EPA-450/4-92-001
A TIERED MODELING APPROACH FOR
ASSESSING THE RISKS DUE TO
SOURCES OF HAZARDOUS AIR POLLUTANTS
by
David E. Guinnup
U. S. ENVIRONMENTAL PROTECTION AGENCY
Office of Air Quality Planning and Standards
Technical Support Division
Research Triangle Park, NC 27711
March 1992
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TABLE OF CONTENTS
DISCLAIMER ii
FIGURES v
TABLES vi
1.0 INTRODUCTION 1
1.1 Background and Purpose 1
1.2 Risk Assessment in Title III 2
1.3 Overview of Document 5
1.4 General Modeling Requirements, Definitions, and Limitations 6
2.0 TIER 1 ANALYSES 9
2.1 Introduction 9
2.2 Long-term Modeling 9
2.2.1 Maximum Annual Concentration Estimation 9
2.2.2 Cancer risk assessment 12
2.2.3 Chronic Noncancer Risk Assessment 13
2.3 Short-term Modeling 13
2.3.1 Maximum Hourly Concentration Estimation 14
2.3.2 Acute Hazard Index Assessment 15
3.0 TIER 2 ANALYSES 19
3.1 Introduction 19
3.2 Long-term Modeling 19
iii
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3.2.1 Maximum Annual Concentration Estimation 19
3.2.2 Cancer Risk Assessment 21
3.2.3 Chronic Noncancer Risk Assessment ................. 22
3.3 Short-term Modeling . 22
3.3.1 Maximum Hourly Concentration Estimation 22
3.3.2 Acute Hazard Index Assessment 24
4.0 TIER 3 ANALYSES 25
4.1 Introduction 25
4.2 Long-term Modeling 25
4.2.1 Maximum Annual Concentration Estimation 25
4.2.2 Cancer Risk Assessment 28
4.2.3 Chronic Noncancer Risk Assessment 29
4.3 Short-term Modeling 30
4.3.1 Maximum Hourly Concentration Estimation 30
4.3.2 Acute Hazard Index Exceedance Assessment 32
5.0 ADDITIONAL DETAILED ANALYSES 35
6.0 SUMMARY OF DIFFERENCES BETWEEN MODELING TIERS 37
REFERENCES 39
APPENDK A - ELECTRONIC BULLETIN BOARD ACCESS INFORMATION .... 41
APPENDIX B - REGIONAL METEOROLOGISTS/MODELING CONTACTS 43
IV
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FIGURES
Number
1 Schematic of Example Facility with Long-Term Impact
Locations 27
2 Schematic of Example Facility with Short-Term Impact
Locations 33
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TABLES
Number
1 Normalized Maximum Annual Concentrations,
11
2 Normalized Maximum 1-Hour Average Concentrations,
(Hg/m3)/(g/s) 16
3 Differences between Modeling Tiers 37
VI
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1.0 INTRODUCTION
1.1 Background and Purpose
Title IH of the Clean Air Act Amendments of 1990 (CAAA) sets forth a framework for
regulating major sources of hazardous (or toxic) air pollutants which is based on the
implementation of MACT, the maximum achievable control technology, for those sources.
Under this framework, prescribed pollution control technologies are to be installed without the
a priori estimation of the health or environmental risk associated with each individual source.
The regulatory process is to proceed on a source category-by-source category basis, with a list
of source categories to be published by the end of 1991, and a schedule for their regulation to
be published a year later. After the implementation of MACT, it will be incumbent on the
United States Environmental Protection Agency (EPA) to assess the residual health risks to the
population near each source within a regulated source category. The results of this residual risk
assessment will then be used to decide if further reduction in toxic emissions is necessary for
each source category (refer to §112(f) of the CAAA). These decisions will hinge primarily on
a determination of the lifetime cancer risk for the "maximum exposed individual" for each
source as well as the determination of whether the exposed population near each source is
protected from noncancer health effects with an "ample margin of safety". The determination
of lifetime cancer risk involves the estimation of long-term ambient concentrations of toxic
pollutants whereas the determination of noncancer health effects can involve the estimation of
long-term and short-term ambient concentrations.
Since the measurement of long-term and short-term ambient concentrations for each toxic
air pollutant (189 pollutants as listed in §112(b)) in the vicinity of each source is a prohibitively
expensive task, it is envisioned that the process of residual risk determination would involve
performing analytical simulations of toxic air pollutant dispersion for all sources (or a subset of
sources) within each source category. Such simulations will subsequently be coupled with health
effects information and compared to available population data to quantify human exposure,
cancer risk, noncancer health risks, and ecological risks.
In addition to mandating the residual risk assessment process, the CAAA provide for the
exemption of source categories and pollutants from the MACT-based regulatory process if it can
be demonstrated that the risks associated with that source category or pollutant are below
specified levels of concern. EPA-approved risk assessments would need to be performed to
justify such an exemption, and the CAAA provide for petition processes to approve or deny
claims that a source category or a specific pollutant should not be subject to regulation.
The purpose of this document is to provide guidance on the use of EPA-approved
procedures which may be used to assess risks due to the atmospheric dispersion of emissions of
hazardous air pollutants. It is likely that the techniques described herein will be useful with
respect to several decision-making processes associated with the implementation of CAAA Title
III (e.g., petition to add or delete a pollutant from the list of hazardous air pollutants, petition
1
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to delete a source category from the list of source categories, demonstration of source
modification offsets, etc.). In addition, these procedures may serve as the basis for the residual
risk determination process described above. The guidance addresses the estimation of long-term
and short-term ambient concentrations resulting from the atmospheric dispersion of known
emissions of hazardous air pollutants, and subsequently addresses the techniques currently used
to quantify the cancer risks and noncancer risks associated with the predicted ambient
concentrations. It describes a tiered approach which progresses from simple conservative
screening estimates (provided in the form of lookup tables) to more complex modeling
methodologies using computer models and site-specific data. In addition to providing guidance
to assist in the CAAA Title HI implementation process, it is being provided to the general public
to assist State and local air pollution control agencies as well as sources of hazardous air
pollutants in their own assessments of the impacts of these sources.
