PB 199 904
EiASIS FOR ESTABLISHING GUIDES FOR SHORT-TERM
EXPOSURES OF THE PUBLIC TO AIR POLLUTANTS
National Academy of Scien ces - National Research Council
Washington, D. C.
May 1971
NATIONAL TECHNICAL INFORMATION SERVICE
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U.S. DEPARTMENT OF COMMERCE
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Basis for Establishing Guides
for
Short-Term Exposures
of the
Public to Air Pollutants
by
The Committee on Toxicology
of the
National Academy of Sciences - National Research Council
, Washington, D. C.
May 1971
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Prepared under Contract No. CPA 70-57 between the
National Academy of Sciences, Advisory Center on
Toxicology and the Air Pollution Control Office of the
Environmental Protection Agency.
Contract Monitor:
Dr. Vaun A. Newill, Director
Division of Health Effects Research
Air Pollution Control Office
Environmental Protection Agency
Durham, North Carolina
The information contained in this
letter is intended only as guidance
for your professional and technical
staff and contractors. It is not for
public distribution or attribution
to the National Academy of Sciences
without prior written approval.
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Issued by the Committee on Toxicology
Herbert E. Stokinger, Chairman
Seymour L. Friess, Vice Chairman
Bertram D. Dinman
Arthur B. DuBois
Robert E. Eckardt
Harold M. Peck
Verald K. Rowe
C. Boyd Shaffer
Prepared with the assistance of the staff of the Advisory
Center on Toxicology, Ralph C. Wands, Director.
National Academy of Sciences - National Research Council
2101 Constitution Avenue
Washington, D. C. 20418
1971
11
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Table of Contents
Preface ' iv
Summary 1
Introduction 2
Assumptions Underlying Guide Preparation 2
Definition of Short-Term Exposures 2
Predictable Short-Term Exposures 3
Unpredictable Short-Term Exposures 3
Factors of Consideration in Guide Preparation 4
Human Factors 4
Plant and Animal Factors 4
Materiel Factors 5
Nuisance Factors 5
Analytical Factors 5
Physical and Chemical Factors 5
Interaction Factors 6
Cause and Effect Relationships 6
Acquisition of Data 6
Interpretation of Data 7
Translation of Animal Data to Man 7
Determination of Safety Factors 7
Selection of Standards: Short-Term Public
Limits (STPL's) and Public Emergency Limits (PEL's) 8
Definitions 8
Balance of Risks 8
Acceptable Risks 9
111
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Preface
The continuing introduction of chemicals into channels of distribu-
tion for industrial, military, space exploration, and various other appli-
cations creates the possibility of relatively brief exposures of both
occupational and non-occupational groups of people to air pollution from
such chemicals.
Such occupational exposures were the subject of a report issued in
1964 by the Committee on Toxicology of the National Academy of Sciences-
National Research Council titled rrBasis for Establishing Emergency
Inhalation Exposure Limits Applicable to Military and Space Chemicals. "
It was prepared at the request of the federal agencies sponsoring the
Committee and its Advisory Center on Toxicology to meet a need within
their organizations for guidance on the protection of the health of their
civilian or military personnel who might briefly be exposed to atmospheric
pollutants arising from the federal facilities. The report dealt only in
general terms with possible similar exposures of the public.
IV
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The Clean Air Act as amended in 1970 places responsibility for
public exposures to air pollutants with the Commissioner of the Air
Pollution Control Office of the Environmental Protection Agency.
The assistance of the Committee on Toxicology and its Advisory
Center on Toxicology of the National Academy of Sciences-National
Research Council was requested in providing guidance on the establish-
ment of standards for various pollutants occasionally released for short
periods of time intpthe atmosphere. Their recommendations, as set
forth in this report, will serve as the basis for a series of "Guides for
Short-Term Exposures of the Public to Air Pollutants," each of which
will be limited to specific materials.
Ralph C. Wands
Director
Advisory Center on Toxicology
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SUMMARY
A. Short-term exposures of the public to air pollutants are
described in two categories; those which are predictable as to time and
place of their occurrence, and those which are unpredictable.
Limits for short-term exposures of either category should not
be established for durations longer than 24 hours. Limits for both
categories should always be subject to review as new data and new concepts
are generated.
B. The factors to be considered in setting limits for predictable
and unpredictable short-term public exposures are discussed. These
factors include the nature and extent of effects on humans, plants and
animals, and materiel. Other subjects discussed are nuisance effects,
the availability of monitoring procedures, physical and chemical
characteristics of the pollutant, and the presence of other pollutants.
