,
in assessing the available data on pulmonary function responses
to N02 in asthmatic individuals, concludes that the most
significant responses to NO2 that have been observed in
asthmatics have occurred at concentrations between 0.2 and 0.5
ppm (CD, pg. 16-3). Patients with COPD experience pulmonary
function changes with brief exposure to high concentrations (5 to
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36
8 ppm for 5 minutes) or with more prolonged exposure to lower
concentrations (0.3 ppm for 3.75 hours).
3, Increa s e d Airway Respons i ye ne s s
There is little, if any, convincing evidence that healthy
individuals experience increases in airway responsiveness when
exposed to NC^ levels below 1.0 ppm. However, studies of
asthmatics have reported some evidence of increased airway
responsiveness caused by acute exposure to NC>2 in the range of
0.2 to 0.3 ppm. A detailed discussion of these responses is
provided in Chapter 15 of the CD.
Responsiveness of an individual's airways is typically
measured by evaluating changes in airway resistance or spirometry
following challenge with a pharmacologically active chemical
(e.g., histamine, methaeholine, carbachol), which causes
constriction of the airways. Airway hyperresponsiveness is
reflected by an abnormal degree of airway narrowing caused
primarily by airway smooth muscle shortening in response to
nonspecific stimuli. Asthmatics experience airway
hyperresponsiveness to certain chemical and physical stimuli and
have been identified as one of the population subgroups which is
most sensitive to acute NC>2 exposure (CD, p. 16-1) .
Evidence of increased airway responsiveness in normal adults
has been reported in very few studies. Mohsenin (1988) found
increased airway responsiveness to methacholine following
exposure to 2 ppm N02 for 1 hour at rest. Frampton et al. (1991)
reported statistically significant airway responsiveness
following a 3-hour exposure of healthy subjects to 1.5 ppm NO2
and carbachol challenge, but no increase in airway responsiveness
was reported with exposure to 0,6 ppm NC^ . In one study (Hazucha
et al., 1992), airway responsiveness, which was subsequently
induced by a 2-hour exposure to 0.3 ppm Oj, was augmented by a 2-
hour preexposure to 0 - 6 ppm NO2. None of these studies,
however, provide clear: evidence of increased airway
responsiveness in normal individuals when exposed only to N02 at
concentrations of 1 ppm or lower.
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37
There is evidence that exposure of mildly allergic asthmatic
patients to N02 levels below l. ppm may cause increased airway
responsiveness. Several controlled exposure studies (Ahmed et
al., 1983a,b; Bylin et al., 1985; Hazucha et al., 1982, 1983;
Koenig et al., 1985; Orehek et al., 1981) of asthmatics showed no
significant effect on responsiveness at very low NQ2
concentrations of 0.1 to 0.12 ppm. Using a mouthpiece and
somewhat higher exposures of 0.3 ppm N02, Bauer et al. (1986)
reported a statistically significant response to N02 after 20
minutes rest followed by 20 minutes of exercise (30 L/min); all
had elevated response to cold air bronchoprevocation. A
subsequent study (Morrow and Utell, 1989), using some of the same
subjects in the same laboratory, showed no change in lung
function, symptoms, or carbachol reactivity following exposure to
0.3 ppm NQ2, It is important to note, however, that the-Morrow
•and Utell (1989) study was a chamber .study; thus, the difference
-• in exposure mode (mouthpiece vs. chamber) could account for the
-significant difference in the .study results. Studies which
investigated concentration-response relationships for exposures
of < 0.6 ppm N02 (Roger et al., 1990; Bylin et al., 1985, 1988;
Avol et al., 1988) found no significant changes in spirometry or
airway reactivity as a result of NQ2 exposure. Even for
exposures to 3.0 ppm (Linn et al., 1986) and 4.0 ppm NQ2 (Linn
and Hackney, 1984)-, no effects of NO2 on SRaw, symptoms, heart
rate, or skin conductance were reported in exercising asthmatics.
Folinsbee (1992) analy2ed data on asthmatics experimentally
exposed to N02 in various studies which used challenges producing
increased airway responsiveness in 96 subjects and decreased
airway responsiveness in 73 subjects. For exposures in the range
of 0.2 to 0.3 ppm NC>2, he found that the excess increase in
airway responsiveness was attributable to subjects exposed to NO^
at rest. Because N02 at these levels does not appear to cause
airway inflammation and the increased airway responsiveness
appears fully reversible, implications of the observed increases
in responsiveness remain unclear. It has been hypothesized that
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38
increased nonspecific airway responsiveness caused by N02 could
lead to increased responses to a specific antigen; however, there
is no plausible evidence to support this.
In summary, there is some evidence that acute exposure to
N02 may cause an increase in airway responsiveness in asthmatic
individuals. This response has been observed only at relatively
low NQ2 concentrations, mostly in the range of 0.2 to 0.3 ppm
NC>2. However, the above findings, taken as a whole, do not
provide any clear quantitative conclusions about the health
effects of short-term exposures to N02.
4• Emphysema
Although -no attempt has been made to quantitatively assess
the potential risk of NO2-induced emphysema to humans, the CD (p.
16-9} clearly states that because emphysema is an irreversible
disease representing an important public health concern, the
potential for risk warrants discussion. Investigations of
chronic exposures of animals to N02 levels much higher than those
found in the ambient air have demonstrated that N02 can cause
morphologic lung lesions that meet the criteria for a human model
of emphysema (which requires the presence of alveolar wall
destruction in addition to enlargement of the airspace distal to
the terminal bronchiole). However, only a few studies cited in
Chapter 13 of the CD document emphysema (according to the human
definition) resulting from chronic N02 exposure. Three studies .
(Haydon et al., 1967; Freeman et al., 1972; Port et al., 1977),
in which rats and rabbits were exposed for periods ranging from 1
month to. more than 30 months to N02 concentrations greater than 8
ppm, showed morphologic lung lesions that meet the criteria for a
human model of emphysema. Another study (Hyde et al., 1978)
provided evidence of emphysema (including alveolar destruction)
in dogs exposed to a mixture of 0.64 ppm NO2 and 0.25 ppm NO for
a period of 68 months> The dogs were examined 2.5 to 3 years
after exposure to the mixture The finding that pulmonary
function decrements progressed post-exposure suggests that
morphological effects may have bean progressive also. In the
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39
same study (Hyde et al,, 1978), a low N02 (0.14 ppm) and high NO
(1.1 ppm) mixture produced no evidence of emphysema, thus
implying that N02 was a significant etiblogic factor. Numerous
other studies cited in Chapter 13 of the CD reported emphysema,
but they either lacked sufficient detail for independent
conclusions to be drawn or did not meet all of the criteria for
human emphysema.
In conclusion, it is clear that at sufficiently high
concentrations of NO2 (i.e., > 8 ppm) for long periods of
exposure, N02 can cause emphysema (meeting the human definition
criteria) in animals. Although current information does not
permit identification of the lowest N02 levels and exposure
periods which might cause emphysema, it is apparent that levels
required to induce emphysematous lung lesions in animals are far
higher than any N02 levels which have been measured in the
ambient air.
D. Populations Potentially at Risk
Two general groups in the population may be more susceptible
to the effects of NO2 exposure than other individuals. These
groups -include persons with pre-existing respiratory disease and
children (5-12 years old). Individuals in these groups appear to
be affected by lower levels of N02 than individuals in the rest
of the population.
With regard to decreased respiratory function caused by NC^,
a reasonable hypothesis of enhanced susceptibility to N02 has
been offered for those with preexisting respiratory disease.
Since these individuals live with reduced ventilatory reserves,
any reductions in pulmonary function caused by exposure to NO2
have the potential to further compromise their possibly marginal
health status. Compared to healthy individuals with normal
ventilatory reserves who may not notice small reductions in lung
function, those with preexisting respiratory disease may be
prevented from continuing normal activity following exposure to
N02.
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40
Numerous epidemiological studies conducted in homes with gas
stoves provide evidence that children (5-12 years old) are at
increased risk of respiratory symptoms/illness from exposure to
elevated.N02 levels (Melia et'al., 1977, 1979, 1983; Ekwo et al.,
1983; Ware et al., 1984; Ogston et al., 1985; Dockery et al.,
1989a; Neas et al., 1990, 1991, 1992; Dijkstra et al., 1990;
Brunekreef et al., 1987; Samet et al., 1993). Because childhood
respiratory illness is very common (Samet et al., 1983; Samet and
Utell, 1990), any impact which NC^ might have of increasing the
probability of respiratory illness in children is a matter of
public health concern. This is particularly true in light of
evidence that -recurrent childhood respiratory disease may be a
risk factor for later susceptibility to lung damage (Glezen,
1989; Samet et al., 1983; Gold et al., 1989) (CD, p. 16-4). In
the United States, there are approximately 35 million children in
the age group 5 to 14 years (Centers for Disease Control, 1990) .
Airway inhalation challenge tests have been used to evaluate
the responsiveness of asthmatics' airways and'have demonstrated
that asthmatics are hyperresponsive to a variety of inhaled
substances (e.g., pollens, cold/dry air, dust, and air
pollutants). With regard to NO2, asthmatics are considered to be
one of the groups in the population most responsive to N00
£t
exposure (CD, p. 16-1) . There is evidence that asthmatics'
exposed to low levels of N02 (0.2 to 0.3 ppm) will experience an
'increase in airway responsiveness (Folinsbee, 1992), The
National Institutes of Health (1991) estimates that approximately
10 million asthmatics live in the U.S. Because asthmatics tend
to be much more sensitive to inhaled bronchoconstrictors than
nonasthmatics, there is the added concern that NC^-induced
increase in airway response may exacerbate already existing
hyperresponsivenees caused by preexposure to other inhaled
materials.
Patients with chronic obstructive pulmonary disease (COPD)
constitute another subpopulntion which is potentially susceptible
to N02 exposure. This group, which is estimated to be 14 million
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41
in the U.S. (U.S. Department of Health and Human Services, 1990),
includes persons with emphysema and chronic bronchitis. One of
the major concerns for COPD patients is that they do not have an
adequate ventilatory reserve and, therefore, would tend to be
more affected by any additional loss of ventilatory function as
may result from exposure to NO2. It is also possible that N02
might further damage already impaired host defense mechanisms,
thus putting COPD patients at increased risk to lung infection.
A final subpopulation group which the CD (p. 1-24) has
identified as potentially susceptible to N02 is immunocompromised
individuals. Persons in this group would have an increased
susceptibility for infectious pulmonary disease as well as other
health effects. Examples of such individuals would be those
suffering from acquired immune deficiency syndrome (AIDS) and
cancer patients being treated with chemotherapy. Approximately 1
million persons were estimated by the Centers for Disease Control
(CDC, 1990) to be infected with the human immunodeficiency virus,
which precedes development of AIDS symptoms (Karon et al., 1990).
There are about 1 million patients diagnosed with cancer each
year (U.S. Bureau of the Census, 1991), about 25% of whom are
prescribed chemotherapy as a first course of treatment (Steele et
al., 1991). Although the above immuno-compromised groups
represent populations potentially at risk for N02 effects, no
human research has examined the effects associated with N02
exposure in these groups. Additionally, no such studies have
been conducted in similarly immune-compromised animals exposed to
N02. Although it is clear that N02 can affect alveolar
macrophages, humoral immunity, and cell-mediated immunity in
otherwise normal animals (Chapter 13), the animal-to-human
extrapolation cannot yet be made quantitatively. Thus, there
only now exists a hypothesized association with increased
susceptibility to N02- Nevertheless, it may be prudent to
consider including such reduced immune function groups as
susceptible subpopulations at potentially increased risk for N02-
induced health effects" (CD, p. 1-25). Table V-5 summarizes
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TABLE V-5
SUMMARY OF POTENTXAIitlf SENSITIVE GROUPS
SENSITIVE GROUP
CBI1.DREM
SUPPORTING EVIDENCE
to NOj in home*
with ga* *tove*
increases risk of
children developing
reepiratary aymptoms and
di*ea«e.
REFEREHCES FOR
SUPPORTING EVIDENCE
Melia et al., 1977,
1979, 1980, 1982
Ware at «!., 1984
Neas «t al., 1991
Elcwo «t al., 1983
Dljkttra at al,r 1990
Kallar at al., 1979
POPULATION ESTIMATES
Aga« S-141
35 million
ASTHMATICS
Atthmatict exhibit
greater functional
dacrenentfl aod nirvay
r««pon«iv«n««« at lower
levels of NO2 than
nonaithmaticfl.
Kerr at al., 1979
Folinsbee, 1992
Bauer et al., 1986
JCoenif et al., 1987a,t5
10 million''
to
Patient* with Chronic
Oh«hructtve Pulmonary
Patient* with COPO
experience pulmonary
function change* with
acute exposures to high
tfOj level* or prolonged
exposures to lower
level*
Morrow and Utali, 1989
Von Nieding et al., 1980
14 Billion-
T mm u n o comp rooi i * ad
Individual*
I mmu o o c otnp ren i a ad
individual* tend to be
more *useaptible to lung
infection and other
health effect*
CD, p. 1-141
1 million infected with
RIV,4
1/4 million using
chemotherapy'
Center* for Di«ea*e Control, 1990.
National Inititutes ef Health, 1991
U.S. Department of Health and Human Services, 1990
Karon et al., 1990
U,S, lureim of the Cen«ti»» 1991, Steels et al», 1991
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43
-evidence and population estimates for groups identified above.
E. Effects of Concern
As discussed in detail in the CD (U.S. EPA, 1993a) and in
earlier sections of this Staff Paper, there is substantial
scientific literature providing evidence.of health effects
associated with exposure to N02. These effects vary widely
depending on the level of exposure, pattern of exposure, and
conditions of subjects during exposure (e.g., ventilation
rate,sensitivity). Although scientific research provides an
essential basis upon which conclusions can be drawn relating
various health effects to exposure patterns and levels of
exposure to NQ2, the specific level, averaging time, and form of
any primary NAAQS are' largely a public health policy judgment of
the Administrator.