While the methods described herein comprise the most up-to-date means for assessing
the impacts of sources of toxic air pollution, they are subject to future revision as new scientific
information becomes available, possibly as a result of the risk assessment methodology study
being conducted by the National Academy of Sciences (NAS) under mandate of section 112(o)
of the CAAA (report due to Congress from NAS in May, 1993).
1.2 Risk Assessment in Title III
As mentioned above, several provisions of CAAA Title III describe the need to consider
ambient concentration impacts and their associated health risks in establishing the regulatory
process for sources of toxic air pollutants. Specifically, these are:
1. A pollutant may be deleted via a petition process from the list of hazardous or toxic
pollutants subject to regulation if the petition demonstrates (among other things) that
"ambient concentrations ... of the substance may not reasonably be anticipated to cause
any adverse effects to the human health." (§112(b)(3)(C))
2. A pollutant may be added to the list if a petition demonstrates that "ambient
concentrations ... of the substance are known to cause or may reasonably be anticipated
to cause adverse effects to human health. "(§112(b)(3)(B))
3. An entire source category may be deleted from the list of source categories subject
to regulation if a petition demonstrates, for the case of carcinogenic pollutants, that "no
source in the category ... emits (carcinogenic) air pollutants in quantities which may
cause a lifetime risk of cancer greater than one in one million to the individual in the
population who is most exposed to emissions of such pollutants from the source,"
(§112(c)(9)(B)(i)) and, for the case of noncarcinogenic yet toxic pollutants, that
"emissions from no source in the category ... exceed a level which is adequate to protect
public health with an ample margin of safety and no adverse environmental effect will
result from emissions from any source." (§112(c)(9)(B)(ii))
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4. Within eight years after a source category has been subject to a MACT regulation,
EPA must determine whether additional regulation of that source category is necessary
based on an assessment of the residual risks associated with the sources in that category.
Based on such an assessment, additional regulation of the source category is deemed
necessary if "promulgation of such standards is required in order to provide an ample
margin of safety to protect the public health" with respect to noncancer health effects,
or if the MACT standards "do not reduce lifetime excess cancer risks to the individual
most exposed to emissions from a source in the category or subcategory to less than one
in one million" with respect to carcinogens, or if a determination is made "that a more
stringent standard is necessary to prevent ... an adverse environmental effect."
In the context of these provisions, decisions are to be made based on whether or not the
predicted impact of a source exceeds some level of concern. For comparison to specified levels
of concern, source impacts are quantified in four ways:
1. lifetime cancer risk;
2. chronic noncancer hazard index;
3. acute noncancer hazard index, and;
4. frequency of acute hazard index exceedances.
These impact measures are discussed in more detail in the next few paragraphs. It is
worth noting at this point that insofar as knowledge is available regarding the effects of specific
hazardous pollutants on the environment, it may be possible to use ecological hazard index
values to quantify such impacts. Such calculations may proceed on a track which is parallel to
the calculation of health hazard index values. However, until specific methodologies for
ecological risk assessment are adopted, the techniques identified in this document will remain
limited to the assessment of human health risks due to inhalation of hazardous air pollutants.
F5r carcinogenic pollutants, the level of concern is the risk of an individual contracting
cancer by being exposed to ambient concentrations of that pollutant over the course of a lifetime,
or lifetime cancer risk. For the purposes of §112(c), the criterion specified in the CAAA is 1
in 1,000,000 lifetime cancer risk for the most exposed individual, or the individual exposed to
the highest predicted concentrations of a pollutant. (For other purposes, the lifetime cancer risk
specifying the level of concern may be higher or lower..) Lifetime cancer risks are calculated
by multiplying the predicted annual ambient concentrations (in /tg/m3) of a specific pollutant by
the unit risk factor or unit risk estimate (UREV for that pollutant, where the unit risk factor is
equal to the upper bound lifetime cancer risk associated with inhaling a unit concentration (1
/xg/m3) of that pollutant. Since predicted annual pollutant concentrations around a source vary
as a function of position, so do lifetime cancer risk estimates. Thus, decisions involving whether
the impact of a source or group of sources is above some level of concern typically focus on the
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highest predicted concentration (and hence the highest predicted lifetime cancer risk) outside the
facility fenceline. The EPA has developed unit risk factors for a number of possible, probable,
or known human carcinogens, and will be developing additional cancer unit risk factors as more
information becomes available. For the purposes of this document, cancer risks resulting from
exposure to mixtures of multiple carcinogenic pollutants will be assessed by summing the cancer
risks due to each individual pollutant, regardless of the type of cancer which may be associated
with any particular carcinogen.2
For pollutants causing noncancer health effects from chronic or acute exposure, the levels
of concern are chronic and acute concentration thresholds, respectively, which would be derived
from health effects data, taking into account scientific uncertainties. For purposes of estimating
potential long-term impacts of hazardous air pollutants, EPA has derived for some pollutants
(and will derive for others) chronic inhalation reference concentration (RfCV values, which are
defined as estimates of the lowest concentrations of a single pollutant to which the: human
population can be exposed over a lifetime without appreciable risk of deleterious effects. For
purposes of specific chronic noncancer risk assessment, EPA may designate the RfC value, or
some fraction or multiple thereof, as the appropriate long-term noncancer level of concern. For
purposes of specific acute noncancer risk assessment, the EPA may designate acute reference
thresholds as the appropriate short-term noncancer level of concern. For the purposes of this
document, long-term noncancer levels of concern will be referred to as chronic concentration
thresholdsT and short-term noncancer levels of concern will be referred to as acute concentration
thresholds. For ease of implementation, acute concentration thresholds will be designated for
1-hour averaging times. This does not necessarily mean that exposure data indicate deleterious
health effects from exposure times of 1 hour, but rather that the 1-hour acute concentration
threshold has been derived such that it is protective of the exposure duration of concern.