C. The establishing of standards for short-term exposures of the
public to atmospheric pollutants cannot be reduced to an arbitrary mathe-
matical relationship. The process requires experienced judgment in
evaluating pertinent information and selecting a balance of risks.
D. The kinds of data required for setting limits for predictable
and unpredictable exposures are identified. Interpretation of the data
and the safety factors involved in applying the data to man are discussed.
E. Short-Term Public Limits for predictable exposures and Public
Emergency Limits for unpredictable exposures are defined. Predictable
short-term exposures must be controlled to the point where their effect
on the public health and welfare will be no greater than is acceptable
under prevailing criteria and standards for ambient air quality.
Precautionary measures must be taken to reduce the possibility
of unpredictable short-term exposures and to minimize their effects if
they do occur. These exposures should be limited so as to be rare and
unique in the lifetime of an individual and should not produce effects of a
severe or irreversible degree.
Nuisance effects such as unpleasant odors might be tolerated
for the brief periods with which these Guides are concerned.
F. Some of the judgmental considerations by a regulatory body for
minimizing the potential overall risk to the public from short-term
exposures are discussed.
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I. Introduction
The Committee on Toxicology during the last several years has
included in its services to the Federal Government recommendations for
safe concentrations of air pollutants to which persons under direct Federal
supervision might safely be exposed for brief periods of time during
emergencies such as the accidental release of volatile or gaseous chem-
icals. The present report is an application of this experience to the
protection of the public health. It contains the rationale in setting
standards for short-term exposures of the public and will be followed by
series of reviews on specific substances.
"Guides for Short-Term Exposures of the Public to Air Pollutants"
will be developed from the concepts presented herein and will be designed
to provide guidance in the control of both single and occasionally repeated,
brief exposures of human communities to air pollutants. This document
emphasizes protection against the adverse effects on human health and
recognizes that some temporary irritation or nuisance may be acceptable
,on occasion. For many pollutants human health will be the only concern,
however, it is recognized that for some the primary concern will be their
effects on vegetation or materiel.
In developing these guidelines, consideration must be given to the
presence and related effects of other pollutants and to the incorporation
of appropriate safety factors.
II. Assumptions Underlying Guide Preparation
The effects of short-term or acute exposures to relatively high
doses of air pollutants may be qualitatively and quantitatively different
from those of long-term or chronic exposures. There may be little or
no similarity between the two sets of effects. Accordingly, the protection
of the public health requires careful consideration of short-term expo-
sures in addition to and differentiated from the long-term exposures which
are the subject of Air Quality Criteria and similar ambient air quality
control documents.
A. Definition of Short-Term Exposures
A short-term exposure to an air pollutant is defined as exposure
experienced by an individual to a pollutant released from a single source
for a brief time.
The duration of such an exposure may be as much as 60 minutes
under favorable conditions, although under stagnant atmospheric condi-
tions it may last up to 24 hours or longer. There seems to be little
practical significance in establishing standards for less than 10-minute
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exposures, since variations in the dynamic proc'ess:es -of air movement
and mixing lead to considerable uncertainty in 'predFeting the amount of
pollutant that might be inhaled by an exposed individual immediately
after its release.
Short-term exposures will involve only a discrete downwind area
and will therefore affect a limited segment of the population in the vicinity
of the source. Such exposures will probably, but not necessarily, occur
in a sparsely populated area.
Short-term exposures are in addition to any ambient conditions on
air pollution such as those described in the documents "Air Quality
Criteria" and similar publications issued by the Air Pollution Control
Office for use in setting air quality standards.
B. Predictable Short-Term Exposures
Predictable short-term exposures are those occurring at pre-
dictable times and may arise from single or from occasionally repeated
events. Such predictable exposures require acceptance of'the concept
that the general public may be required to be exposed briefly to relatively
high concentrations of air pollutants (in the course of approved activities
which cannot be carried out without such pollution) and that such exposures
may occur again.
Such exposures occurring at predictable times can in no sense be
considered as accidents. There is no justification for submitting the public
to any appreciable risk in a predictable exposure. Advance provisions
should be made prior to such releases of pollutants to control the public
exposure. Under any set of normal operating conditions, such as rocket
launching or blowing tubes in a steam boiler, it should be possible to
predict reliably the timing, the duration of exposure, the concentration
of pollutant, and the movement of the contaminated air mass. In many
instances the time of release can be chosen to coincide with weather
conditions which are optimum for minimizing the exposure.
Predictable exposures that regularly occur with high frequency
will be subject to emission or ambient air quality standards rather than
to short-term public exposure limits.