In Chapter 16 of the CD (U.S. EPA, I993a), key information
and conclusions of the health assessment chapters are summarized
• and integrated to provide a framework for decisions regarding
health risks of NQ2. That discussion provides the basis for the
following conclusions:
1. Evidence ...is insufficient^at this time to establish
clear.,quantitative conclusions regarding airway responsiveness
and pulmonary function changes resulting from short-term
exposures, of... healthy _and as throat ic ..indiyi duals _ to near-ambient
N02 levels. (CD, pp. 16-3 and 16-4)
Evidence of health effects occurring below 1.0 ppm N02 is
equivocal at best in healthy individuals. There is limited
evidence of increased airway responsiveness in asthmatics
following short-term exposures as low as 0.2 to 0.3 ppm NC<2
{Folinsbee et al., 1992), even though changes in airway
responsiveness have not been observed at much higher
concentrations of up to 4 ppm (CD, p. 16-3). The implications of
this increased responsiveness in asthmatics, however, remain
unclear due to the apparent total reversibility of acute effects
and the apparent lack of airway inflammation at low-level
exposures.
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44
Although no consistent pattern of pulmonary function
response in normal subjects has been reported for short-term
exposure studies conducted at < 1.0 ppm N02 (CD, p. 15-36), there
is limited evidence of small changes in lung function in
asthmatics following short-term exposures to N02• Most of the
responses to N02 in asthmatic individuals have been observed at
concentrations between 0,2 and 0.5 ppm (CD, pg. 16-3). The
significance of these findings, however, is somewhat diminished
by the absence of changes reported at higher (3 to 4 ppm) N02
concentrations (Bylin et al., 1985; Linn et al., 1984, 1985 a,b,
1986) and the apparent lack of a concentration-response
relationship for pulmonary function changes.
2• Although there.issubstantial evidence which suggests
that increased exposure to NCU is associated with increased
respiratory illness in 5- to 12-year old children, exposure
estimates appear to be inadequate to establisha quantitative
relationshipbetween estimated exposures and symptoms. (CD. 16-5)
As described in detail in Chapter 14 of the CD, the meta-
analysis of nine "gas stove" epidemiological studies (Melia et
al., 1977, 1579, 1980, 1.982; Ware et al., 1984; Neas et al. ,
1991; Ekwo et al., 1983;•Dijkstra et al., 1990; Keller et al.,
1979) is supportive of a relationship between estimates of
exposure to NC2 and respiratory symptoms and disease in children
aged 5 to 12, but not in infants (< 2 years old). Furthermore,
underlying mechanisms which support N02 exposure as a cause of
increased susceptibility tc respiratory infection in animals
provide a plausible biological hypothesis for such effects
occurring in humans (CD, 16-5}. However, the "gas stove" studies
do not provide sufficient exposure; information, including human
activity patterns, to establish whether the health effects are
related primarily to peak, repeated peak, or long-term exposures
to NO2 • Also,, extrapolation of indoor exposure to outdoor
exposure is difficult-/according to the CD (p. 16-5) and CASAC
(19S3) .
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45
3. Animal toxicology studies provide qualitativeevidence
~induced morphologic lung lesions •indicative of emphysema;
however,NQ2 exposures whichhavebeen reported to cause
emphysema inanimals are far greater than levels currently
measured in ambient air. (CD, p. 16-11)
Morphologic lung lesions that meet the NIH (1985) workshop
.criteria for a human model of emphysema (i.e., alveolar wall
destruction occurs in addition to other characteristic changes)
have been reported in rats and rabbits following exposures of 1
to 30 months to NQ2 concentrations of > 8 ppm (Haydon et al.,
1967; Freeman et al., 1972; Port et al., 1977). Another study
(Hyde et al, , 1978) in which dogs were exposed to a mixture of
0.64 ppm NC>2 and 0.25 ppm NO for up to 68 months suggested that
NC>2 was the cause of morphologic lesions that meet the criteria
for human emphysema 32 to 36 months after a 68-month exposure.
Although there are numerous other NO^ investigations in
which emphysema has been reported and several which have found no
emphysema, it is not possible at this time to establish a "no-
observed effect" level. Reasons for this cited by the CD (p. 16-
11) include: complexity of changes occurring with NO^ exposure,
the lack of published papers utilizing appropriate morphometric
techniques, interspecies differences in response, and inadequate
description of methods and findings in some published reports.
Although it is clear that at sufficiently high concentrations NC>2
can induce emphysematous lesions, the levels of NO2 which are
known to cause emphysema are far higher than those which have
been reported in the ambient air.
4. There are other factors which staff believes should be
considered in evaluating the adequacy of current NAAOS for NO*,.
First, the Administrator should take into consideration the
range of health effects associated with NO2 exposure and that
these effects vary widely depending on the level of exposure,
pattern of exposure, and condition of subjects during exposure
(e.g., ventilation rate, sensitivity). Second, ethical
considerations in selecting subjects for studies suggest that the
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46
most sensitive individuals have not been studied. Finally, the
staff must reemphasize that most effects of N02 which have been
reported in animal studies have not yet been demonstrated in
human studies (e.g., immunological effects).
VI. AVERAGING TIME AND FORM OF THE PRIMARY NAAQS
A. Averaging Time of the Primary NAAQS
When the EPA promulgated the N02 primary NAAQS in 1971 (36
FR 8186, April 30, 1971), an annual (arithmetic mean) averaging
time was selected. The annual standard was selected in part
because epidemiology studies (Shy et al.. , 1970 a,b) conducted in
Chattanooga reported small, but statistically significant,
decreases in FEV for children (ages 7 to 8) living in areas with
relatively high (> 0.06 ppm) annual average N02 levels. Follow-
up studies (Shy et al., 1973, 1978; Pearlman et al., 1971) failed
to support the earlier findings, and the measurement method used
in the studies was called into question.
When the decision was made on June 19, 1985 to retain then-
existing primary NAAQS for N02 (50 FR 25532), emphasis was placed
on animal studies which provide evidence of health effects
associated wi^h chronic high-level N02 exposures (e.g.,
emphysematous-like lesions in the .lungs and damage to host
defense mechanisms resulting in increased susceptibility to
infection). Other evidence suggesting health effects associated
with long-term, low-level N02 exposures, such as the Chattanooga
community epidemiology studies and uhe studies oi' children living
in homes with gas stoves, were viewed as providing limited
qualitative support for ei long-term standard. Although there was
substantial debate regarding the need for a short-term (1- to 3-
hour) primary NAAQS, it was concluded that there was insufficient
evidence to set a short-term standard until further research
could be conducted. Based on th^ data available in 1985..
retaining the annual NAAQS of 0.053 ppm was seen as a means of
providing protection from long-'..erm health effects and some
measure of protection against possible short-term health, effects
(50 FR 25541, June .1.9.. 1S85) .
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47
Subsequent research has provided little, if any, evidence of
.significant health effects of short-term exposures to N02. No
••consistent pattern of response in normal subjects -has been
reported for short-term exposure studies conducted at < i.o ppra
N02 (CD, p. 15-36). There is only limited evidence of small
changes in spirometry or plethysmography in asthmatics following
short-term exposures to 0.2 to 0.5 ppm N02. The-significance of
these findings, however, is diminished by the absence of changes
reported at higher (up to 4 ppm) NOj concentrations (CD, pg. 16-
3). Patients with chronic obstructive pulmonary disease
experience pulmonary function changes with brief exposures to
high concentrations (i.e., 5 to 8 ppm for 5 minutes) or with more
prolonged exposure to lower concentrations (0.3 ppm for 3.75
hours). As the criteria document discussed at some length,
several questions remain unanswered regarding the correlation of
disease state with exposure variables for both persons with
asthma and COPD patients (CD, pg. 15-62).
The meta-analysis presented in Chapter 14 of the CD (U.S.
EPA, 1993a) provides reasonably good evidence of a quantitative
relationship between indoor NQ2 exposure in homes with gas stoves
and increased respiratory illness in children living in those
homes. However, as noted by the CASAC at the July 1993 meeting
(CASAC, 1993), it is very difficult to generalize this
relationship to outdoor N02 exposures. The exposure patterns
encountered in the homes studied with gas stoves are different
from exposure patterns which would be encountered in the ambient
air. Also, in spite of the large number of studies which have
focused on respiratory effects experienced by children living in
homes with gas stoves, there remains considerable uncertainty
with regard to the averaging time(s) associated with these
effects. Although it is reasonable to hypothesize that repeated
exposure to peak levels of N02 may be responsible for increased
respiratory illness, the contribution of low-level chronic
exposures to NO2 cannot be ruled out at this time. It is,
therefore, not feasible for staff to recommend a NAAQS for N02
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48
with a short-term averaging time due to the lack of adequate
information indicating that acute health effects of concern occur
at or near ambient exposures.
With regard to chronic effects, a relatively large data
base, including epidemiology'and animal toxicology studies,
provides evidence of significant biological effects resulting
from long-term (months to years) exposure to N02. Among the best
documented of these effects is damage to host defense mechanisms
which can result in increased susceptibility to respiratory
illness. These effects are discussed in detail in Chapters 13,
14 and 16 of the CD (U.S. EPA, 1993a). In addition, chronic
exposure to NCu has been shown to produce changes in lung
biochemistry, pulmonary function, and a variety of extrapulmonary
changes. Finally, chronic NC>2 exposure has induced -emphysema-
like lesions in the lungs of experimental animals but only at
concentrations much higher than ambient levels.
In light of the above information, staff concludes that
there is a need to protect public health against short-term and
long-term exposures to NO 2 • This has been accomplished
previously by setting the NAAQS for Nt>2 at an annual average,
which if met would adequately control most peak NCu exposures.
Staff again -believes that this approach is appropriate.
B. Form of the Primary NAAns
The current annual primary NAAQS for N02 is-based on the
arithmetic mean of all valid hourly averages in a calendar year.
The arithmetic mean is more sensitive to repeated short-term
peaks than the alternative, which is the geometric mean, and its
use is consistent with other standards.
If the Administrator chooses to establish a short-term
(e.g., 1-hcar, 8-hour, 24-hour) primary standard to protect the
public frou; exposure to repeated NC>2 peak concentrations, then
the staff recommends that the standard be set in a statistical
form -rather'than a deterministi-' form. (The statistical form of
the standard offers a more stable target for control programs and
is less sensitive to truly unusual meteorological conditions than
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49
a deterministic form). This could be accomplished by (1) setting
a NAAQS with an allowable number of exceedances of the standard
level which would be expressed as an average or expected number
per year, or (2) setting a NAAQS where a given percent of the
daily maximum hourly values would be expected to be less than or
equal to the standard level. The emissions reductions achieved
in the required control implementation program would be based on
a statistical analysis of the monitoring data over a multi-year
period (e.g., the preceding 3-year period),
VII. SUMMARY OF STAFF CONCLUSIONS AND RECOMMENDATIONS FOR THE
NITROGEN DIOXIDE PRIMARY NAAQS
The key findings presented below concerning the .NC^ primary
NAAQS draw upon discussions contained, in previous sections of
this Staff Paper. The key findings are:
1) The staff concludes that exposure to NC^ is associated
with a variety of acute (short-term) and chronic (long-term)
health effects. Clearly adverse health effects (e.g., pulmonary
edema, death) have been reported following accidental short-term
exposures of humans to very high concentrations (150-200 ppm or
greater) of N02 (CD, pg. 14-55) and animal studies have provided
evidence of emphysema caused by long-term exposures to greater
than 8 ppm N02. However, these health effects are caused by
exposures which are much higher than those found in the ambient
air and only serve as indicators of the most extreme effects of a
potentially very noxious oxidant,
2) Based on the assessment .of available information
regarding health effects associated with exposure to N02» the
staff concludes that the two potentially sensitive population at
risk are children 5 to 12 years old and individuals with pre-
existing respiratory diseases.
3) The available data indicate that short-term exposure to
N02 may cause an increase in airway responsiveness in asthmatic
individuals at rest. This response has been reported only at
relatively low concentrations (mostly within the range of 0.2 to
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50
0.3 ppm N02) which are of concern in the ambient environment.
Similarly, N02 induced pulmonary function changes in asthmatic
individuals have been reported at low, but not high, NO2
concentrations. For the most part, the small changes in
pulmonary function observed in asthmatic individuals have
occurred at concentrations between 0.2 and 0.5 ppm, but not at
much higher concentrations (i.e., up to 4 ppm) (CD, pg. 16-3) .
These findings contrast with the findings "for healthy individuals
exposed to low NC^ concentrations. In healthy individuals, there
is no evidence of lung function decrements or changes in airway
responsiveness at concentrations below 1.0 ppm NC^
The epidemiological evidence includes a meta-analysis of
nine epidemiological studies of children (5-12 years old) living
in homes with gas stoves. The results of the meta-analysis show
the children (5-12 years old) living in homes with gas stoves are
at increased risk for developing respiratory diseases and
illnesses compared to children living in homes without gas
stoves, Typical mean weekly N02 concentrations in bedrooms in
studies report-ing NOj levels were predominately between 0.008 and
0.065 ppm NQ2 , Most of the N02 measurements reported in the
studies are for indoor 1- to 2-week averages, but very little
information is provided regarding either peak hourly exposures or
measurements providing estimates of annual exposures. Because of
this, the studies and, therefore, the meta-analysis cannot
distinguish between relative contributions of peak and longer
term exposure and their reloi-ior.ship with the observed health
effects, Giver, the uncertainty associated with determining
actual exposure patterns in these homes and the difficulty in
extrapolating the data to ambient exposures, results of the meta-
analysis provide insufficient data co support specific limits for
either short-tar rr 01 long-term standards for nitrogen dioxide.
' 4) For the reasons discussed in section VI of this Staff
Paper, !~he s;;aff recommends i-.iat er, annual arithmetic mean
averaging time be retained for the primary NO- NAAQS.
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51
5) Based on the assessment of the available scientific and
technical information (which remains largely unchanged since the
1985 review), the staff again recommends that consideration be
given to setting the level of the annual primary standard within
the range of 0.05 to 0.08 ppm N02. In the staff's judgement,
•selecting a standard within this range would provide adequate
protection against the health effects associated with long-term
N02 exposure. In reaching a determination on the standard, the
staff also recommends that consideration be given to the degree
of protection that would be provided against repeated short-term
peak exposures. Based on air quality analyses, a standard
selected from -the lower portion of the range suggested would
effectively limit the frequency and magnitude., of 1-hour NO.-,
' £*
concentrations, McCurdy (1994) estimated that if the existing
standard of 0.053 ppm N02 is attained, the occurrence of 1-hour
N00 values greater than 0.2 ppm would be unlikely in most areas
<<£
of the country. At the upper end of the range, the frequency of
1-hour N02 peaks of 0.2 or higher could increase significantly.