The risk with respect to long- or short-term deleterious noncancer health effects
associated with exposure to a pollutant or group of pollutants is quantified by the hazard index.
The chronic noncancer hazard index is calculated by dividing the modeled annual concentration
of a pollutant by its chronic concentration threshold value. The acute noncancer hazard index
is calculated by dividing the modeled 1-hour concentration of a pollutant by its acute
concentration threshold value. If multiple pollutants are being evaluated, the (chronic or acute)
hazard index at any location is calculated by dividing each predicted (annual or 1-hour)
concentration at that location by its (chronic or acute) concentration threshold value and
summing the results.2 If the hazard index is greater than 1.0, this represents an exceeclance of
the level of concern at that location. For pollutants which can cause deleterious health effects
from acute exposures, exceedances of a level of concern may occur at any location and at any
time throughout the modeling period. Thus, the frequency with which any location experiences
an exceedance also becomes a measure of the risk associated with a modeled source. Frequency
of acute hazard index exceedances is only addressed by the most refined analysis methods
referred to in this document.
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Information on UREs and RfCs is accessible through the Integrated Risk Information
System (IRIS), EPA Environmental Criteria and Assessment Office (ECAO) in Cincinnati, Ohio,
(513) 569-7254.
1.3 Overview of Document
This document is divided into three major sections, each section addressing a different
level of sophistication in terms of modeling, referred to as "tiers". The first tier is a simplified
screening procedure in which the user can estimate maximum off-site ground-level
concentrations without extensive knowledge regarding the source and without the need of a
computer. The second tier is a more sophisticated screening technique which requires a bit more
detailed knowledge concerning the source being modeled and, in addition, requires the execution
of a computer program. The third tier involves site-specific computer simulations with the aid
of computer programs and detailed source parameters. Since the effects of toxic air pollutants
may be of concern from both a long-term and a short-term perspective, each tier is divided into
two parts. The first part addresses dispersion modeling to assess long-term ambient
concentrations (important from a cancer-causing or chronic noncancer effects standpoint) and the
second addresses dispersion modeling for the estimation of short-term concentrations (important
from an acute toxicity perspective).
It should be noted that this document is intended to be used in conjunction with the
User's Guides for the models described: SCREEN3, TOXST4, and TOXLT5. It is not intended
to replace or reproduce the contents of these documents. In addition, the reader may wish to
consult the "Guideline on Air Quality Models (Revised)"6 for more detailed information on the
consistent application of air quality models. Modelers may also wish to use the EPA's
TSCREEN7 modeling system to assist in the Tier 2 computer simulation of certain toxic release
scenarios. It should be noted, however, that toxic pollutant releases which TSCREEN treats as
heavier-than-air are not to be modeled using techniques described herein. Atmospheric
dispersion of such pollutants requires a more refined analysis, such as those described in
Reference 8. Model codes, user's guides, and associated documentation referred to in this
document can be obtained through the Technology Transfer Network (TTN) of the EPA's Office
of Air Quality Planning and Standards (OAQPS), and access information is provided in
Appendix A.
The modeling tiers are designed such that the concentration estimates from each tier
should be less conservative than the previous one. This means that, for a given situation, a Tier
1 modeled impact should be greater than, or more conservative than, the Tier 2 modeled impact,
and the Tier 2 modeled impact should be more conservative than the Tier 3 modeled impact.
Progression from one tier of modeling to the next thus involves the use of levels of concern, as
defined above. For example, if the results of a Tier 1 analysis indicate an exceedance of a level
of concern with respect to either (1) the maximum predicted cancer risk, (2) the maximum
predicted chronic noncancer hazard index, or (3) the maximum predicted acute hazard index,
the analyst may wish to perform a Tier 2 analysis. If all three of these impact measures are
below their specified levels of concern, there should be no need to perform a more refined
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simulation, and thus, there should be no need to progress to the next tier of modeling. Since
the establishment of levels of concern for each specific hazardous air pollutant is not a. part of
this effort, this document will refer to generic levels of concern, and users will need to consult
subsequent EPA documents to determine the specific levels of concern for their particular
pollutant or pollutant mixture and for the particular purpose of their modeling efforts.
1.4 General Modeling Requirements. Definitions, and Limitations
This document describes modeling methodologies for point, area, and volume sources
of atmospheric pollution. A point source is an emission which emanates from a specific point,
such as a smokestack or vent. An area source is an emission which emanates from a specific,
well-defined surface, such as a lagoon, landfarm, or open-top tank. Sources referred to as
having "fugitive" emissions (e.g., multiple leaks within a specific processing area) are t^ically
modeled as area sources. The methods used in this document are generally considered to be
applicable for assessing impacts of a source from the facility fenceline out to a 50 km radius of
the source or sources to be modeled. There is no particular upper or lower limit on emission
rate values for which these techniques apply.
For the purposes of this document, "source" means the same thing as "release",
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Since many sources of hazardous air pollutants are intermittent in nature (e.g., batch
process emissions), the techniques in this document have been developed to allow the treatment
of intermittent sources as well as continuous types of sources. It is important to understand the
different treatment of emission rates for both types of sources when carrying out either the
analysis of a long-term impact or a short-term impact. In a long-term impact analysis, the
emission rate used for modeling is based on the amount of pollutant emitted over a 1 year
period, regardless of whether the emission process is a continuous or intermittent one. In
addition, to assess the worst-case impact of a source or group of sources, long-term emission
rates used in model simulations should reflect the emission rates for a plant or process which
is operating at full design capacity. In a short-term impact analysis, the emission rate used for
modeling is based on the maximum amount of pollutant emitted over a 1 hour period, during
which the source is emitting. The Tier 1 and Tier 2 procedures evaluate the combined worst-
case impacts of intermittent sources as if they are all emitting at the same time, whereas the Tier
3 procedures incorporate a more realistic treatment of intermittent sources by turning them on
and off throughout the simulation period according to user-specified frequency of occurrence of
each release. This frequency of occurrence should reflect the normal operating schedule of the
source when operating at maximum design capacity.