C. Unpredictable Short-Term Exposures
Situations will occur in which pollutants are released in an un-
controlled manner at unpredictable times and places as the result of
accidents such as rHmage to transportation equipment or fire in a chemi-
cal storage facility. Even for such circumstances it should be possible
to predict with fair accuracy the variety of probable conditions of an
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accident and hence to predict the corresponding exposure parameters.
Safeguards should be taken against such accidents so that the
probability of such an unpredictable release affecting an individual will
be rare in his lifetime. Such an exposure would be considered an
accident in its effect upon a given individual, and within this limitation,
there is some justification for acceptance of a reasonable risk of
reversible injury from potential public exposures.
III. Factors for Consideration in Guide Preparation
Although primary consideration must be given to factors affecting
human health, secondary consideration must be given to the effects of
short-term pollution episodes on plants, animals, and materiel. One
or more of these latter categories may be more sensitive than humans
and, thus, may constitute the limiting circumstances. Consideration of
factors other than human health may require different criteria or standards.
A. Human Factors
Most of the reliable data on the toxicity of any material are the
results of carefully controlled experiments on animals. Occasionally,
data may be available on controlled exposures of human volunteers.
Comprehensive epidemiological data can only rarely be found. Therefore,
the prediction of the effects of human exposures to toxic materials re-
quires a large measure of educated and experienced judgment.
The use of data from animal testing for predicting the effects of a
substance on humans carries with it several sources of uncertainty
which include:
(a) differences between individuals of the same animal
species,
(b) differences between animal species,
(c) extrapolation of the data from animals to humans,
(d) differences between humans,
(e) non-uniformity of the contaminated air mass,
(f) deviations from the predicted movement of the
contaminated air mass.
B. Plant and Animal Factors
There are numerous well-known examples of susceptibility of plants
and animals greatly in excess of that of humans to the same pollutants.
Many plants are readily damaged by traces of ethylene which have no
effect upon humans or animals. Grazing animals, especially sheep and
cattle, are severely affected by airborne arsenic, lead, and molybdenum
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participates that are deposited on forage and then ingested. Instances of
this type are sufficiently common to justify their consideration when
developing standards for short-term exposures.
C. Materiel Factors
There have been a few episodes in which brief releases of air
pollutants have had significant effects upon materials in addition to
generally reversible effects on the health of the exposed humans, plants,
and animals. The release of gaseous sulfur compounds has caused dis-
coloration of buildings painted with lead based paints. Corrosive gases
and vapors will attack stone and metal; although the effect may not be
immediately noticeable, these exposures can produce significant damage
when sufficiently prolonged or repeated. The possibility of such effects
should also be taken into account in arriving at short-term exposure
standards.
D. Nuisance Factors
Undesirable esthetic effects such as unpleasant odors or reduced
scenic visibility, and associated economic losses, should.be considered
in connection with predictable exposures. They are not deemed to be
important factors in standards to be applied to short-term unpredictable
exposures.
E. Analytical Factors
The maintenance and monitoring of air quality standards requires
that suitable methods be available for sampling and analyzing air for the
pollutant in question. If adequate procedures are not available the
immediate need for research and development should be emphasized.
Recommendations for "zero11 levels are technically meaningless. If it
is desirable to control a material to the lowest possible level, a phrase
should be used such as "non-detectable by the most sensitive method
of analysis available. "
F. Physical and Chemical Factors
The physical form of a pollutant, i. e. , gas, vapor, dust, or mist,
may have a pronounced effect upon the route and extent of its attack.
Particles up to 10 micra in diameter can be inhaled into the lungs,
whereas larger particles are filtered out in the upper respiratory
passages. Such properties as solubility and chemical composition will
also alter the nature and degree of the effect of a pollutant. Consequently,
it is essential that *he pollutant be well characterized.
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G. Interaction Factors
Exposure of the public to atmospheric pollutants is seldom, if ever,
to a single compound. The effects of any pollutant involved in a short-
term episode may be modified by interaction with one or more-ambient
pollutants. The interaction may be physical, as in the case of adsorption
of gases on solid particulates; it may be chemical, as in photochemical
smog; it may be biological, where the toxic effects are modified either
in degree or in nature as in thickening of the alveolar barrier by NOŁ«
It is,therefore, important that information be obtained on the composition
of the ambient air at the anticipated site of the short-term exposure.