However, because all areas of the country reporting NC>2 air
quality data are attaining the existing standard and because of
the nonlinear relationship of l-hour peaks and annual averages,
it is not possible to estimate with any degree of confidence what
the frequency and magnitude of 1-hour peaks would be if the
standard was selected from the upper end of the suggested range.
Given this uncertainty, the staff recommends consideration be
given to selecting the standard from the lower portion of the
range in order to provide a reasonable measure of protection
against repeated 1-hour peaks of potential health concern.
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52
VIII. Environmental Effects of Nitrogen Deposition
This section describes the environmental effects attributed
to nitrogen deposition and, where possible, sets forth judgments
as -to which levels of effects may be defined as adverse for
standard setting purposes. In addition to the environmental
features identified for protection by the secondary standard in
the definition of public welfare (see Section 302(h) [42 U.S.C.
7602(h)]), the 1990 Amendments to the Clean Air Act express a new
determination on the part of Congress to investigate through
research "...short-term and long-term causes, effects, and trends
of ecosystems damage from air pollutants..," (see Title IX, Sec.
901(e)). In keeping with this expanded scope, the Staff Paper
will address as part of the secondary standard review short- and
long-term effects of nitrogen deposition on biological, physical
and chemical components of ecosystems and the resulting effect of
changes in these components on ecosystems structure and function,
as well as the traditional issues of visibility impairment, and
materials damage.
A. Introduction
1. Context of Current Standard Review
In the 1982 OAQPS Staff Paper for nitrogen oxides, the staff
concluded that "there is inadequate evidence to demonstrate that
exposure to N02 alone at low levels will lead to significant
impacts on growth and yield for commercially important crops and
indigenous vegetation...," Additionally, chough studies on
pollutant combinations showed that synergistic responses could
occur., this type of response was extremely variable and was not
adequately documented, Therefore, the staff concluded that there
existed "insufficient data on combined effects of NO2 and SO2 to'
do a quantitative evaluation cf-yield reduction for various
ambient exposure levels." CASA.C concurred that the data did not
suggest significant effects of K02 on vegetation at or below
current atnfo"i ent levels suid that- a standard in the form of an
annual arithmetic mean at the If-vel of 0.053 ppm would provide
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53
sufficient protection against significant adverse effects on
vegetation (50 PR 25532, June 19, 1985).
More recent research in the above areas, as discussed in the
1993 NOV Criteria Document, continues to demonstrate that
J\.
pollutant levels for NO alone and in combination with other
pollutants in the United States, except in very rare cases, are
well below those required to produce direct phytotoxic effects on
plant yield or growth. The evidence linking nitrogen deposition
with ecological impacts, however, has been growing since the mid-
1980's (Skeffington and Wilson, 1988). Since these new concerns
have not been evaluated in the context of a secondary NAAQS for
nitrogen oxides,.the main focus of this Staff Paper will be on
the possible cumulative impacts of nitrogen at the ecosystem
level.
This Staff Paper is an effort to identify for the
Administrator any existing or potentially sensitive ecosystems or
ecosystem structures or functions which may be adversely affected
by ambient levels of. the criteria pollutant, NC^, its oxides, and
other nitrogen inputs. The definition of ecosystems set forth in
the discussion on page 10-2 of the NOX Criteria Document is used.
2 • At mo spheri q_._ Ni t rogen .Input s
Little, if any, research has been initiated to determine
what percentage of total nitrogen deposition can be attributed to
emissions of nitrogen oxides. However, it is known that there
are many natural and anthropogenic sources of nitrogen that are
not nitrogen oxide based.
Of the different species of nitrogen oxides present in the
atmosphere (CD, Table 9-12, p. 9-141), NO and N02 are generally
present in highest concentrations in the lower troposphere.
Thus, they are considered the most likely to have a potential
impact on vegetation. Research on interactions of NOX species,
other than NO and NQj, with vegetation is sparse. Some other
nitrogen-containing species do participate - in photochemical
reactions and may convert to NOX in the atmosphere. According to
some researchers, this is the case with ammonia (NH3), which
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54
originates on a global scale through both natural and
anthropogenic processes. Another source of atmospheric nitrogen
inputs to the environment, and one that has been the focus of
growing concern, is nitric acid (HN03).
Under ambient air quality conditions observed to date (see
Section IV, Air Quality Trends), the current secondary standard
for NC>2 is rarely exceeded. However, an analysis performed by
McCurdy (1994), .shows that areas attaining the current N02 NAAQS
might still experience multiple exceedances of 1 hour .values
greater than or equal to 0.15 ppm, and in Los Angeles, have
multiple hourly or daily exceedances greater than 0.20 ppm. As
will be discussed in Section B.l.a, (Vegetation) it does not
appear that the concentration times duration scenarios discussed
by McCurdy would be likely to adversely affect vegetation in
those areas, though given other factors, some sensitive species
might experience temporary foliar injury.
To have an effect, nitrogen that has been released to the
atmosphere must enter the ecosystem by either wet or dry
deposition. Deposition is primarily as nitrate. One-third to
one-half of the emissions of NOX in the United States are
estimated to be removed by wet deposition (Levy and Moxim, 1987;
Hicks et al. , 1991). The measurement of dry deposition is still
very much a contentious isSUP. Most researchers estimate rates
of dry deposition by assuming they are some fraction of wet
deposition rates. The assumption that; dry deposition is equal to
wet deposition is probably reasonable for areas directly adjacent
to emissions sources (Summers »t al,, 1986), but the ratio of dry
deposition to the sum of wet and dry deposition may fall as low
as 0.2 in locations remote from sources (CD, p. 206). A third,
and rarely measured mechanism of deposition that is locally
- important is the interception or capture of fog or cloud droplets
by vegetation. Two studies (Love'ct et al. , 1982; Woodin and Lee,
1987) recorded the ohenomenon c.r higher concentrations of
nitrogen within the cloud drop], --cs than were measured by bulk
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55
deposition gauges, suggesting that bulk precipitation samplers
underestimate total deposition.
This paper will limit its focus to a discussion of the
mechanisms by which concentrations of nitrogen in the air may
affect deposition and how, in turn, nitrogen deposition affects
.sensitive ecosystems. Thus, the paper will not specifically
address how the nitrogen component of acid rain influences
ecosystem response.
3. Nitrogen Cycle
Nitrogen is unique among nutrients in that its retention and
loss within ecosystems is regulated almost exclusively by
biological processes. The processes which make up the "nitrogen
cycle" and transform nitrogen as it moves through an ecosystem
include: (l) assimilation (2) nitrification (3) denitrification
(4) nitrogen fixation and (5) mineralization (see Figure'VIII-1
and definitions in Appendix C). In general, the nitrogen cycle
is identical in terrestrial, freshwater, and oceanic habitats;
only the microorganisms that mediate the various transformations
are different (Alexander, 1977).
Mature natural ecosystems are essentially self-sufficient
and independent of external additions (CD, p. 10-6).
Anthropogenic inputs, by altering amounts of nitrogen moving
though the cycle, can upset the relationships that exist among
the various components and thus alter ecosystem structure and
function. Under natural conditions, nitrogen is added to
ecosystems by fixation of atmospheric nitrogen, deposition in
rain, from windblown aerosols containing both organic and
inorganic nitrogen, and from the absorption of atmospheric NH^ by
plants and soil (Smith, 1980). However, modern technology is
altering the cycle by changing the amounts and fluxes of nitrogen
in the various portions of the cycle. The effects that these
perturbations are having on the nitrogen cycle are outlined in
the discussion below.
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CTl
Figure vm-lSchematlc representation of the nitrogen cycle, emphasizing human activities that affect fluxes of nitrogen.
The figure depict*! possible sources of nitrogen fluxes. Transform at tons are qualitative, not quantitative,
Source: Modified from National R«wi«h Council (1978).
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57
B. Ecosystem Response
A great degree of diversity exists among ecosystem types as
well as in the mechanisms by which each system assimilates
nitrogen inputs from various sources. Because the various
ecosystem components are chemically related, stresses placed on
the individual components, such as those caused by nitrogen
loading, can produce perturbations that are not readily reversed
and will significantly alter an ecosystem. How each ecosystem
responds to stress will depend on the response of the organisms
that comprise that particular system. The significant effects of
nitrogen inputs on terrestrial (i.e., plant communities),
wetland, and aquatic ecosystems are evaluated in the following
discussions.
The following discussion focuses primarily on how soil
acidification and nitrogen saturation affect terrestrial
ecosystems. In the terrestrial ecosystem, nitrogen may enter the
plants by several different mechanisms: (1) through the roots by
absorption of ammonia and ammonium; (2) by absorption of nitrate
and nitrite; and, (3) through nitrogen fixation by symbiotic
organisms (CD, p. 10-6), Therefore, uptake of nitrogen deposited
to the soil, direct adsorption of gaseous NOX through the plant
foliage, and interactions.at the soil/root interface will be
addressed in the following discussion.
Exposure for the wetland ecosystem is chiefly through
deposition of nitrogen to the water surface. Additional nitrogen
may be added to the system from the watershed if the wetlands
were formed and maintained by drainage. The discussion focuses
on impacts to the waterlogged soils as well as on the affects of
direct deposition on the leaves of plants within the system.
Acidification and eutrophication are issues of concern for
aquatic systems. The basic concern is that deposition of
nitrates alters the availability of nitrogen to the organismal
constituents and thereby can cause changes in species composition
within the system. The scientific evidence documenting these
changes is summarized below.
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1. Terrestrial Systems
a. Vegetation
Nitrogen is an essential nutrient in plant metabolism and
vital to the photosynthetic process. Previous studies have
demonstrated that the occurrence, type, and magnitude of N02
effects on terrestrial vegetation depend on the pollutant
species, the concentration of pollutant, the duration of
exposure, the length of time between exposures, and the various
environmental and biological factors that influence the response.
Foliar injury from N02 is rarely found in the field (CD, p. 9-1).
Though the relationship between reduced plant yield, altered
biochemical processes, or foliar injury and N02 exposure
(concentration x duration) is complex and distinctly non-linear,
common features from many studies do suggest a general form for
the relationship between exposure to NOX and effect on growth or
yield. These features include: (1) a threshold exposure that
must be exceeded for an effect (i.e., a deviation from the
unexposed state) to occur; (2) an increase in growth or yield at
exposures above the threshold but below those that produce a
decrease; (3) an increasingly greater reduction in growth or
yield with increasing concentration of NOX or duration or
frequency of exposure (greai-.er than those that produce an
increase in growth), yielding a nonmonotonic but unimodal
relationship; and (4) within the same species, the exposure-
effect relationship can be different for reproductive and
vegetative development and it can vary among different organs of
the same plant (e.g., an effect on the growth of roots could
occur at a lesser or greater exposure than what would produce the
same degree .of effect in the growth of stems or leaves)
. (CD, p. 3-89}, Studies also show that -at sufficiently high
concentrations and durations, plants experience irreparable cell
or tissue damage and ultimately, plant death.
Once ? particular plant species is exposed to NO2
concentrations above i-he lowest effects concentration x duration
for that particular -species, the observable physiological effects
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.of NO 2 are significant and include changes in carbon dioxide
^fixation (photosynthesis), alterations in specific enzymes,
.changes in metabolite pools, and alterations in the allocation
and translocation of photosynthate. Biochemical changes within
..the plants may become expressed as visible foliar injury,
-premature senescence, increased leaf abscission, and altered
plant growth and yield (CD, p. 9-1), These changes at the
individual plant level may lead to altered reproduction,
reduction of plant vigor, or other changes in competitive ability
which could have far reaching impacts 'at the ecosystem level.
Gaseous concentrations of nitrogen in the United States are
generally not 'high enough to cause the adverse effects on plants
listed above. Therefore, if the current NAAQS for NC^ continues
top be attained on an annual basis, the likelihood that plants
being exposed to direct atmospheric concentrations of nitrogen
oxides through their leaves will cause adverse effects is
minimal. Single exposures of 24 hours or less only produce acute
or adverse effects at concentrations of N02 greater than what
have been shown to* occur in ambient concentrations in the United
States. In experiments of- 2 weeks or more, with intermittent
exposures of several hours per day, adverse effects on growth or
yield start to appear when the concentration of NCX. reaches the
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standard provides adequate protection for vegetation from direct
ambient exposure to NO2-
b. Plant/Soil Interactions
Plants obtain their nutrients through the soil. Nitrogen
present in the soil is an essential element for healthy plant
growth and development. As discussed above, biological processes
which make up the nitrogen cycle regulate how much and what form
nitrogen will be available to plants. Because it is the most
common limiting nutrient for vegetative growth, deposition of
nitrogen at non-toxic levels and durations in any biologically
available form to most uncultivated areas is likely to produce
some growth increase. When heterotroph and plant demand for
nitrogen are substantially satisfied (i.e., when nitrogen
saturation occurs) nitrification and nitrate leaching will become
significant. If there is an excess of nitrogen available, the
result will be alternation of the plant/soil interaction. This
alteration is the most important aspect of excess nitrogen in a
terrestrial ecosystem.
Vegetative nitrogen demand varies according to stand age,
stand type, the availability of other nutrients, temperature, and
moisture. The effects of stand age and type on vegetative
nitrogen demand are illustrated in the facts that uptake rates of
nitrogen for conifers are generally maximal around the'time of
canopy closure, whereas in deciduous forests, maximum nitrogen
uptake rates occur somewhat later (and at higher rates) due to
the annual replacement of canopy foliage in these ecosystems
(Turner et al., 199C).
As forests mature, and nitrogen uptake rates decline, soils
begin to acidify naturally. Processes, such as forest fires and
harvesting, *hieh can release nitrogen and increase ecosystem
nitrogen demand can reverse this trend in the long-term.
However, it should be pointed out that because of the lack of
biomass to requester the nitrogen, these events may also lead to
episodic acidification. Also, depending on the intensity of the
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fire, nitrogen may be redistributed.from the forest floor to the
surface mineral soil rather than being lost from the system.