In addition to emission rate estimates, it is necessary to have quantitative information
about the sources to conduct a detailed impact assessment. Tier 1 analyses require information
about the height of the release above ground level and the shortest distance from the release
point to the facility fenceline. Higher tiers of analysis require additional information including,
but not limited to:
Stack height
Inside stack diameter
Exhaust gas exit velocity
Exhaust gas exit temperature
Dimensions of structures near each source
Dimensions of ground-level area sources
Exact release and fenceline location
Exact location of receptors for determining worst-case impacts
Land use near the modeled facility
Terrain features near the facility
Duration of short-term release
Frequency of short-term release
Where appropriate, this document will address the best means of obtaining these input data. In
some more complex cases, the modeling contact at the nearest EPA Regional Office may need
to be consulted for specific modeling guidance (see listing in Appendix B).
Depending on the specific purpose of the impact assessment, it may be difficult for the
modeler to decide which sources (or release points) and which pollutants should be included in
a particular analysis or simulation. Since these questions pertain to the particular purposes for
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which the impact assessment is being performed, they are not addressed by this document.
Instead, this document refers to and provides guidance for modeling various scenarios including
single-source, multiple-source, single-pollutant, and multiple-pollutant scenarios. Subsequent
EPA documents will address the questions of which sources and which pollutants should be
included in an impact analysis for a specific regulatory purpose.
8
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2.0 TIER 1 ANALYSES
2.1 Introduction
Tier 1 analysis of a stationary source (or group of sources) of toxic pollutant(s) is
performed to address the question of whether or not the source has the potential to cause a
significant impact. This "screening" analysis is performed by using tables of lookup values to
obtain the "worst-case" impact of the source being modeled. The analysis is performed to assess
both the potential long- and short-term impacts of the source. If the predicted screening impacts
are less than the appropriate levels of concern, no further modeling is indicated. If the predicted
screening impacts are above any levels of concern, further analysis of those impacts at a higher
Tier may be desirable to obtain more accurate results.
The Tier 1 "lookup tables" have been created as tools which may be easily used to
estimate conservative impacts of sources of toxic pollutants with a minimal amount of
information concerning those sources. The normalized annual and 1-hour concentration tables
were created based on conservative simulations of toxic pollutant sources with Gaussian plume
dispersion models. In this context, "conservative" simulations use conservative assumptions
regarding meteorology, building downwash, plume rise, etc.
2.2 Long-term Modeling
Long-term modeling of toxic or hazardous air pollutants is aimed at the estimation of
annual average pollutant concentrations to which the public might be exposed as the result of
emissions from a specific source or group of sources. From the EPA regulatory viewpoint, this
"public" does not include employees of the facility responsible for the emissions (this is the
jurisdiction of the Occupational Safety and Health Agency, OSHA). Thus, the impact
assessment focuses on estimating concentrations "off-site", or outside the facility boundary. For
carcinogens, the calculation of cancer risk proceeds by multiplying annual concentrations by
pollutant-specific cancer potency factors derived from health effects data. The impacts of
pollutants with chronic noncancer effects are generally assessed by comparing predicted annual
concentrations with chronic threshold concentrations which are again derived from experimental
health data. For the purposes of protecting the general public against "worst-case" pollutant
concentrations, the analysis is focused on predicting the worst-case, or maximum annual average
concentrations.
2.2.1 Maximum Annual Concentration Estimation
A long-term Tier 1 analysis requires the following information:
1. annual average emission rate of each pollutant from each source included in the
simulation (T/yr). These emissions do not have to be continuously emitted, but rather
should represent the total amount of pollutant which is generated by this source in a year.
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Note that the tons used in this regard are English tons (1 T. = 2000 lb.). Also note that,
for Tier 1 analyses, the emission rate from an area source represents the total emissions
from the area, not the emissions per square unit area.
2. height of the release point above ground (m), for each point source.
3. source types (point or area). Point sources typically include exhaust vents (pipes or
stacks), or any other type of release that causes toxic materials to enter the atmosphere
from a well-defined location, at a well-defined rate. Area sources may also be well-
defined, but differ from point sources in that the extent over which the release occurs is
substantial.
4. maximum horizontal distance across each area source (m).
5. nearest distance to property-line (m). Concentration estimates are needed at locations
that are accessible to the general public. This is typically taken to be any point at or
beyond the property-line of a facility. Estimate the distance from the point of each
release to the nearest point on the fenceline. (This need not be the same fenceline point
for each release.) If the source is characterized as an area source, this distance should
be measured from the nearest edge of the area source, not from the center.
Once these five items are determined for each release (or source), screening estimates
of normalized maximum annual concentrations resulting from each release are obtained from
Table 1 using the following procedure.
1. For an area source, select the "side length" in the table (10m, 20m, 30m) which is
less than or equal to the maximum horizontal distance across the source.
2. For a point source, select the largest "emission height" in the table (Om, 2rn, 5m,
10m, 35m, or 50m) that is less than or equal to the estimated height of release.
3. Select the largest distance in the table (10m, 30m, 50m, 100m, or 200m) that is less
than or equal to the nearest distance to the property-line.
4. Take the appropriate normalized maximum annual concentration for this release
height and distance from the table, and multiply by the emission rate of each toxic
substance (T/yr) in the release to obtain the concentration estimate (jig/m3). DO NOT
INTERPOLATE TABLE VALUES.
For example, consider the situation in which a toxic pollutant A is released at a rate of 11.6 T/yr
from a vent-pipe that is 40m tall, and which is attached to a building that is 4m tall, 10m long,
and 5m wide. The nearest boundary of the facility is located 65m from the pipe. A value of
35m should be selected for the emission height, because all larger entries in the table exceed the
10
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actual height of release of 40m. Concentrations should be estimated for a distance of 50m,
because once again, all greater entries in the table exceed the actual distance of 65m. The
appropriate normalized maximum annual concentration is 1.13 0*g/m3)/(T/yr). Multiplying by
the emission rate of 14.6 T/yr results in a maximum annual concentration estimate for screening
purposes equal to 16.5 /*g/m3.