IV. Cause and Effect Relationships
As far as is known all living systems have some ability to withstand
injury from toxic materials. This resistance may be due to mechanisms
which prevent absorption of the toxicant, rapidly excrete it, metabolize
and detoxify it, or increase the rate of repair of injured tissues. These
mechanisms are sometimes inherent in the organism and are sometimes
enhanced or acquired in response to toxic stress. Whenever the capacity
of these protective mechanisms is exceeded by the applied toxic stress
the effect will become observable. Beyond this point of no biologically
significant effect from a finite dose, the extent of the effect, i. e. , the
degree of injury, will increase with an increasing dosage of toxicant
with death of the organism as the upper limit. The relationship between
causative dosage and resultant effect is not necessarily a constant
proportionality over the entire range. This lack of proportionality in
dosage-effect relationships makes extrapolations much beyond the range
of available data unreliable.
A. Acquisition of Data
The data necessary to evaluate the relationship between exposure
to a pollutant and its effects on the population at risk are not always
available. For many air pollutants further study will be needed and the
following kinds of information are desired as a minimum. Experimental
data should be derived from studies on at least two susceptible species.
They are presented in relationship to human health concerns, but the
principles involved apply to other concerns.
1. The most sensitive target organ(s) or body system(s) to be
affected by the short-term air contaminant;
* .
2. A full characterization of the nature of the effect upon-the
target(s);
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3. The range of the time-concentration relationship for the
target(s) from no effect to severe effects;
4. The rate of recovery from reversible effects;
5. The nature and severity of injury at which the effect ceases
to be reversible;
6. Identification of cumulative effects, if any;
7. The combined effects, if any, of the toxicant with other air
pollutants and the concentrations at which the combined effects occur;
8. Identification of types of functional abnormalities and pathologi-
cal states among the potentially exposed population which may render
such individuals more susceptible to the pollutant.
B. Interpretation of Data
The interpretation of the information derived from animal
experiments requires mature, experienced, scientific judgment from a
variety of professional disciplines. The evaluation should consider the
conditions under which the data were obtained and, in particular, their
relevance to the conditions of human exposure. How closely do the test
species and the target organtcompare in morphology, sensitivity of
response, and metabolism with that of man? Were the observed animal
responses the consequences of exposure conditions to which the public
may be subjected?
C. Translation of Animal Data to Man and Determination of
Safety Factors
Development of Guides for Short-Term Public Exposures requires
that animal data must be translated as quantitatively as can be estimated
to human response. The response of the "most sensitive1' species
should be used to arrive at the preliminary baseline for ultimate deter-
mination of the appropriate exposure level. The use of data from animal
experiments, tissue culture, and the like may yield values surrounded
by considerable uncertainty. If data from human exposures are available
they may result in levels of considerable reliability. Obviously, reliable
human information is the one of choice and should be obtained and utilized
whenever possible except when the response indicated from animal experi-
ments is life-threatening (e.g. cancer).
There are genetic variations among humans that may have a potential
effect on their susceptibility to air pollutants. Examples are: allergic
asthma, familial pulmonary emphysema and serum antitrypsin deficiency;
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susceptibility to lead poisoning and deficiency of the enzyme glucose-6-
phosphate dehydrogenase in the red blood cells; male susceptibility to
chronic granulomatous disease and leucocyte deficiency of glucose-6-
phosphate dehydrogenase; sickle cell anemia and enhanced risk from
effects of carbon monoxide.
In the absence of dependable values for maximum no-effect doses,
resort must be had to the incorporation of safety factors. These safety
factors should be of a magnitude commensurate Mth (1) the severity of
the response, (2) degree of hypersusceptibility related to (a) preexisting
disease, such as respiratory disease, (b) heredity, (c) nutritional state,
or (d) age, (3) extent of physical exertion, and (4) uniqueness of man's
response, e. g. , hypersensitivity of the respiratory tract.
V. Selection of Standards; Short-Term Public Limits (STPL's) and
Public Emergency Limits (PEL's)
A. Definitions
Short-Term Public Limits are those for predictable exposures
known to be safe for man. They differ from air quality standards only in
that the concentration may be higher since the exposure is shorter and
less frequent.
Public Emergency Limits relate to unpredictable exposures and they
differ from short-term limits in that the levels selected are those known
to cause only minor and fully reversible injury. They are levels such as
those the public would voluntarily tolerate if necessary as a part of civil
defense, or defense of neighborhood property'as in the case of a fire.
B. Balance of Risks
Selecting an upper limit for the concentration of an air pollutant
for a short-term exposure of the public entails a choice of the least risk
to the public health of all the risks to the public associated with the opera-
tion releasing the pollutant. It thus involves a study not only of the effects
such as those which have already been described but also an equally
careful review of the source of the pollutant and the reasons for its
presence.