Additional factors that influence ecosystem response to
nitrogen inputs include the rate of the nitrogen additions,
initial nitrogen status of the receiving system, the competitive
advantage between species, and the size of the initial population
of nitrifying bacteria. When nitrogen fertilizer is added in one
or two large doses to sites where the initial population of
nitrifiers is low, as is typical of nutrient poor areas, trees
out compete soil heterotrophs 'and show increased growth, while a
significant lag time occurs before the onset of nitrate
production (CD, p. 10-49) . On the other hand, slow, steady
nitrogen inputs typical of atmospheric pollution to the same
system would favor a buildup of nitrifying bacteria populations
and thus - cause a much earlier commencement of nitrate leaching.
In nitrogen rich sites, researchers find much higher soil
solution nitrate concentrations in general, no delay in the onset
of nitrate leaching, and tree growth under both pollutant and
fertilizer regimes (Tschaplinski et al., 1991). In some cases
where atmospheric nitrogen deposition acts as a fertilizer and"
leads to increased plant production, the benefits of
fertilization may be considered to outweigh the detrimental
effects of soil acidification (see soil discussion below).
However, in systems in which the pre-existing balance among
species is mediated by competition for nitrogen, additional
inputs 'may also bring about an alteration in species mix. For
example, Van Breeman and van Dijk (1988) found a substantial
displacement of heathland plant by grasses from 1980-1986 and
noted increases in nitrophilous plants in forest herb layers (see
wetlands discussion). In the United States, nitrogen deposition
has been occurring since the 1920's. However, because we have
only recently become concerned about the effects of this
deposition, there are no documented accounts in the United States
of ecosystems undergoing species shifts due to atmospheric
nitrogen deposition.
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In addition to direct effects of nitrogen on growth, several
studies have examined the effects of nitrogen deposition on
forest species sensitivity to drought, cold, or insect attack.
Though some studies show that increased nitrogen deposition can
alter tree susceptibility to insect and disease attack and plant
community structure, results are mixed and often contradictory,
and currently do not provide a sufficient basis on which to make
a decision on a secondary standard (CD, pp. 10-92 to 10-93) .
Climate is thought to play a major role in the severe red spruce
decline in the northeastern United States, perhaps with some
additional exacerbation due to the direct effects of acid mist on
foliage (Johnson et, al., 1992). There is also some evidence that
suggest that indirect effects of nitrogen saturation (i.e,
nitrogen input in excess of total combined plant and microbial
nutritional demands), namely nitrate and Al leaching, may be
contributing factors to red spruce decline in the southern
Appalachians (CD, p. 10-74).
While the available data in the U.S. is limited, the staff
is concerned that excess nitrogen deposition to terrestrial
ecosystems may be modifying interplant competitive balances
leading to future changes in species composition and/or diversity
(see following discussions on wetlands and aquatics), Because it
is difficult to ascertain the potential for ecosystem
simplification from the current concentrations of nitrogen in the
atmosphere, any appropriate regulatory control option would need
to take into account the current nitrogen status of the.
ecosystem-(s) of concern and. the specific dynamics of nitrogen
inputs into the area,
c. Soils
Soils are the largest nitrogen pool in terrestrial
ecosystems. The most obvious and immediate effect of pollutant
nitrogen inputs on soils is an increase in the activity of
heterotrophs and nitrifiers associated with an increase in
decomposition and nitrification. The foremost concern about
long-term, capacity-controlled effects of excessive nitrogen
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deposition and N03~ leaching is soil acidification and
mobilization of A13 + into soil solution and surface waters,
Base saturation is the primary measure of soil mcidity (CD,
p. 10-66) . Base saturation refers to the degree to which soil
cation exchange sites (i.e., negatively charged sites to which
-positively charged ions (cations) are adsorbed) are occupied with
base cations. Base cations are calcium ions (Ca^+) , magnesium
ions (Mg2"1") , and potassium ions (K+) , Aluminum ions {Al^*5 and
hydrogen ions (H+) are acid cations. Lower base saturation
values correlate with more acidic soils. Figure VIII -2 below
shows a schematic diagram" of a soil with 50% base saturation
versus a soil -with 10% base saturation.
Input
Input
Mineral
Weathering
50%
Base
Sat.
Mineral
Weathering
Leaching
Leaching
Figure VIII-2 Schematic diagram of cation exchange for base cations, aluminum ions,
and hydrogen ions in circumneutral (50% base saturation, left) and acid
(10% base saturation, right) soils.
Source: Figure 10-15, p. 10-66 of the 1993 Air Quality Criteria for Oxides of Nitrogen
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Soils go through various buffering ranges as they acidify
(CD, p. 10-67). First is the base cation buffering range, where
incoming acid and base cations are exchanged primarily for base
cations. In this buffering range, very little H+ and Al^ + are
leached from the soil (see Figure VIII-2, left). As soils
acidify, exchangeable base cations are replaced by exchangeable
Al3* and H*, and soils are said to be in the aluminum buffering
range. In this range, incoming cations (acid and base) are
exchanged primarily for H4 and Al^+ and acid cations become more
prevalent in the soil leachate (see Figure VIII-2, right).
Atmospheric additions of nitrogen speed acidification of
soils and increase aluminum mobilization if they are at or in
excess of plant and microbial demand. However, the levels of
nitrogen input necessary to produce measurable soil -acidification
are quite high. In the few studies cited (Tamm and Popovic,
1974; Van Miegroet and Cole, 1984), nitrogen inputs ranged from
50 to 3,900 kg/ha for' 50 and 10 years respectively to effect a
change in soil pH of 0.5 pH units. Currently, there are no
documented cases in the United States in which excessive
atmospheric nitrogen deposition has caused soil acidification;
however, the potential exists :i E additions are high enough for a
sufficiently long period of time. Concerns may also be greater
for nitrogen deposition in certain sensitive areas of the
country.
Results of the National Acid Precipitation Assessment
Program (NAPAP) and other research'efforts showed that the
acidification of soils can alter: the availability of plant
nutrients (i.e., calcium and magnesium) and may expose tree roots
to toxic levels of aluminum and manganese. While the impact on
tree growth may be a significant cause of concern, only limited
information is available on which to make quantitative
assessments of damage.
In addition to anthropogenic inputs, the type and size of
soil nitrogen pools are also significantly changed by living
organisms (e.g., heterotrophs (decomposers such as fungi and
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65
bacteria), autotrophs (plants), and nitrifying bacteria) in the
system. These organisms compete for the biologically available
portion (mineralizeable or organic pools) of the total soil
nitrogen. As heterotrophic bacteria decompose organic matter,
they release NH4+ which can subsequently be nitrified to N0-j~ by
nitrifying bacteria. Because NC>3~ is poorly adsorbed to soils,
it is readily leached from the system, taking nutrients essential
to plant health with it. The mobilization of aluminum which
follows, can be toxic to plants, and if transported to the
waterways, toxic to various aquatic species.
There are a few areas in the U.S. where soils are exposed to
nitrogen addit-ions without the benefit of vegetative cover, as in
some desert systems or in areas temporarily denuded by fire or
flood. Because arid and semiarid soils are more alkaline than
most forest and agricultural systems and have less water
available than in more humid regions, they are not as susceptible
to soil acidification and groundwater N0j~ pollution and will,
therefore, not be considered as a sensitive ecosystem on the
basis of short-term (under 3 years) exposure to NOj pollution
(CD, p. 10-41).
Since natural disasters such as fires or floods occur
randomly, it seems impractical to consider their impacts. In
intensively managed forests, fire may be used as a tool and
fertilizer applied regularly. In these cases, the effects of
pollutant deposition are overwhelmed by manmade manipulation of
nitrogen inputs.
2. Wetlands
Wetlands function as habitats for plant and wildlife, flood
control systems, stabilizers and sinks for sediments, storage
reservoirs for water, and biological filters that maintain water
quality. Since many different types of communities can be
classified as wetlands, there is often significant disagreement
over what constitutes a wetland and what terminology is used to
describe them. For the purposes of this discussion, we will use
four terms defined in the criteria document for wetlands: marsh;
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fen; bog; and heathland or shrub bog (see definitions in CD, p.
10-110 to 10-116) . The discussion that follows evaluates how
nitrogen deposition from anthropogenic sources may alter the
nitrogen cycle in the wetland system and analyzes the possible
effects that may occur as a result of these changes.
In general, the significance of atmospheric nitrogen inputs
as a percent of total nitrogen inputs from other sources to
wetlands can be said to increase as rainfall increases as a
fraction of the total water budget for a wetland. Salt marsh
ecosystems which depend on tide and groundwater for most of their
nitrogen inputs are relatively unresponsive to additional
atmospheric nitrogen, while ombrotrophic bogs, which receive
nutrients exclusively from precipitation, are much more
responsive.
a. Vegetation
Nitrogen assimilation by wetland plant communities varies
from 38 to 66 kg nitrogen/ha/year in bog ecosystems to 225 to 274
kg nitrogen/ha/year in the salt marsh and fen ecosystems,
respectively. The nitrogen cycle in bog and heathland ecosystems
is largely closed while in, salt marshes and fens it is open,
permitting a great exchange with adjacent systems. Using rates
of nitrogen application ranging from 7 to 3,120 kg/ha/yr,
numerous field experiments have documented standing biomass
increases in a year of 6 to 413% (CD, Table 10-18, p, 10-112),
showing that primary production in wetland ecosystems is most
commonly limited by the availability of nitrogen. Other short-
term (3 years or less) fertilization studies performed in
different; wetland communities demonstrate that (1) stimulation of
primary production by nitrogen applications is not a linear
function, with the greatest increase in standing biomass
occurring in studies where the control biomass was low, and (2)
the response of leaf growth is much greater than the response of
roots. In addition to stimulating growth rates of wetland
species, several fertilisation studies demonstrated a tendency
toward a change in species composition or dominance. Vermeer
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(1986) found that in fen and wet grassland communities, grasses
tended to increase in dominance over other species. Jefferies
and Perkins {1977} also found a species-specific change in stem
density at a Norfolk, England salt marsh after fertilizing
monthly with 610 kg N03~ nitrogen/ha/year or 680 kg NH^+
nitrogen/ha/year over a period of 3 to 4 years,
Long-term studies (greater than three years) of increased
nitrogen loadings to wetland systems further demonstrate that
increases in primary production can result in changes in species
composition and succession. Changes in species composition may
occur from increased evapotranspiration (Howes et al., 1986;
Logofet and Alexander, 1984) leading to a changed water regime
that favors different species or from increased nutrient loss
from the system through incorporation into or leaching from
aboveground vegetation. In parts of Europe, historical data
-seems to implicate pollutant nitrogen in altering the competitive
relationships among plants and threatening wetland species.
adapted to habitats of low fertility (Tallis, 1964,- Ferguson et
al., 1984;'Lee et al., 1986).
What makes these findings.important to the policymaker is
that wetlands are often home to many rare and threatened plant
species. Some of these plants adapt to systems low in nitrogen
or with low nutrient levels. For some species, these conditions
can be normal for growth. Therefore, excess nitrogen deposition
can alter these conditions and thus alter species density and
diversity. In what was formerly West Germany, Ellenberg (1988)
surveyed the nitrogen requirements of 1,805 plant species and
concluded that 50% have adapted well to habitats with low
nitrogen supplies. Furthermore, of the threatened plants, 75 to
80% grow only in habitats where nitrogen availability is low. In
eastern Canadian wetlands, nationally rare species are found
principally on low fertility sites (Moore et al., 1989; Wisheti
and Keddy, 1989). In the conterminous United States, wetlands
also harbor 14% (18 species) of the total number of plant species
that are formally listed as endangered. Several species on this
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68
list, such as the insectivorous plants, are widely recognized to
be adapted to nitrogen-poor environments. Based on the evidence
reviewed in the criteria document, the staff believes we can
anticipate similar effects from atmospheric nitrogen deposition
in the United States, in areas where deposition is exceeding an
area's absorptive capacity. However, there is no documentation
provided in the studies reviewed that provides sufficient
evidence to support the theory that species changes have occurred
in the past or are occurring currently in the United States.
b. Soils
The feature of wetlands that sets them apart from
terrestrial ecosystems is the anaerobic (oxygen-free) nature of
their waterlogged soils. Because there is no oxygen,
decomposition of organic matter in wetland soils is incomplete
(CD, p. 10-110}. Consequently, organic carbon is built-up in the
system. This influences the various microbial transformations in
nitrogen cycling. Anoxic soils favor denitrification which
results in important'losses of nitrogen from wetland ecosystems.
Furthermore, the hydrology of wetlands favors diffusive exchanges
of nitrogen compounds to and from sediments and water within the
system carries nitrogen compounds between ecosystems.
Single additions of ^ N-labeled mineral nitrogen to
vegetated soils at rates of about 100 kg nitrogen/ha/year
indicate that up to 93% of applied NH4+ is rapidly assimilated
into organic matter within a single growiriy season. The majority
of the labeled nitrogen is lost from the system after 3 years by
the combined processes of advective transport in water of
particulate organic matter, advective and diffusive transport of
dissolved nitrogen, and denitxification. In the absence of
plants, the major fate of inorganic nitrogen applied to wetland
soile is loss to the atmosphere, by denitrif ication (CD, pp. 10-
247 to 10-248).
3 = Aquatic Systems
Some aquati-: systems are potentially at risk from
atmospheric aitrogev- additions through the processes of
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69
eutrophication and acidification. Both processes can
sufficiently reduce water quality making it unfit as a habitat
for most aquatic organisms and/or human consumption. Atmospheric
nitrogen can enter aquatic systems either as direct deposition to
water surfaces or as nitrogen deposition to the watershed. In
-northern climates, nitrate may be temporarily stored in snow
packs and released in a more concentrated form during snow melt
(see discussion on acidification). Nitrogen deposited to the
watershed is then routed (e.g., through plant biomass and soil
microorganisms) and transformed (e.g., into other inorganic or
organic nitrogen species) by watershed processes, and may
eventually run" off into aquatic systems in forms that are only
indirectly related to the original deposition. The contributions
of direct and indirect atmospheric loadings has received
increased attention. Growing evidence does indicate that the
impact of nitrogen deposition on sensitive aquatic systems may be
significant. However, there is still uncertainty with regard to
the quantification of the relationship between atmospheric
deposition of nitrogen and its appearance in receiving waters..
a. Acidification
1. Chronic Acidification. In the United States, the most
comprehensive assessment of chronic acidification of lakes and
streams comes from the National Surface Water Survey (NSWS)
conducted as part of the National Acid Precipitation Assessment
Program (NAPAP). The NSWS selected a single season of the year
that exhibited low spatial and temporal variability to be the
"index period", so that the general condition of surface waters
could be assessed. For lakes, the index period selected was
autumn overturn (a time when most lakes are mixed uniformly from
top to bottom). Henriksen (1S88) has proposed that the ratio of
N03~iNO3~ + SO4^2~^ in surface waters be used as an index of the
influence of N03" on chronic acidification status. The results
of. the Eastern Lake Survey using measurements taken during the
autumn overturn index period, suggest that nitrogen compounds
make only a small contribution to chronic acidification in North
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70
America. Other studies (cited in Henriksen, 1988) which did not
use this index, but monitored intensively over the course of the
year, however, showed that N03" may be important in chronic
acidification. Since annual mean values include high spring NO3~
concentrations in runoff waters and will, therefore, be higher
than concentrations measured only in the autumn, these study
results cannot be compared. Regional lake surveys with
representative annual or spring values for the United States were
not available for review.