2.2.2 Cancer risk assessment
Once the maximum annual concentration has been estimated for each release: being
modeled, upper bound lifetime maximum individual cancer risk may be estimated by multiplying
the maximum annual concentration estimates of each carcinogenic pollutant by the unit cancer
risk factor for that pollutant and then summing results. This approach assumes that all cancer
risks are additive, regardless of the organ system which may be affected. It should be noted that
this approach assumes that all worst-case impacts occur at the same location. While this
assumption may not be very realistic, it does help to insure that Tier 1 results are conservative,
and, therefore protective of the public.
As an example of this approach, suppose one is simulating a plant which emits 2
pollutants, A and B, through 4 different stacks such that pollutant A is released from stacks 1
and 2, and pollutant B is released from stacks 2, 3, and 4. In this example, stack 1 is the same
as that described in the example above. After going through the above procedure to estimate
the maximum annual concentrations of each pollutant from each stack, the results are:
Source Compound Max impact
Stack 1 Pollutant A 16.5 jtg/m3
Stack 2 Pollutant A 5.49 jtg/m3
Stack 2 Pollutant B 2.35 /xg/m3
Stack3 Pollutant B 4.13 /ig/m3
Stack 4 Pollutant B 24.9
Suppose that the unit cancer risk factors for pollutants A and B are known to be 1.0 X 10"7 and
2.0 X 10"7 Oig/m3)-1, respectively. The Tier 1 maximum cancer risk is calculated for the
individual releases and pollutants and summed as follows:
Source Compound Max impact Max risk
j
Stack 1 Pollutant A 16.5 /ig/m3 1.65 X 1Q-6
Stack 2 Pollutant A 5.49 /xg/m3 5.49 X 10'7
Stack 2 Pollutant B 2.35 /*g/m3 4.70 X 10'7
Stack3 Pollutants 4.13^g/m3 8.26 X 10'7
Stack 4 Pollutant B 24.9 /tg/m3 4.98 X 10"6
Total risk 8.48 X 10"*
12
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If we are assessing the impact of this group of sources in relation to the CAAA specified
level of concern of 1 X 10* lifetime cancer risk, and since the maximum Tier 1 risk is greater
than the CAAA specified concern level of 1 X 10"*, this source warrants further modeling on
the basis of cancer risk (note that this does not rule out the need to investigate acute or chronic
noncancer risks).
2.2.3 Chronic Noncancer Risk Assessment
For all pollutants which pose a chronic noncancer threat to health, an assessment of the
magnitude of this threat is made using the hazard index approach. The chronic noncancer
hazard index is calculated by summing the maximum annual concentrations for each pollutant
divided by the chronic threshold concentration value for that pollutant. If the calculated hazard
, index is greater than 1 .0, the release or releases being simulated may pose a threat to the public,
and further modeling may be indicated. It should again be noted that, for the sake of erring
conservatively, this approach assumes that the worst-case impacts of all releases occur at the
same location.
As an example of the above procedure, suppose that pollutants A and B in the example
above pose a chronic noncancer health risk, and their respective chronic concentration threshold
values are 20.0 and 5.0 jtg/m3, respectively. The chronic noncancer hazard index would be
formulated as follows:
Source Compound Max impact Hazard index
Stack 1 Pollutant A 16.5 /*g/m3 0.825
Stack 2 Pollutant A 5.49 /xg/m3 0.275
Stack 2 Pollutant B 2.35 jtg/rn3 0.470
Stack3 Pollutant B 4.13 /*g/m3 0.826
Stack 4 Pollutant B 24.9 ^g/m3 4.980
Total hazard
index 7.376
In this case, one of the individual hazard index values exceeds 1.0, the total hazard index
for the modeled facility exceeds 1.0, and further modeling at a higher Tier may be desired.
2.3 Short-term Modeling
Short-term modeling of toxic or hazardous air pollutants is aimed at the estimation of
1-hour average pollutant concentrations to which the public might be exposed as the result of
emissions from a specific source or group of sources. Again, from the EPA regulatory
viewpoint, this "public" does not include employees of the facility responsible for the emissions
(this is the jurisdiction of OSHA). Thus, the impact assessment focuses on estimating
concentrations "off-site", or outside the facility boundary. From the short-term perspective, the
13
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health effects of most concern vary, but they are those which create detrimental health effects
as the result of short-term exposure to toxic pollutants. The risks associated with such exposures
are generally assessed by comparing 1-hour predicted concentrations with acute threshold
concentrations which are derived from experimental health data. For the purposes of protecting
the general public against "worst-case" pollutant concentrations, the analysis is focused on
predicting the worst-case, or maximum 1-hour average concentrations.
2.3.1 Maximum Hourly Concentration Estimation
A short-term Tier 1 analysis requires the following information:
1. maximum 1-hour average emission rate of each pollutant from each source included
in the simulation (g/s). If the release is a continuous, constant-rate emission, then this
value is equivalent to the release rate for long-term modeling, except that it is expressed
in g/s instead of T/yr. (To convert from T/yr to g/s, divide by 34.73; to convert from
g/s to T/yr, multiply by 34.73.) If the release is intermittent, such as a batch process,
this value is equivalent to the maximum number of grams emitted during any hour when
the release is occurring divided by 3600. Again note that, for Tier 1 analyses, the
emissions from an area source represent the total emissions from that source, not just the
emissions per unit surface area.
2. height of each release above ground (m), for point sources.
3. source types (point or area). Point sources typically include exhaust vents (pipes or
stacks), or any other type of release that causes toxic materials to enter the atmosphere
from a well-defined location, at a well-defined rate. Area sources may also be well-
defined, but differ from point sources in that the extent over which the release occurs is
substantial.
4. maximum horizontal distance across each area source (m).