Studies may be needed in order that the Commissioner can relate
the risk to health of short-term exposures to the alternate risks which
might accrue to the public if the operation were to be abandoned or
conducted at varying degrees of control.
An example is the firing of a rocket motor during research,
development, training, or launch operations by a federal agency such as
8
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A
etfpH
STANDARD TITLE PAGE
FOR TECHNICAL REPORTS
1. Report No.
APTD-0684
3. R
,V
iNMrt'S Catalog NO.
«. Title and Subtitle „
Basis for Establishing Guides for Short-Term Exposure:!1
of the Public to Air Pollutants
,5. Report Date
i May 1971
6, Pttwmmg Organization Code
> 4
Autnone)
8. Performing Organization Rept. No.
9. Performing Orientation Name and Address
The Committee on Toxicology of the National Academy
of Sciences - National Research Council
2101 Constitution AVenue
Washington, D. C. 20418
TI'Sponse7lrTgl\iency NMM and Ad****' '• !" ~~*""
Division of Health Effects Research
Air Pollution Control Office
Environmental Protection Agency
Durham, North Carolina 27701
10. Project/Task/Work Untt No.
IT. don»8Ct7ffrii*He7
CPA 70-57
13. Tyflpe of Raport & Period Covered
. Sponsoring AjgencylpJe '
15. Supplementary Notea
16. Abstracts
:The basic rationale used in the preparation of a series of guides for
short-term exposure to air pollutants are presented. The assumptions
underlying guide preparation are given. The factors and considerations
in guide preparation are outlined. The evaluation of the relationship
between exposure to a pollutant and its effect on the population are dis-
cussed. Also included is a discussion of the selection of short-term
Public Limits and Public Emergency Limits.™—
.M
17. Key Words and Document Analyst*, (a). Descriptors
Air pollution
Contaminants
Toxicology
Exposure
Standards
Public Health
l?b. IdentlfIwa/Open-Endtd Terms
Public Emergency Limits
Short-Term Public Limits
Air Pollution effects (humans)'
17C. COSATI Field/Cm* 13 B , 6 T
18. Distribution Statement
Unlimited
19.Security ClasstThls Report)
UNCLASSIFIED
ZO.Secuflty Class. (This Page)
UNCLASSIFIED
21. No. of Pages
16
22. Price
FORM
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DISCLAIMER
This report was furnished to the Air Pollution
Control Office by
The Committee on Toxicology of the National
Academy of Sciences - National Research Council
2101 Constitution Avenue
Washington, D. C.. 20418
in fulfillment of Contract fta. CPA lOp-57
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the National Aeronautics and Space Administration or the Department of
Defense. When the risks have been evaluated, the Commissioner can
provide the agency with the appropriate options to reduce those risks to
the public to acceptable levels by setting short-term limits foe predictable
exposures.
An example involving the private sector is the testing of a new
firefighting technique or the training of firemen. The extent of public
exposure to smoke, the firefighting agent, and its pyrolysis products
are predictable. Among the risks to be evaluated and balanced would be
the health and other effects which are the subject of these Quides, and the
risk to the public of being deprived of adequate fire protection.
Associated with both of the foregoing examples is another risk
which is the possibility of the accidental release of the rocket fuels or
firefighting agents as the result of a spill. These would be unpredictable
exposures and would be the subject of Public Emergency Limits.
C. Acceptable Risks
The risk to the public health must be appraised from the view of
establishing -what, if any'? is an acceptable level of effect. One of the
important factors to be considered is the duration and frequency of the
exposure. Certainly, any severe or permanent disability cannot be
tolerated. Even a minor effect such as a mildly unpleasant odor or slight
irritation of the eyes and nose becomes sufficiently objectionable, if of
frequent occurrence, to be a consideration in the selection of a Short-
Term Public Limit. There is no justification for subjecting the public
to any appreciable risk in a predictable exposure.
The risks associated with unpredictable exposures require for their
full evaluation a consideration of the probability of an accident. Such
risks can never be reduced absolutely to zero but by proper planning of
operations and equipment design they can be minimized. Exposure to an
accidental release of an air pollutant should be a rare and unique event
in the lifetime of any individual. In some instances the objective may be
sufficiently important to justify accepting some probability of an accident
associated with some degree of risk of reversible effects from exposure
to a pollutant. There should be no acceptance of any possibility of
irreversible injury from accidental exposure to a pollutant.
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