The index period for streams was chosen as the spring base
flow (the period after spring snowrnelt and before leaf-out) .
Unlike the-lake surveys, the National Stream Survey (NSS) data
have the advantage of having been collected during a spring base-
flow index period. This period has been shown to be a good index
of mean annual condition for streams.. Though several stream
regions exhibit ratios of N03~:N03" + SO4'2~' as high as those
reported for the Adirondacks, this is in part because some of the
stream regions have SO4^2~^ concentrations that are relatively
low. The stream data do suggest that the Catskills, Northern
Appalachians. Valley and Ridge Province, and Southern
Appalachians all show some potential for chronic acidification
due to NO3". Two efforts (Kaufmann et al., 1991; Driscoll et
al., 1989) have also been undertaken to determine if atmospheric
deposition is the source of the NO3~ leaking out of these
watersheds. Data from the NSS (Kaufmann et al,, 1991} suggest a
strong correlation between concentrations of stream water and
levels of wet nitrogen deposition in each of the NSS regions.
Secondly, Driscoll et al, (1989) collected input/output budget
data for a large number of watersheds in the United States and
Canada, and summarized the relationship between nitrogen export
and nitrogen deposition at all the.sites. Though the
relationships discovered should not be over-interpreted or
construed ar- nn illustration of. cause and effect, they do show
that watersheds in many regions of North America are retaining
less than 75% of the nitrogen that enters them, and that the
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amount of nitrogen being leaked from these watersheds is higher
in areas where nitrogen deposition is highest.
In Europe, many sites show chronic increases in nitrogen
export from their watersheds (e.g., Henriksen and Brakke, 1988;
Hauhs, 1989). • Chronic acidification attributable to ammonium
deposition has also been demonstrated in the Netherlands (Van
Breemen and Van Dijk, 1988; Schuurkes, 1986, 1987). It has been
suggested that chronic nitrogen acidification is more evident in
Europe than in North America because nitrogen saturation is
further progressed in Europe.
On a chronic basis in the United States, especially in the
eastern part d'f the country, nitrogen deposition does play a role
in surface water acidification. However, there are significant
uncertainties with regard to the long-term role of nitrogen
deposition in surface water acidity and with regard to the
quantification of the magnitude and timing of the relationship
:between atmospheric deposition and the appearance of nitrogen in
surface waters.
2. Episodic Acidification. Acidic episodes have been
registered in surface waters in the Northeast, Mid-Atlantic, Mid-
Atlantic Coastal Plain, Southeast, Upper Midwest, and West
regions (Wigington et al., 1990). In the Mid-Atlantic Coastal
Plain and Southeast regions, all of the episodes catalogued to
date have been associated with rainfall. In contrast, most of
the episodes in the other regions are related to snowmelt,
although rain-driven episodes apparently can occur in all regions
of the country. It is important to stress that even within a
given area, such as the Northeast, major differences can be
evident in the occurrence, nature, location (lakes or streams),
and timing of episodes at different sites. A number of processes
contribute to the timing and severity of acidic episodes
(Driscoll and Schaefer, 1989). The most important of these are;
1) dilution of base cations (Galloway et al., 1980) by high
discharge; 2) increases in organic acid concentrations (Sullivan
et al., 1986) during periods of high discharge, 3) increases in
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72
gQ^(2-) concentrations (Johannessen et al., 1980) during periods
of high discharge, and 4) increases in N03~ concentrations
(Galloway et al., 1980; Driscoll and Schafran, 1984; Schofield et
al., 1985) during periods of high discharge. In many cases, all
of these processes will contribute to episodes in a single
aquatic system. In addition to these factors, the likelihood of
an acidic episode is also influenced by the chemical conditions
before an episode begins. For example, the magnitude of the
episodes experienced by lakes depends strongly on their lake's
acid neutralizing capacity (ANC) which is the same as the base
cation concentration. Lakes with lower baseline ANC are affected
more by nitrate pulses, and lakes with higher baseline ANC are
affected more by base cation dilution from snowmelt.
Some broad geographic patterns in the frequency of episodes
in the United States, are now evident. Episodes driven by NO3~
are common in the Adirondacks and Catskill Mountains of New York,
especially during snowmelt, and also occur in at least some
streams in other portions of the Northeast (e.g., Hubbard Brook).
Nitrate contributes on a smaller scale to episodes in Ontario,
and may play, some role in episodic acidification in the western
United States. There is little current evidence that N03~
episodes are important in the acid-sensitive portions of the
southeastern United States'outside the Great Smoky Mountains. We
*i
have no information on the relative contribution of N03~ to
episodes in many of the subregions covered by the NSS, including
those that exhibited elevated N03~ concentrations at spring base
flow (e.g., the Valley and Ridqe Province and Mid-Atlantic
Coastal Plain), because temporally-intensive studies have not
been published for these areas
There is a great need to emphasize the importance of
examining nitrogen episodes in a truly long-term context.
Although the data reported her -• for the Catskills can be
considered i- ruly loag-t«?rm (up to 65 years of record), data for
the Adirondacks iDriscoil and /an Dreason, in press) and other
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73
areas of the United States. (Smith et al., 1987) span only 1 to 2
.sdeeades, .and should be interpreted with caution.
Many of the data discussed above suggest that N0g~ episodes
are more severe now than they were in the past. These surface
.water nitrogen increases have occurred at a time when nitrogen
deposition has been relatively unchanged in the northeastern
United States (Husar, 1986; Simpson and Olsen, 1990). If we
accept the idea that an increase in the occurrence of N03"
episodes is evidence that nitrogen saturation of watersheds is
progressing, then current data suggest that current levels of
nitrogen deposition are too high for the long-term health of
aquatic systems in the Adirondacks, the Catskills, and possibly
elsewhere in the Northeast. It is important to note that this
supposition is dependent on our acceptance of NO^" episodes as
evidence of nitrogen saturation. While some data suggest this to
be the case, because of the lack of adequate air monitoring and
deposition data, there are still significant uncertainties with
respect to the quantification of the linkage and the timing of
the relationship between the atmospheric deposition of nitrogen
and its episodic or chronic appearance in surface waters.
Because the capacity to retain nitrogen differs from one
watershed to another, the levels of nitrogen deposition that
produce adverse effects will also vary. For example, the
Northeast, because of the presence of aggrading forests and
deeper soils in comparison to those of the west, may be able to
absorb higher rates of deposition without serious adverse effects
than areas of the mountainous West, where soils are thin in
comparison and forests are often absent at the highest elevations
(CD, p. 10-179) . The data of Silsbee and Larson (1982) suggest
strongly that forest maturation is also linked to the process of
N03~ leakage from Great Smoky Mountain watersheds.
It should also be noted that acidification of lakes from
nitrogen deposition may have some effect on the increased
leaching and methylation of mercury in the aquatic ecosystem.
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Current scientific evidence point to nitrogen as the
principal contributor to episodic acidification of streams and
lakes in the Northeast. Additionally, if further research
indicates that indeed some of the watersheds of the Northeast and
the mid-Appalachians are nearing nitrogen saturation, nitrogen
deposition will become a more direct cause of chronic surface
water acidification. However, regional and ecosystem differences
will make it difficult for the Administrator to set a secondary
standard which will address the problem in all areas,
b, Eutrophication
Eutrophication is the process by which aquatic systems are
enriched with"the nutrient(s) that are•presently limiting for
primary production in that system. Eutrophication may produce
conditions of increased algal biomass and productivity, nuisance
algal populations, and decreases in oxygen availability for
heterotrophic organisms. Another effect of chronic
eutrophication is increased algal biomass shading out
ecologically valuable estuarine seagrass beds. During severe
episodes of eutrophy, fish kills can occur and under chronic
conditions, sensitive species may be permanently lost. Though
this process often occurs naturally over the long-term evolution
of lakes, it can be significantly accelerated by the additional
input of the limiting nutrients from anthropogenic sources. A
theory .is that atmospheric nitrogen deposition is causing
nutrient enrichment in several freshwater systems. In order to
establish-a liiih between nitrogen deposition and the
eutrophication of aquatic systems, one must first demonstrate
that the productivity of the system is limited by nitrogen
availability, and second, that nitrogen deposition is a major
source of nitrogen to the system.
1. Freshwater Eutrophication. In most lakes and streams,
phosphorus, not nitrogen, in the limiting nutrient. It is
thought that surplus inorganic nitrogen in lakes and streams is
.maintained through nitrogen j:ixation by various algae and aquatic
plants, which -.mist counteract; the significant loss of nitrogen
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75
that occurs in aquatic ecosystems from denitrification.
Eutrophication by nitrogen inputs will only be a concern in lakes
that are chronically nitrogen limited and have a substantial
total phosphorous concentration. This condition is common only
'In lakes that have received excessive inputs of "anthropogenic
phosphorous, or in rare cases, have high concentrations of
natural phosphorus. In the former case, the primary dysfunction
of the lakes is an excess supply of phosphorous, and controlling
nitrogen deposition would be an ineffective method of gaining
water quality improvement. In the latter case, lakes with
substantial total phosphorous concentrations would experience
measurable increases in biomass from increases in nitrogen
deposition.
Good predictions of nutrient limitation can now be made
from ratios of total dissolved inorganic nitrogen (DIN) to total
phosphorous (TP) (Morris and Lewis, 1988). Morris and Lewis '
found that lakes with DIN:TP values less than 9 could be limited
by either phosphorous or nitrogen, whereas lakes with DINjTP
values less than 2 were always limited by nitrogen. This ratio,
is a refinement .from the earlier nutrient ratio -of 16:1 put forth
by Redfield (1934) and excludes the forms of nitrogen and
phosphorus that are not biologically available. Other methods
are available for measuring nutrient limitation. Bioassay
experiments, for example, are more direct than nutrient ratios as
various known concentrations of potentially limiting nutrients
are added to an enclosed small volume of lake water and the
growth response of the biomass measured. However, the results of
such experiments are available for only a few selected nutrient-
poor lakes and no clear pattern of nitrogen or phosphorous
limitation is discernible.
Though significant percentages of lakes in the Pacific
Northwest (27.7%) and the Upper Midwest (19%) indicated nitrogen
limitation when the critical DIN:TP value less than 2 was applied
to lakes from the Eastern Survey and Western Lake Survey, the
number of lakes which meet the criteria of chronic nitrogen
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76
limitation is likely to be small. Therefore, the potential for
nitrogen deposition to contribute to the eutrophication of
freshwater lakes is limited,
2. Estuaries and Coastal Waters. Estuaries come in many
different shapes and sizes with many different sets of factors
influencing algal production. This variety is partially captured
by the classification scheme designed by Boynton et al., (1982) .
Because estuaries and coastal waters receive substantial amounts
of weathered material from terrestrial ecosystems and from
exchange with sea water, acidification is not a concern,
However, this same load of weathered material and anthropogenic
inputs makes these same areas prone to the effects of
eutrophication. Few topics in aquatic biology have received as
much attention in the past decade as the debate over whether
estuarine and coastal ecosystems are limited by nitrogen,
phosphorus, or some other factor. Numerous geochemical and
experimental studies have suggested that nitrogen limitation is
much more common in estuarine and coastal waters- than in
freshwater systems. Experiments to confirm widespread'nitrogen
limitation in estuaries have not been conducted, however, and
nitrogen limitation cannot be assumed to be the rule. However,
taken as a whole, the productivity of estuarine waters of the
United States correlates more closely with supply rates of
nitrogen than of other nutrients (Nixon and Pilson, 1983) .
Specific instances of phosphorus limitation (S^ith, 1984) and of
seasonal switching between nitrogen and phosphorus limitation
(D'Elia et al., 1986; McComb et al., 1981) have been observed,
however, and stand as exceptions to the general rule. Nitrogen-
fixing blue-green algae are rarely abundant in estuarine waters
(Howarth et a3 ,, 1988), and so nitrogen-deficient conditions may
continue indefinitely in these systems.
Estimation of the contribution of nitrogen deposition to the
eutrophiuation of estu-arine and coastal waters is made difficult
by the multiple direct anthropogenic sources (e.g., from
agriculture and sewage) of nitrogen. In the United States, only
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77
a few systems have been studied with enough intensity to develop
predictions about the contribution of atmospheric nitrogen to
total nitrogen inputs. One example is the Chesapeake Bay, where
a large effort has been made to establish the relative importance
of different sources of nitrogen to the total nitrogen load
entering the bay (e.g., D'Elia et al,, 1982; Smullen et al.,
1982; Fisher et al., 1988a; Tyler, 1988). Though estimates for
each individual source are very uncertain, three attempts to
determine the proportion of the total N0j~ load to the bay
attributable to nitrogen deposition all produced estimates in the
range of 18 to 39%. If this is the case, supplies of nitrogen
from deposition exceed supplies from all other non-point sources
(i.e., farm runoff) to the bay and only point-source inputs
(i.e., discharges to water, emissions from industrial facilities)
represent a greater input than deposition.
From the few available examples, it is clear that
atmospheric nitrogen inputs to aquatic ecosystems are of concern.