5. nearest distance to property-line (m). Concentration estimates are needed at locations
that are accessible to the general public. This is typically taken to be any point at or
beyond the property-line of a facility. Estimate the distance from the point of each
release to the nearest point on the fenceline. (This need not be the same fenceline point
for each release.) If the source is characterized as an area source, this distance should
be measured from the nearest edge of the area source, rather from than the center of the
area source.
Once these five items are determined for each release, screening estimates of maximum
1-hour average concentrations resulting from each release are obtained from Table 2 using the
following procedure.
14
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1. For area sources, select the "side length" in the table (10m, 20m, 30m) which is less
than or equal to the maximum horizontal distance across the source.
2. For point sources, select the largest "emission height" in the table (Om, 2m, 5m,
10m, 35m, or 50m) that is less than or equal to the estimated height of release.
3. For each source, select the largest distance in the table (10m, 20m, 50m, 100m, or
200m) that is less than or equal to its nearest distance to the property-line.
4. Take the normalized maximum 1-hour average concentration for this release and
fenceline distance, and multiply by the emission rate of each toxic pollutant (g/s) in the
release to obtain the maximum off-site 1-hour average concentration estimates (/tg/m3).
DO NOT INTERPOLATE TABLE VALUES.
For example, again consider the situation in which a toxic material A is released from a vent-
pipe that is 40m tall, and which is attached to a building that is 4m tall, 10m long, and 5m wide.
The nearest boundary of the facility is located 65m from the pipe. For the short-term
assessment, it has been determined that the maximum emissions of A that can occur during any
hour of the year is ISOOg, therefore the emission rate for short-term assessment is 1800g/3600s
= 0.50 g/s. A value of 35m is again selected for the emission height, because all larger entries
in the table exceed the actual height of release. Concentrations are estimated for a distance of
50m, because once again, all greater entries in the table exceed the actual distance of 65m. The
appropriate normalized maximum 1-hour average concentration is 3.94E+2 (^g/m3)/(g/s).
Multiplying by the emission rate of 0.50 g/s results in a maximum hourly concentration estimate
for screening purposes equal to 197 /zg/m3.
2.3.2 Acute Hazard Index Assessment
For all pollutants which pose a threat to health based on acute exposure, an assessment
of the magnitude of this threat is made using the acute hazard index approach, similar to that
used in .chronic noncancer risk assessment. In this case, however, the acute hazard index is
calculated by summing the maximum 1-hour concentrations for each pollutant divided by the
acute concentration threshold value for that pollutant. It should again be noted that, for the sake
of erring conservatively, this approach assumes that the worst-case impacts of all releases can
occur simultaneously at the same location. Similar to the chronic risk assessment, if the
calculated hazard index is greater than 1.0, the release or releases being simulated may pose a
significant threat to the public, and further modeling at a higher Tier may be indicated.
15
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As an example of the acute hazard index approach, consider the same plant being
simulated in Section 2.2.2, but this time the maximum 1-hour concentrations are determined
using the procedure in Section 2.3.2 to be the following:
Source Compound Max 1-hr impact
Stack 1 Pollutant A 197 jtg/m3
Stack 2 Pollutant A 257 /tg/m3
Stack 2 Pollutant B 110/ig/m3
Stack 3 Pollutant B 301 jtg/m3
Stack 4 Pollutant B 367
Further suppose that pollutants A and B pose health problems from acute exposures with acute
threshold concentration values of 200 and 100 ^g/ro3, respectively. The acute hazard index is
calculated as follows:
Source Compound Max 1-hr impact Hazard index
Stack 1 Pollutant A 197 jig/m3 0.985
Stack 2 Pollutant A 257 jig/m3 1.285
Stack 2 Pollutant B 110/ig/m3 1.100
Stack 3 Pollutant B 301 /zg/m3 3.010
Stack 4 Pollutant B 367 /tg/m3 3.670
Total hazard index 10.050
In this case, 4 of the individual hazard index values exceeds 1.0, the total hazard index
for the modeled plant exceeds 1.0, and further modeling at a higher Tier may be desired.
17
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18
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3.0 TIER 2 ANALYSES
3.1 Introduction
Tier 2 analysis of a stationary source (or group of sources) of toxic pollutant(s) may be
desired if the results of a Tier 1 analysis indicate an exceedance of a level of concern with
respect to one or more of the following: (1) the maximum predicted cancer risk; (2) the
maximum predicted chronic noncancer hazard index, or; (3) the maximum predicted acute
hazard index. Note that in situations where only one or two of the Tier 1 criteria are exceeded,
only those analyses which exceed the Tier 1 criteria may need to be performed at the higher
Tier. For example, if the Tier 1 analysis showed cancer risk and chronic noncancer risks to be
of concern while the acute risk analysis showed no cause for concern, only long-term modeling
for cancer risk and chronic noncancer risk may need to be performed at Tier 2. Tier 2 analyses
are slightly more sophisticated than Tier 1 analyses, and therefore require additional input
information as well as a computer for their execution. Tier 2 analyses are structured around the
EPA's SCREEN model and its corresponding documentation3. The SCREEN model source code
and documentation is available through the OAQPS TTN (see Appendix A).
Again, similar to the Tier 1 analysis, if any of the predicted impacts from Tier 2 are
above the appropriate levels of concern, further modeling is indicated at a higher Tier.
3.2 Long-term Modeling
Long-term Tier 2 modeling utilizes the SCREEN3 model to estimate 1-hour maximum
concentrations, and then utilizes a conservative conversion factor to derive maximum annual
concentration values from the SCREEN predictions16-17. These maximum annual concentration
estimates are used to assess cancer risk and chronic noncancer risk exactly as in Sections 2.2.2
and 2.2.3 of this document.
3.2.1 Maximum Annual Concentration Estimation
In addition to the information required to perform a Tier 1 long-term analysis, a Tier 2
analysis requires the following information:
1. the inside diameter of the stack at the exit point (m).