The significance of these inputs will vary from site to site and
will depend on the availability of other growth'nutrients, the
flushing rate through the system, the sensitivity of resident
species to added nitrogen, the types and chemical forms of
nitrogen inputs from other sources, as well as other factors.
c. Direct Toxicity
In a report titled "Ambient Water Quality Criteria for
Ammonia-1984", developed by the United States Environmental
Protection Agency, high NH-j concentrations are associated with
lesions in gill tissue, reduced growth rates of trout fry,
reduced fecundity (number of eggs), increased egg mortality, and
increased susceptibility of fish to other diseases, as well as a
variety of pathological effects in invertebrates and aquatic
plants. Given the serious nature of these effects, EPA developed
regulations requiring 4-day average concentrations of NH3 not to
exceed the limits set more often on average than once every 3
years, nor can 1 hour average concentrations exceed one-half of
the limit set more often on average than once every 3 years.
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78
Because critical concentrations of NH^ that cause the various
effects are wide ranging and are related to site specific
temperature and pH values, no single toxic concentration can be
established. Rather the limits (final chronic value and final
acute value) must be calculated as a function of pH, temperature,
and ionic strength of the water source, Because of the serious
nature of the effects, it is important to determine whether
nitrogen deposition could potentially contribute directly to
toxic effects in surface waters. Given current maximal
concentrations of NH4+ in deposition and reasonable maximum rates
of dry deposition, even if all nitrogen species were ammonified,
the maximum potential NH44 concentrations attributable to
deposition would be approximately 280 nmol/L and would be
unlikely to be toxic except in unusual circumstances. Therefore,
it appears that the information reviewed suggests that the
potential for directly toxic effects attributable to nitrogen
deposition in the United States is very limited.
C. Critical Loads and Other Potential-Regulatory Actions
1. Critical Loads Concept
Concerns regarding nitrogen saturation have led to efforts
to develop "critical loads", of nitrogen for various ecosystems.
One widely applicable, general definition of a "critical load"
is: "a quantitative estimate of an exposure to one or more
pollutants below which significant harmful effects on specified
sensitive elements of the environment do not occur according to
present knowledge" (Nilsson and Grennfelt.- 1988} . Other more
specific definitions have been developed for particular
ecosystems to include relevant receptors and measurable
parameters.
The critical load concept assumes a certain acceptable
lifetime for a' particular ecosystem resource to function at a
predetermined capacity For example, a critical load may be set
so that at a given rate of nitrogen into the system, the
buffering capacity of a certain soil will not be exceeded for the
next 50 or 100 years. Below j.s a partial list of endpoints that
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79
have been suggested as useful for defining appropriate critical
nitrogen loads on ecosystems:
* prevent nitrate levels in drinking or surface waters
from rising above standard levels,
* ensure proton production less than weathering rate,
. ' .* maintenance of a fixed NH3-base cation balance,
* maintain nitrogen inputs below nitrogen outputs (the
nitrogen saturation approach), and
* minimize accelerations in the rates of ecological
succession (vegetation changes due to altered
interspecific competition),
Critical load ""values are often expressed as weight of nitrogen
(kilogram) deposited to a certain size area (hectare) over a set
time period (year).
In 1986,' a series of international meetings and workshops to
address critical loads of nitrogen and sulfur deposition to
soils, forest, vegetation, surface water, and groundwater were
initiated. The goal of'these meetings was to provide a technical
basis for determining how extensively deposition, and hence
emissions, need.be reduced to preserve valuable-ecological and
cultural resources (i.e., develop target loads). One such
workshop was conducted in Sweden by Skokloster in 1987. The
Skokloster workshop report provided valuable information the
United States used to start evaluating how the critical loads
approach would work in this country.
Estimates of critical loads may be based upon a number of
different methods and, to a great extent, the selection of a
method depends upon the receptor chosen and the availability of
relevant data for the calculations. Research is underway in.
several different countries, including the United States, to
develop acceptable quantitative methods. However, after
evaluating various options for setting a critical nitrogen load,
Skeffington and Wilson (1988) concluded that "we do not
understand ecosystems well enough to set a critical load for
nitrogen deposition in a completely objective fashion."
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Limited data from the National Acid Deposition Program
. (NADP) and other sources indicate that total deposition of
nitrogen in parts of North America, particularly the eastern
United States, is comparable to that found for many areas of
Europe, North American sites would appear to have total nitrogen
deposition rates less than 25 kg nitrogen/ha/year (Tables VIII-1
and VIII-2), Other studies indicate that wet NOj~ deposition
alone exceeds 15 kg nitrogen/ha/year over most of the midwest and
20 kg nitrogen/ha/year in portions of the northeast United States
(Zemba et al., 1988).
Furthermore, it is clear that ecosystems differ in their
ability to assimilate and buffer excess nitrogen inputs depending
on-the region they are located in. Hence, there is probably no
universal critical load definition that could be applied
effectively to all ecosystems. Therefore, any attempt to set a
critical loads standard(s) would need to take into account
regional differences in sensitivity, as well as other policy
considerations.
Congress included Section 404 in Title IV of the 1990 Clean
Air Act Amendments (Appendix B of the Act) which requires the
Agency to provide a report to Congress on the feasibility and
effectiveness of an acid deposition standard to protect sensitive
and critically sensitive aquatic and terrestrial resources. The
results of this Acid Deposition Standard Study should .begin to
answer some of thp. questions necessary for the Administrator to
make future policy decisions related to setting critical loads.
Specifically, the study seeks to accomplish the following goals:
(1) to identify sensitive ecosystems in the United States and
Canada; (2) to determine the Clean Air Act's (Title IV - Acid
Rain) level of protection for sensitive ecosystems; (3) to
determine the possible need for an acid deposition standard(s)
(e.g., further emission reductions to protect these 'systems);
and, (4) to assess the feasibility and costs of implementing an
acid deposition standard.
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81
MEASUREMENTS OF VARIOUS FORMS OF
ANNUAL NITROGEN DEPOSITION TO NORTH AMERICAN AND
EUROPEAN ECOSYSTEMS
Forms of Nitrogen Deposition (kg/ha)*
Site Location/
Vegetation
United States
California, Chaparral
California, Sierra Nevada
Georgia. Loblolly pine
North Carolina, Loblolly pine
North Carolina, Hardwoods
North Carolina, White pine
North Carolina, Rod spruce
New Hampshire, Deciduous
New Hampshire, Deciduous
New York, Red spruce
New York, Mixed deciduous
Tennessee, Mixed deciduous
Tennessee, Oak forest #1
Tennessee, Oak forest #2
Tennessee, Oak forest tt\
Tennessee, Oak forest ffl
Tennessee, Oak forest
Tennessee, Loblolly pine
Washington, Douglas fir
Washington, Douglas fir
"U.SxRepicfaS" ••—
Adirondack:
Midwest
Northeast
Northwest
Southeast
__fcwheast Appalachians
Wei
Ooud Rain
8.2
_ —
-
3.7
8.7
- 4.8
3.7
8.7 6.2
- 7,0
9.3
7.3 6.1
_ 4.8
- 2.9
3.2
2.9
6.9
6.0
4.5
4.3
2.9
- 1.0
6.3
4.2
— 21.7
- 16.6
20.6
4.2
Dry
Panicles Gases
_ -
- -
1.0 4.2
2.2 4.1
0.5
0.9 2:7
3.6 8.6
— —
— —
0.2 2-3
0.8 2.5
4.1 6.1
>
4.4 ' 4.0
4.4 4.0
1.3
1.2
1.8 3.8
0.6 1.4
1.3 0.6
— —
4.7 -
2.9
_ —
_ _
_ _
3.1
Total*1
23C
a)
9
15
5.3
7
27
(7)
(9)
16
8
13
12
11
8
7
10
9
5
(I)
11
7.1
22
17
21
7.3
Reference
Riggan et al. (1985)
Williams and Melack
(I991a)
Loveit (1992)
Loveu(1992)
Swank and Waide (1988)
Lovett (1992)
Lovett (1992)
Ukens et al. (1970)
Ukens (1985)
Lovett (1992)
Lovett (1992)
Kelly and Meagher
(1986)
Kelly and Meagher
(1986)
Kelly and Meagher
(1986)
Kelly (1988)
Kelly (1988)
Undberg et al. (1986)
Lovett (1992)
Lovett (1992)
Henderson and Harris
(1975)
Driscoll et al. (1989a)
Driscoll et al. (1989a)
Mungfir and Fucmrcich
(1983)
Mungerand EUearekh
(1983)
Munger and Bseoreidi
(1983)
Driscoll et al. (I989a)
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82
Table VJJJ-1 (coni'd). MEASUREMENTS OF VARIOUS FORMS OF
ANNUAL NITROGEN DEPOSITION TO NORTH AMERICAN AND
EUROPEAN ECOSYSTEMS
Site Location/
Vegetation
Forms of Nitrogen Deposition (kg/ha)
Wet
Dry
Cloud
Rain
Particles Gases Total Reference
Canada
Alberta (southern)
British Columbia
Ontario
Ontario (southern)
7.3
5.5
3.7
2J
12.2
1.4
19.5 FeaJke and Davidson
(1990)
(5) Fdler(l987)
(4) Linscy et «1. (1987)
3,7 Ro et id. (1988)
Federal Republic of Germany
Spruce (Southeast slope)
Spruce (Southwest slope)
^etheriands
Oik4»irdi
Deciduous/spruce
Soots ploe
Douglas far
Douglas fu
Morwey
Spruce
United Kingdom
Spruce forest
moor
16.5
24.3
19.3
W.3
16.5 HantscfaeJ el al. (1990)
- 24.3 Hantschol a. al. (1990)
— - 24-56* Van Brecojeo and Vas
Dijk (1988)
.,- 2M2C Van Breeinea and Van
Dijk (1988)
— — 17-64* Van Broeroco and Van
Dijk (1988)
- ; - 17-64* Van Breoa» and Van
D*yk(1988)
9S.7*5 - 115 Draaijeis«al. (1989)
1.9
0,4
0.7 0.2 11.2 icvctl (1992)
- - 3~I9C Royal Society (1983)
.8-0 - 13.5 23.4 Fowler etal.(I989a)
JM) - 4.0 12.4 Fowler«id. (1989a)
— Symbolizes d*U not available or, in the case of cloud deposition, not present.
Measurements of total deposition data that do ool include both * wet and dry estimate probably underestimate
total nitrogen deposition tad are enclosed in parentheses.
ibtsl nltrof en deposition was based on bulk deposition *nd ttuxHigbfal! tueasuremetrts and does include
ooaipooeats of wet sod dry deposition.
Includes deposition from gaseous forms.
Soutve: Table 10-14, pp. 10-99 10 10-100 of
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83
Table VIII-2
NITROGEN INPUT/OUTPUT RELATIONSHIPS
FOR SEVERAL ECOSYSTEMS
Efflux*
Site/Vegetation
Inputs
(kg/ha/year) (kg/ha/year)
Reference
States
Florida, Slash pine
Georgia, Loblolly pine
Minnesota, Spruce
North Carolina. Loblolly pine
North Carolina, Oak/hickory
North Carolina, Rod spruce
North Carolina, White pine
North Carolina, White pine
New Hampshire, N. hardwood
New Hampshire, N. hardwood
New York. Deciduous
New York, Red spruce
Oregon, Douglas fir
Tennessee, Loblolly pine
- Tennessee, Hardwood
Tennessee, Hardwood
Tennessee, Hardwood
Tennessee, Oak forest
Tennessee, Oak forest
Tennessee, Shortleaf/pine
Tennessee, Yellow/poplar
Washington, Douglas fir
Washington, Douglas fir
Washington, Red alder
Washington. Silver fir
Wisconsin, N. hardwoods
Canada
Ontario Maple
Federal Republic of Germany
Norway spruce
Beech
Netherlands
Oak
Oak/bircb
Oak
Mixed deciduous
5.9°
9.0"
7.5b
15.0"
8.2C
27.1b
S.SC
7.4"
6.5
73.6
8.0b
I5.9b
2,0
8.7b
13.2b
13.0
8.7
7.0-8.0*
11.5*
8,7
7.7
1.7
4.7"
70.0b
1.3
5.6
7.8
21.8
21.8
45.0
54.0
56.0
63.0
0
0
0
0
3.2
U.0-20.0
0.2
0
4.0
17.4
1.0
3.0
1.5
0-2-0
4.4
3.1
1.8
1.25
3.2
1.8
3.5
0.6
0
/ 71.0
2,7
0.05
18.2
14.9
4.4
22.0
78.0
28.0
6S.O
Van Miegtoet et al. (1992)
Van Miegroet et al. (1992)
V»n Miegroet ct al. (1992)
Van Miegroet et al. (1992)
Cole and Rapp (1981)
Van Miegroet et al. (1992)
Cole and Rapp (1981)
Van Miegroet ct al. (1992)
Bormann et al. (1977)
likens et al. (1977)
Van Miegroet et al. (1992)
Van Miegroet et al. (1992)
Sollins et al. (1980)
Van Miegroet et al. (1992)
Kelly and Meaner (1986)
;V Henderson and Harris (1975)
Cole and Rapp (1981)
Kelly (1988)
Kelly and Meagbcr (1986)
Cole and Rapp (1981)
Cole and Rapp (1981)
Cole and Rapp (1981)
Van Miegroet et al. (1992)
Van Miegroet and Cole (1984)
Turner and Singer (1976)
Pastor and Bockbeim (1984)
Foster and Ntoolsoa (1988)
Cole and Rapp (1981)
Cole and Rapp (1981)
Van Brcemen el al. (1987)
Van Breeoeo et al. (1987)
Van Breemen et al. (1987)
Van Breemen el al. (1987)
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84
Table VIII-2 (coni'd). NITROGEN INPUT/OUTPUT RELATIONSHIPS
FOR SEVERAL ECOSYSTEMS
Site/Vegetation
Norway
Spruce
Sweden
Coniferous
United Kingdom
Mixed hardwood
U.S.S.R.
Norway spruce
Inputs
(kg/ha/yeaf)
11. 2*
2,1
5,8
1.1
Efflux*
(kg/ha/year)
0
0.6-1.0
12.6
0.9
Reference
Van Miegroet et al. (1992)
Rosen (1982)
Cole and Rapp
Cole and Rapp
(198!)
(1981)
An estimate based on nitrogen losses from the soil profile or from stream flow out of • watershed.
Includes precipitation, cloud (where appropriate), particulate, and gaseous forms of nitrogen deposition.
^Includes nitrogen inputs from precipitation and paniculate forms of deposition.
Mean of two oak forests in eastern Tennessee.
Source: Table 10-15, pp. 10-101 to 10-102 of the 1993 Air Quality Criteria for Oxides
of Nitrogen
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85
•: It is clear that the critical loads concept has potential as
'-a tool in the regulatory process. Most countries of western
"Europe have adopted a system for estimating critical loads and
are moving closer to implementing emission reduction programs
"based on the concept. However, while some progress in the
science has occurred in the past few years, numerous critical
loads related issues remain unresolved, including and
particularly, the long-term effects of nitrogen alone and in
combination with other pollutants. In the United States, no
national critical load level has been proposed, due in part to a
lack of data on multiple forms of nitrogen deposition and a
general need to examine the implications of- this form of effects-
based regulations.
2. Target Loads
A modification of the critical loads concept is that of
target loads. As discussed above, critical loads are developed
dominantly, although not purely, as a -matter of science. In
" contrast, target loads are only partially based'on science and
take into account legal, social, cultural, economic and political
: realities. A target load is the level that is believed
attainable given the above constraints. The Administrator could
decide to set a target-• load-.for, NOX, however it would also be
necessary to exercise significant judgement in identifying the
resources of concern and the' acceptable levels of impact to those
resources if a critical load or target load standard is set.
3• Applicability to U.S. Situation,
The information discussed in the CD and summarized in this
document suggest that effects to ecosystems from deposition of
atmospheric nitrogen may have already occurred in the U.S. and
are therefore a very real future concern. While rough estimates
of total atmospheric nitrogen loads in the United States approach
levels recommended as critical loads for some European countries,
- further research is needed to better estimate appropriate
critical loads and to develop the process for setting critical or
target load'values in the United States. Furthermore, the staff
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86
feels that it would be premature to establish a long-term
national ambient air quality standard based on the current
tentative critical loads estimates.
D. Visibility
1' Maior Categories
Air pollution can degrade the visual appearance of distant
objects to an observer and reduce the range at which they can be
distinguished from the background. While visibility impairment
can occur naturally, it is well documented that anthropogenic air
pollution in the form of fine suspended particles or NO2
exacerbates the problem. The effects are manifested both in
local impairment (e.g., plumes) and in large-scale, hazy air
masses. For purposes of discussion, we are separating visibility
impairment into two major categories: local scale or "reasonably
attributed impairment" and large scale "regional" haze,
Reasonably attributed impairment, which may be defined as a
coherent, identifiable impairment, can be seen as an optical
entity against the background sky or a distant object. This
definition assumes that a single source or a small group of
sources produce pollutants that are not widely dispersed. Thus,
reasonably attributable impairment is considered a near source
problem, although it can be part of a larger, regional
impairment.
The second category of impairment, regional haze, is
produced from a multitude of sources and impairs visibility in
every direction over a large area, such as an urban area, or
possibly over several states. Objects on the horizon are masked
and the contrast of nearby objects is reduced. In some cases,
the haze may be elevated and appear as layers of discoloration.
Multiple sources may combine over many days to produce haze,
which may be regional in scale. The fate of haze Is a function
of meteorological processes that occur concurrently on larger
scales of time and distance.
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87
2• Contributors to Visibility _Reduction
Visibility impairment is caused by the scattering and
-absorption of light by particles and gases in the atmosphere and
depends on the concentrations and properties of the gases
•present. Under typical ambient conditions, light scattering by
particles dominates total extinction, which is related to
reduction of contrast and visual range. The most significant
optical effect of NO2, however, involves discoloration. NO2
appears as a yellow to reddish-brown gas because it strongly
absorbs blue light/ allowing red wavelengths to reach the eye,
The extent to which NO2 filters out blue light is determined by
the integral of N02 concentration along the sight path. The
Criteria Document reports that less than 0,1 ppm-km N02 is
sufficient to produce a color shift which is distinguishable in
carefully controlled, color-matching tests. Reports from one
laboratory using N02 containing sighting tubes indicate a
possible visible color threshold of 0.06 ppm-km for the typical
observer. These values refer to the effect of NO2 in the absence
of atmospheric aerosol because the visibility impairment caused
by aerosol usually overwhelms the effect caused by N02.
Although the physical properties of NQ2 are well known and
its coloration effect.in a controlled environment recognized,
there are relatively little data available for judging the actual
importance of N02 to visual air quality. .The data needed to make
such a judgment are potentially complex, including wavelength
dependence of scattered light at different angles which can also
cause discoloration (Charlson et al., 1978). In addition to
being modified by particle scattering, discoloration of plumes
and haze layers by NO2 also is affected by a number of other
factors such as sun angle, surrounding scenery, viewing angle,
human perception parameters, and pollutant concentrations.
One unresolved issue concerns the relative contribution of
N02 and particles to atmospheric discoloration. Although the
color of urban haze (often termed "brown") was originally
ascribed to NO2, more recently it has been shown that the brown
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88
color can result from particles alone. The coloration effect of
particles depends on particle size, composition, scattering angle
between observer and illumination, and optical characteristics of
the background target. The overall impact of aerosol haze is to
reduce visual range and contrast and possibly, to change color.
Nitrate aerosols do contribute to the overall impact on
visibility impairment. It is difficult, however, to make a
quantitative assessment of their contribution. The draft staff
paper on particulate matter will consider the relative
contribution of nitrate to fine particle mass.
EPA must assess whether there is any supportable
relationship between NC>2 concentrations at a given point and
visibility impairment due to a plume or regional haze. The
present N02 standards are intended to protect against effects at
or near ground level, and monitoring for NC>2 is generally
performed at or near ground level. In the case of visibility
impairment due to a plume from a stationary source, there is no
reliable relationship between ground, or near ground, level
concentrations at any given point and discoloration caused by the
plums. The plume, trapped in an atmospheric inversion, would
disperse slowly and mix to the ground far downwind of the source.
Concentrations -taken at ground level while a coherent plume was
clearly visible would not necessarily exceed an ambient standard.
For this reason it would be difficult to set a NAAQS for NOj,
based on ground level monitoring, that would insure an acceptable
level of visibility.
Another approach to establishing a visibility standard would
be to monitor plume level concentrations. Because of the
difficulty in making plume measurements, it may be possible to
measure the discoloration Itself using an optical device,- e.g., a
telephotometer. This instrument could measure the color contrast
between the background and the plume. The measurements could be
used as a possible index of the effect of NO2« Measuring the
actual discoloration would avoid the problems encountered in the
ordinary approach, where a monitor near ground level might not
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89
pick up a violation of the standard, but a plume would be clearly
visible, or where high N02 levels detected by remote sensing did
not result in a perceptible plume. One problem in this approach
is the uncertainty as to whether the discoloration that would
-trigger the telephotometer is caused by N02 or by particles.
Although measurement of discoloration would be a unique way of
expressing the standard, it would be worthy of consideration once
these problems are resolved.
Another regulatory mechanism, provided under sections 169A
and 165(d) of the Clean Air Act (CAA), may provide some control
over the more noticeable brown plumes appearing in otherwise
pristine areas. In December 1980, EPA promulgated a phased
approach to visibility protection. Phase I applies to pristine
(Class I) areas (i.e., all international parks and certain
national parks and wilderness areas as described in section-
162(a) of the CAA) and requires control of visibility impairment
caused by visible plumes from individual sources. EPA has
initiated steps to control this aspect of visibility impairment.
The Agency is now beginning its development of rules to address
regional haze in Class I areas.
In regard to regional haze, because the effect of NCX,
depends on the product of the pollution concentration and the
viewing path length, the impression of severity is greater the
farther away the viewer can see past (or around) the haze layer.
(The coloration of 0.05 ppm NO^ over 10 km is the same as 0.5 ppm
over 1 km.) When NOj is dispersed over a large area, as in the
case of urban emissions, ground level concentrations at
individual points may be less than a national standard but
because an observer views the entire NC«2 mass, the urban plume
would appear discolored.
In summary, the scientific evidence indicates that light
scattering by particles is generally the primary cause of
degraded visual air quality and that aerosol optical effects
alone can impart a reddish brown color to a haze layer, thus
raising the question as to the appropriateness of a NAAQS for NO2
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90
to protect against visibility impairment. While it is clear that
particles and NC>2 contribute to brown haze, it is the staff's
judgement that the improvement in visual air quality to be gained
by reducing NO^ concentrations seems uncertain at best.
Therefore, the staff concludes that an ambient secondary standard
for NO2 to protect visibility is not warranted at this time.
E» Non-Biological Materials
Nitrogen oxides (NOX), including nitric oxide (NO), nitrogen
dioxide (N02), and nitric acid (HN03), are known to enhance the
fading of dyes; diminish the strength of fabrics, plastics, and
rubber products; assist the corrosion of metals; and reduce the
use-life of electronic components, paints, and masonry. Compared
to studies on sulfur oxides (SOK), however, there is only limited
information available quantifying the effects of nitrogen oxides.
Especially in the outdoor environment, it is difficult to
distinguish a single causative agent for observed damage to
exposed materials because many agents, together with a number of
environmental stresses, act on a' surface throughout, its life.
Oxides of' nitrogen are deposited on material surfaces through
both wet and dry deposition processes. (Tombaeh, 1982). In
addition, particles containing NOX can be transported to a
material surface through gravitational settling or inertial
impaction of the particles on the surface. The rate at which
deposition processes transport NOX to the surface is dependent on
the NOX concentrations in the environment, the chemistry and.
geometry of the surface, the concentrations of other atmospheric
constituents, and the turbulent transfer properties of the air
(Lipfert, 1989)- There is wide variation in the deposition of '
NOX to different surfaces and NOX species themselves are reactive
and their interactions with other atmospheric constituents are
complex.
It is the staff's conclusion that insufficient evidence
exists thai, NO2 is currently playing a significant role in damage
to non-biological materials. Better experimental techniques are
needed, both for investigating materials damage on the whole and
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91
for determining the role played by NOX. Care should be exercised
in the extrapolation of data and conclusions based on chamber
studies to effects expected from ambient exposures. Franey and •
Graedel (1985) have suggested that for any chamber study to be
realistic, moisture, radiation, carbon dioxide, reduced sulfur,
and a nitrogen-containing gas must be included. Therefore, we
conclude that an ambient secondary standard for NO2 to protect
against materials damage is not warranted at this time.
IX. Summary of^.Staf.f ^Conclusions and Recommendations for
Secondary Nitrogen Dioxide NAAPS
This summary of staff conclusions and recommendations for
the NCU secondary NAAQS draws from the discussions contained in
previous sections of this Staff Paper. The-key findings are;
1} The staff concludes that the impact on terrestrial
vegetation from short-term exposures to NC^ under existing
ambient levels is insignificant and does not warrant a change in
the secondary NAAQS at this time. As presented above in section
B.I.a., studies of short-term, acute effects of nitrogen
additions to vegetation have demonstrated that NOo in mixtures
with other pollutants or at higher than ambient concentrations
can produce adverse effects on plant growth or reproduction.
Though the sensitivity of vegetation to NC>2 varies both within
and between species and can- be modified by season, developmental
stage, or other climate or environmental factors, taking all of
these variables into account, the. ambient levels of NO2 in the
United States are considered below those that evoke a short-term,
acute response.
2) The staff concludes there is little potential for
concentrations of nitrogen to be sufficient to acidify soils to
the extent that it would cause direct phytotoxic effects in
plants.
3} The staff concludes, based on existing European studies
and information in the United States, that addition of nitrogen
to ecosystems in the U.S. (specifically in mature forests and
wetlands which host a number of endangered species adapted to
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92
nitrogen-poor habitats) may lead to shifts in species competition
and composition and may represent a significant environmental
problem. However, there is not sufficient evidence to conclude
that changes in plant communities have occurred in the past or
are now occurring in the United States due to nitrogen inputs.
Furthermore, there is only limited long-term data available on
plant community composition. Additional research is needed to
more accurately characterize any potential threat to mature
forests and wetland species from atmospheric nitrogen. Once this
information is available, the need for additional ecosystem
protection should be reassessed by the Administrator.
4) The staff concludes that nitrogen deposition .is a factor
in chronic acidification of surface waters in certain sensitive
regions. However, at this time, the staff finds insufficient
evidence to quantify the magnitude or timing of the relationship
between atmospheric deposition of nitrogen and its appearance in
surface waters, Additional research should be conducted so that
such information will be available for the next standard review.
Once this relationship can be reasonably quantified, the staff
recommends that the Administrator consider appropriate regulatory
approaches to address the problem. These actions could take the
form of a uniform national standard or regional standards which
account- for differences in regional sensitivities. •
5) Based on the review of the available scientific
information in the CD, the' staff concludes that nitrogen
deposition is a contributor to the episodic acidification of some
streams and lakes in the United States. Thus, it can also be
concluded that some freshwater ecosystems will benefit from
reductions in nitrogen inputs from man-made sources. However,
the scientific information reviewed in the CD is insufficient, at
this time, to quantify how much of a contribution nitrogen
deposition is making to the acidification problem and what levels
of reduction are neces.sary to remedy the situation.
When additional scientific information is available that
adequately quantifies the relationship between nitrogen
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93
deposition and episodic acidification, the staff recommends that
the Administrator consider appropriate regulatory approaches to
address the problem. Key considerations will be the major
differences evident in the occurrence, nature, location, timing
of episodes at different sites, and the subsequent effects
produced. Such differences will make setting a single national
standard which would adequately protect all areas of the country
difficult and complex. Therefore, staff believes regional
approaches that take into account such variations should be
considered for providing the level of control needed to address
the problem of episodic acidification.
6) The staff concludes that it is very unlikely that
eutrophication of freshwater systems is due to atmospheric
nitrogen additions since phosphorus, not nitrogen, is generally
the limiting factor for algae growth.
7) The staff concludes that both upland and direct
atmospheric nitrogen deposition can significantly affect the
trophic status of estuarine and coastal waters. However, it is
difficult to quantify the effect that nitrogen deposition is
-having either within a system or across systems.
As mentioned before, once a relationship can be reasonably
quantified, staff recommends that the Administrator consider
appropriate regulatory approaches, including a uniform national
standard or regional standards, to protect our valuable natural
resources. Regional standards may provide a better mechanism to
address the problem because of the variability in responses to
increased nitrogen inputs between regions.