2. the stack gas exit velocity (m/s)
3. the stack gas exit temperature (K)
4. a determination of whether the area surrounding the modeled facility is urban or
rural. This is usually assessed on the basis of land use in the vicinity of the facility.
19
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Refer to the "Guideline on Air Quality Models .(Revised)"6 for additional guidance on this
determination.
5. downwash potential. Downwash effects must be included in dispersion estimates for
point (stack) sources whenever the point of release is located on the roof of a building
or structure, or within the lee of a nearby structure. The potential for downwash is
determined in the following way. First, estimate the heights and maximum horizontal
dimensions* of the structures nearest the point of release. For each structure, determine
which of these two dimensions is less, and call this length L. If the structure is less than
5L away from the source, then this structure may cause downwash. For every structure
satisfying this criterion, calculate a height by multiplying L by 1.5, and adding this to
the actual height of the structure. If any calculated height exceeds the height of the
release, then downwash calculations must be made for that release.
Once these items are determined for each release being modeled, estimates of maximum
concentrations from each release are obtained through individual SCREEN runs for each release.
Recommendations for each SCREEN run are as follows:
1. The emission rates used for Tier 1 long-term modeling should be converted from T/yr
to g/s (divide T/yr by 34.73). Area source emission rates should be converted to g/s/m2
by dividing by the total area of the source.
2. Choose the default atmospheric temperature of 293K.
3. For each release, exercise the automated distance array choosing as the minimum
receptor distance the appropriate nearest fenceline distance for that release, and choosing
50 km as the maximum receptor distance. The maximum concentration for that: release
will then be chosen as the maximum at or beyond the nearest fenceline distance.
4. The option for flagpole receptors should not be used.
5. For each release, the maximum 1-hour concentration should be noted.
6. Maximum annual concentrations should be calculated for each release by multiplying
predicted maximum 1-hour concentrations by 0.08.
As an example of the Tier 2 long-term analysis, consider Stack 1 from the Tier 1
example. To consider downwash possibilities, the maximum horizontal dimension is first
estimated as {(10m)2 + (5m)2}m = 11.2m. The dimension L is then 4m, and the maximum
stack height for which downwash is possible would be 4m + 1.5 X 4m = 10m. Since the
* Note: The maximum horizontal dimension is defined as the largest possible alongwind
distance the structure could occupy.
20
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actual stack height is 40m, downwash need not be considered in the SCREEN simulation. The
emission rate specified in the example of 14.6 T/yr is converted to g/s to be used in the
SCREEN simulation, resulting in an emission rate of 14.6/34.73 = 0.42 g/s. In addition to the
actual stack height (40m) and minimum fenceline distance (65m), input parameters for the
SCREEN simulation are:
Inside stack diameter 0.5m
Stack gas exit velocity 5.6 m/s
Stack gas exit temperature 303 K
Plant location urban
The results from the SCREEN simulation indicate that the maximum 1-hour concentration at or
beyond 65m is 32.5 fj.g/m*, occurring 165m downwind. Using the recommended conversion
factor of 0.08, the maximum annual concentration is estimated as 2.6 /xg/m3 (this value can be
contrasted with the Tier 1 estimation of 16.5 /tg/m3).
3.2.2 Cancer Risk Assessment
Maximum annual concentrations for all releases of carcinogens should be multiplied by
the appropriate unit cancer risk factor and summed to estimate the maximum cancer risk. It
should be noted that this approach, as in Tier 1, presumes that all worst-case impacts occur at
the same location. While this assumption may not be very realistic, it does help to insure that
the results of a Tier 2 analysis are conservative and therefore protective of the public. More
receptor-specific risk calculations are addressed in the Tier 3 analyses.
Borrowing again from the Tier 1 example, maximum annual impacts for each source and
pollutant combination are estimated using the SCREEN model. Risk estimates are then made
by summing the risk due to each release, regardless of downwind distance to maximum impact.
The results are:
Source Compound Max impact Max risk
Stack 1 Pollutant A 2.60 /xg/m3 2.60 X 10"7
Stack 2 Pollutant A 1.34 /xg/m3 1.34 X 10'7
Stack 2 Pollutant B 0.58 /xg/m3 1.16X10'7
Stack3 PollutantB 0.62 /xg/m3 1.24 X 10'7
Stack 4 Pollutant B 3.70 /xg/m3 7.40 X 10'7
Total risk 1.38 X
21
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For this example, the maximum lifetime cancer risk estimated using the Tier 2 methods
is a factor of 6 lower than that estimated in the Tier 1 analysis. However, the cancer risk level
still exceeds 1 X 10"*, indicating that modeling at a higher Tier may be desirable.
3.2.3 Chronic Noncancer Risk Assessment
As in Tier 1, maximum annual concentrations are divided by their chronic concentration
threshold values and summed to calculate the hazard index values. Again, this approach
conservatively assumes that all worst-case impacts occur at the same location.
Continuing with the example, the chronic noncancer hazard index is recalculated using
the Tier 2 estimated long-term impacts. Threshold concentration values for chronic noncancer
effects again are taken as 20.0 and 5.0 /ig/m3 for pollutants A and B, respectively. The
following results:
Source Compound Max impact Hazard index
Stack 1 Pollutant A 2.60 /ig/m3 0.130
Stack 2 Pollutant A 1.34 jtg/m3 0.067
Stack 2 Pollutant B 0.58/tg/m3 0.116
Stack3 Pollutant B 0.62 jtg/m3 0.124
Stack 4 Pollutant B 3.70/tg/m3 0.740
Total hazard
index 1.177
The chronic noncancer hazard index estimated in Tier 2 is a good deal less than that
estimated for the same sources in Tier 1. Even though none of the individual source/pollutant
combinations exceeds a chronic threshold concentration value, the total hazard index exceeds
1.0, and further analysis at Tier 3 is indicated for chronic noncancer effects.
3.3 Short-term Modeling
Short-term Tier 2 modeling utilizes the SCREEN3 model to estimate 1-hour maximum
concentrations directly. These maximum 1-hour concentration estimates are used to assess acute
hazard index values exactly as in Section 2.3.2 of this document.