8) Because of nitrogen saturation, nitrogen levels in
surface waters could increase even if nitrogen deposition does
not increase. Therefore, episodic and chronic acidification of
freshwater, as well as increased loadings to estuaries, could
worsen even without increases in nitrogen deposition. Further
research is needed, however, to determine the direct effects of
nitrogen deposition on acidification or eutrophication, the rate
at which this increase would occur, and in what time frame.
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94
Therefore, staff recommends that further research be conducted to
provide the information needed to evaluate this issue in its next
standard review.
9) The staff concludes that it would be premature to
consider establishment of a long-term national critical loads
secondary ambient air quality standard for NC>2 given the state of
the science of critical loads estimation that exists at the
current time. However, this methodology has the potential to
provide a useful tool for making regulatory decisions at the
regional level in the future. The Acid Deposition Standard
Study, which is designed to fully evaluate the issues associated
with this approach, should assist the Administrator further in
evaluating the potential application of both critical and target
loads.
10) With regard to visibility impairment, the scientific
evidence indicates that light scattering by particles is
generally the primary cause of degraded visual air quality.
Furthermore, aerosol optic effects alone can impart a reddish-
brown color to a haze layer. While it is clear that particles
and NOj contribute to brown haze, in the staff's judgement
reducing ground level NO^ concentrations would result in little
improvement in visual air quality. Therefore, the staff
concludes that an ambient secondary standard for N02 to protect
visibility is not warranted at this time.
11) Based on the available data, the staff concludes that
it is unlikely that NO-, is playing a significant role in the
damage to non-biological materials and therefore does not
recommend consideration be given to setting a secondary standard
to protect against material damage.
12) If the Administrator determines that a separate
secondary standard is appropriate, the staff recommends that it
be set identical to the primary standard for NC>2 - A secondary
standard set within the range recommended for the primary
standard would, in the staff's judgement, provide adequate
protection against the 'iirect effects of NG>2 on the environment.
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95
13) It is clear that additional studies need to be
conducted to reduce the uncertainty in the relationship between
nitrogen deposition and forest health and in determining the
adverse effects on surface waters and estuaries arising from the
long-term accumulation of nitrogen. The staff recommends
substantial efforts be undertaken to illuminate the quantitative
relationships between nitrogen deposition and changes in forest
nitrogen cycling, episodic and chronic acidification of surface
waters, and estuarine eutrophication, either through ongoing
research or new initiatives. Such efforts will provide an
improved scientific basis for the next standard review.
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APPENDIX A: CASAC CORRESPONDENCE
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UNITED STATES ENVIRONMENTAL PROTECTION AGENCY
WASHINGTON, D.C, 20460
^o
September 30, 1993 EPA-SAB-CASAC-LT8.-93-O15
OFFICE Of
Honorable Carol M Browner THS ADMINISTRATOR
Administrator
U.S. Environmental Protection Agency
401 M Street, S.W.
Washington, DC 20460
Subject: Clean Air Scientific Advisory Committee Closure on the Air Quality
Criteria Document for Oxides of Nitrogen.
Dear Ms. Browner:
The Clean Air Scientific Advisory Committee (CASAC) of EPA's Science Advisory
Board (SAB) at a meeting on July 1, 1993, completed its review of the draft document entitled
National Ambient Air Quality Standards for Oxides of Nitrogen (NCO. The Committee notes
with satisfaction the improvements made in the scientific quality and completeness of the criteria
document. It has been modified in accordance with the recommendations made by the CASAC
in April, 1993.
The document has organized the relevant information in a logical fashion and the
Committee believes that it provides a scientifically adequate basis regulatory decisions on oxides
of nitrogen based on present scientific data on health effects of such exposure.
The Committee looks forward to reviewing the upcoming staff decision paper on the NO,
standard.
Sincerely,
George T. Wolff,
Chairman
Clean Air Scientific Advisory Committee
^ Recyded/Recydabi*
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XT—I/"-/ »»-..!.
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UNITED STATiS ENVIRONMENTAL PROTECTION AGENCY1
WASHINGTON. O.C. 204tO
OFFICE OP THE ADMINISTRATOR
6CIENC* ADVISORY BOARD
August 22, 1995
EPA-SAB-CASAC-lTR-95-004
Honorable Carol M, Browner
Administrator
U.S. Environmental Protection Agency
401 M St., SW
Washington, .DC 20460
Subject: CASAC Review of the Staff Paper for the Review of the
National Ambient Air Quality Standards for Nitrogen Dioxide:
Assessment of Scientific and Technical Information
Dear Ms. Browner:
The Clean Air Scientific Advisory Committee (CASAC) of EPA's Science
Advisory Board (SAB) at a meeting on December 12,1994, reviewed the document
entitled Review of the National Ambient Air Quality Standards for Nitrogen Dioxide:
Assessment of Scientific and Technical Information. Office of Air Quality Planning and
Standards (OAQPS) Staff Paper. At that meeting and in subsequent written comments,
the Committee made a number of recommendations for improving the document. On
June 2, 1995, a revised Staff Paper was mailed to the CASAC members for review with
a letter response. The resulting comments by the Committee members note with
satisfaction the improvements made in the scientific quality and compltteness of the
staff paper. It has been modified in accordance with the CASAC recommendations.
The document is consisttnt with all aspects of the scientific evidence presented
in the criteria document for oxides of nitrogen, It has organized the relevant
information in a logical fashion and the Committee believes that it provides a
scientifically adequate basis for regulatory decisions on nitrogen dioxide. The staff
paper concludes, and the CASAC concurs, that an annual primary standard of the
present form and with a numerical value between 0.05 to 0.08 ppm would be supported
by the present scientific data on chronic health effects of exposure to nitrogen dioxide.
In addition, the CASAC concurs with the rationale that if the existing annual standard of
0.053 ppm is attained, it will provide adequate protection against the occurrence of
short term 1-hoyr peak concentrations of 0.2 ppm or higher.
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The staff paper also concludes and the CASAC concurs, that the present
primary standard protects against the direct effects of nitrogen oxide exposure to
vegetation. Further, the CASAC agrees that there is insufficient scientific evidence
available to warrant a secondary standard designed to protect aquatic and terrestrial
ecosystems from the adverse effects of nitrogen deposition including eutrophication
and acidification. Finally. CASAC concurs with the Staff conclusion that setting an N02
secondary standard to protect against visibility impairment is not warranted.
The Committee looks forward to receiving notice of the revised or reaffirmed
nitrogen dioxide standard when it is proposed.
Sincerely,
Dr. GeorgeT. Wolff,
Clean Air Scientific Advisory
Committee
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APPENDIX B: ANALYSIS OF CURRENT NOO NAAQS ATTAINMENT
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APPENDIX B
The following is an excerpt from the report "Analysis of
High l Hr NCu Values and Associated Annual Averages Using 1388-
1992 Data" ' (McCurdy, 1994).
"Attaining the current N02 annual average NAAQS of 0,053 ppm
is expected to keep 1 hour and daily exceedances of various N02
concentration levels below the values shown in Table 16. It
should be noted that personal judgement was used to develop some
of the minimum and maximum estimates since the 95PI [e.g., the
95th percentile prediction interval around the mean estimate! did
not always appear for an indicator. Most of the "problem
intervals" involved very small exceedances (i.e., <2), so any
mis-estimates that may exist are small.
1. Non-Los Angeles CMSA Sites
As seen in Table -16, attaining the current NO2 NAAQS results
in no expected (mean) exceedances of 1 hour or daily exceedances
>0.20 ppm. The worst area might have 10 exceedances of the EX20-
[e.g., the number of observed 1 hour N02 values j> 0.20 ppm],
indicator and 5 exceedances of the DAY20 [e.g., the number of
days with >. 1 hour NC>2 concentrations 2. 0.20 ppm] indicator under
this situation. Multiple exceedances of EX15 [e.g., the number
of observed 1 hour NC^ values >. 0.15 ppm] and DAY15 [e.g., the
number of days with >. 1 hour NC^ concentrations >_ 0.15 ppm]
indicators can be expected, however, even if the 0.053 ppm N02
NAAQS is attained.
2. Los Angeles Consolidated Metropolitan Statistical Area
(CMSA) Sites
Attaining the current NO2 NAAQS in this area does not
guarantee that numerous hourly or daily exceedances of a 0.15 or
0.20 ppm indicator will be prevented. The CMSA has an N02
pattern of numerous short-term peaks for relatively low NO,
annual averages. This finding may indicate that more emphasis
should be placed on reducing short-term N02 levels in that major
metropolitan area. This may mean a change in the areas's N02
control strategy to emphasize peak hour reductions that are not
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much impacted by area-wide programs, such as motor vehicle
emission controls.
The peak levels that we are discussing are 0.15 and 0.20
ppm. Even the Los Angeles CMSA would not be expected to see
numerous 1 hour or daily exceedances of a 0.25 ppm peak
concentration if the NO2 NAAQS were attained,"
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Table 16
ESTIMATED NUMBER OF EXCEEDANCES OF VARIOUS N00 SHORT-TERM AIR
£t
QUALITY INDICATORS GIVEN ATTAINMENT OF THE CURRENT NO2 NAAQS OF
0.053 PPM
B.
D.
Concentration
Level (ppm)
1 h Exceedances in
0.15
0.20
0.25
0,30
1 h Exceedances in
0.15
0.20
0,25
0.30
Daily Exceedances
0.15
0.20
0.25
0.30
Daily Exceedances
0.15
0.20
0.25
0.30
Minimum
Estimate
Los Angeles CMS A
10
0
0
0
Other Areas
0
0
0
0
in Los Angeles CMSA
5
0
0
0
in Other Areas
0
0
0
0
Mean
Estimate
75
13
1
0
5
0
0
0
27
6
1
0
3
0
0
0
Maximum
Estimate
195
38
17
2
44
10
•3
.1
57
21
8
2
9
5
2
1
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APPENDIX C: DEFINITIONS
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APPENDIX C
DEFINITIONS:
(These definitions were derived from the welfare discussions in the
Criteria Document.)
1) Short-term exposures:
a) organismal: the organism is exposed for periods less than or
equal to 24 hours in duration.
b} ecosystem: the ecosystem is exposed consistently for less
than or equal to 3 years in duration.
2) Long-term exposures:
a) organismal: the organism is exposed for periods greater than
24 hours in duration and typically greater than or equal to two weeks in.
duration.
b) ecosystem: the ecosystem is exposed consistently for greater
than three years in duration.
3) Assimilation; the uptake and metabolic use of nitrogen by either
aquatic or terrestrial plants. While terrestrial plant species generally
favor uptake of NH4+ over NO^', the form of nitrogen used will strongly
affect the effect nitrogen deposition will have on pH.
4) Nitrification: the oxidation of NH4 + to NO3~ by aquatic or
terrestrial bacteria or fungi and is a strongly acidifying process.
5) Denitrification: an anaerobic process which reduces NO3~ to K2»
NO, or ^0, High rates of N20 production may be a concern in some areas
because of its significance as a greenhouse gas. Denitrification is also
always a acidifying process.
6) Nitrogen fixation: the process by which a variety of single-celled
organisms and aerobic and, anaerobic bacteria fix gaseous atmospheric
nitrogen into NH4+, These organisms may be found singly or in symbiotic
nitrogen-fixing nodules on roots of some plant species.
7) Mineralization: the bacterial decomposition of organic matter,
releasing NH4+ that can subsequently be nitrified to N03~. The effect of
mineralization on the acid/base status of draining waters will depend on
the form of nitrogen produced.
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APPENDIX D: REFERENCES
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TECHNICAL REPORT DATA
(Please read Instructions on reverse .before completing)
EPA-452/R-95-005
3. ttCXPXSMT'S ACCESSION HO,
4, TITLE AND SUBTITLE
Review of National Ambient Air Quality Standards for
Nitrogen Dioxide: Assessment of Scientific mnd
Technical Information (OAQPS Staff Paper)
5. REPORT DATE
1995
t. »ERPOWiII»3 OkliWJIiATION CODE
7. MTTWORIS)
Edwards, Chebryll C.; McKee, David J,
V.
Atwell, Victoria
B. MRKRMXNS OISJUIIZATIOH REPORT NO.
3, SSRKmmXS ORGANISATION NAMF AMI! ADDRESS
U.S. Environmental Protection Agency
Office of Air and Radiation
Office of Air Quality Planning and Standards
Research Triangle Park, NC 27711
1C. PROGRAM ELEMENT NO.
11. CWIRACT/OIUUfT DO.
12. SPONSORING AGENCY HUME ANB ABBMSS
Director
U.S. Environmental Protection Agency
Office of Air and Radiation
Office of Air Quality-Planning and standards
Research Triangle Park, NC 27711
13- TYPB OF RBPOJJT AMD PERIOD COVERED
Final
14. StONSORINO A08NCY CODB
EPA/200/04
15. SaWUSMSHTARY MOTES
16. ABSTRACT
This paper evaluates and interprets the updated scientific and technical information
that EPA staff believes is most relevant to the review of the primary (health) and
secondary (environment; welfare) national ambient air quality standards (NAAQS) for
nitrogen dioxide. This assessment is intended to bridge the gap between the scientific
review in the 1993 revised criteria document and the judgements .required of the
Administrator in setting ambient air quality standards for nitrogen dioxide. The major
conclusions of the staff paper include the following: (l) the Administrator should
consider setting the level of the annual primary standard within the range of 0.05 to
0,08 ppm NO-, to provide adequate protection against the health effects associated with
long-term NO2 exposure,- (2) air quality analyses indicate that a standard selected from
the lower portion of the range suggested would effectively limit the frequency and
magnitude of l-hour peak N02 concentrations of potential health concern; (3) the annual
secondary standard for N02 should be set identical to the primary standard in order to
provide adequate protection against the direct effects of N02 on the environment; (4)
additional research is needed to determine the quantitative relationships between
nitrogen deposition and changes in forest nitrogen cycling, episodic and chronic
acidification of surface waters, and estuarine eutrophication.
KEY MQRP5 AKP OOCTMBdT ANALYSIS
DliCIIFTORS
fc. IPmrriFIBRS/Ot'lK KNEED THUS
COSATI Fi«ld/Gro«£i
nitrogen dioxide, national ambient air
quality standards (NAAQS), respiratory
illness/disease, acidification,
eutrophication
air pollution control,
environmental protection,
ambient air
16. DISTRIBUTION STATBMSNT
Release Unlimited
19. SECURITY CLASS
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