3.3.1 Maximum Hourly Concentration Estimation
In addition to the information required to perform a Tier 1 short-term analysis, a Tier
2 analysis requires the following information for stack sources:
1. the inside diameter of the stack at the exit point (m).
22
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2. the stack gas exit velocity (m/s)
3. the stack gas exit temperature (K)
4. a determination of whether the area surrounding the modeled facility is urban or
rural. This is usually assessed on the basis of land use in the vicinity of the facility.
Refer to the "Guideline on Air Quality Models (Revised)"6 for additional guidance on this
determination.
5. downwash potential. Downwash effects must be included in dispersion estimates for
point sources whenever the point of release is located on the roof of a building or
structure, or within the lee of a nearby structure. The potential for downwash is
determined in the following way. First, estimate the heights and maximum horizontal
dimensions of the structures nearest the point of release. For each structure, determine
which dimension is less, and call this length L. If the structure is less than 5L away
from the source, then this structure may cause downwash. For every structure satisfying
this criterion, calculate a height by multiplying L by 1.5, and adding this to the actual
height of the structure. If any calculated height exceeds the height of the release, then
downwash calculations must be used for that release.
Once these items are determined for each release being modeled, estimates of maximum
concentrations from each release are obtained through individual SCREEN runs for each release.
Recommendations for each SCREEN run are as follows:
1. Choose the default atmospheric temperature of 293K.
2. Area source emission rates reflect the total emission rate from divided by the area of
the source.
3. For each release, exercise the automated distance array choosing as the minimum
receptor distance the appropriate nearest fenceline distance for that release, and choosing
50 km as the maximum receptor distance. The maximum concentration for that release
will then be chosen as the maximum at or beyond the nearest fenceline distance.
4. The option for flagpole receptors should not be used.
5. For each release, the maximum 1-hour concentration should be noted.
Using this approach with the Stack 1 example, the SCREEN model is exercised with the
stack parameters specified in Section 3.2.1. The maximum short-term emission rate of 0.50 g/s
(see Section 2.3.1), however, is used to estimate the maximum 1-hour source impact. The
results of the SCREEN model indicate that the maximum 1-hour concentration is 38.8
again occurring 165m downwind.
23
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3.3.2 Acute Hazard Index Assessment
As in Tier 1, maximum 1-hour concentrations are divided by their acute threshold
concentration values and summed to calculate the acute hazard index values. Again, this
approach conservatively assumes that all worst-case impacts can occur simultaneously at the
same location.
To illustrate this procedure, short-term impacts from the example plant are assessed using
the hazard index approach. Again the acute threshold concentration values are taken as 200 and
100 /ig/m3, respectively. The results are:
Source Compound Max 1-hr impact Hazard index
Stack 1 Pollutant A 34.8 jig/m3 0.174
Stack 2 Pollutant A 70.5 ^g/m3 0.352
Stack 2 Pollutant B 29.9 jtg/m3 0.299
Stack3 Pollutants 50.0/xg/m3 0.500
Stack 4 Pollutants 60.4/xg/m3 0.604
Total hazard index 1.925
For this example, the acute hazard index estimated in Tier 2 is roughly 20% of that
estimated for the same sources in Tier 1. However, since the total hazard index exceeds 1.0,
further analysis at Tier 3 is indicated for health effects resulting from acute exposures.
24
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4.0 TIER 3 ANALYSES o
k
J.' Introduction st
a
, Tier 3 analysis of a stationary source (or group of sources) of toxic pollutant(s) may be st
esired if the results of a Tier 2 analysis indicate an exceedance of a level of concern with
Aspect to one or more of the following: (1) the maximum predicted cancer risk; (2) the
laximum predicted chronic noncancer hazard index, or; (3) the maximum predicted acute >e
azard index. Tier 3 analysis of a stationary source (or group of sources) of toxic pollutant(s)
performed to provide the most scientifically-refined indication of the impact of that source.
his Tier involves the utilization of site-specific source and plant layouts as well as ie
eteorological information. In contrast to the previous Tiers, Tier 3 allows for a more realistic h
mulation of intermittent sources and combined source impacts. In addition, results from short-
rm analyses indicate not only if a risk level of concern can be exceeded, but how often that
vel of concern might be exceeded during an average year. Dispersion modeling for the Tier 2-
analysis procedure is based on use of the EPA's Industrial Source Complex (ISC2) model18, - is
id as such utilizes many of the same techniques recommended in the "Guideline on Air Quality ta
todels (Revised)"6 approach to the dispersion modeling of criteria pollutants. is
cs
To facilitate the dispersion modeling of toxic air pollutants, the EPA has developed s)
OXLT (TOXic modeling system Long-Term)5 for refined long-term analyses, and TOXST s.
^OXic modeling system Short-Term)4 for refined short-term analyses. The TOXLT system
corporates the ISCLT2 (long-term) directly to calculate annual concentrations and the TOXST
'Stem incorporates the ISCST2 (short-term) model directly to calculate hourly concentrations.
odes and user's guides for both TOXLT and TOXST are available via electronic bulletin board
ee Appendix A).
2 Long-term Modeling
Long-term Tier 3 modeling uses the TOXLT5 modeling system to estimate maximum
nual concentrations and maximum cancer risks. The TOXLT modeling system uses the
CLT2 model to calculate these annual concentrations at receptor sites which are specified by
^ user. A post-processor called RISK subsequently calculates lifetime cancer risks and chronic
mcancer hazard index values at each receptor.
2.1 Maximum Annual Concentration Estimation
In addition to the information required to perform a Tier 2 long-term analysis, the
er 3 long-term analysis requires the following information:
1. five years of meteorological data from the nearest National Weather Service (NWS)
station. These data are for the most recent, readily-available consecutive five year period.
NWS data are available through the electronic bulletin board (see Appendix A).
Alternatively, one or more years of meteorological data from on-site measurements may
25
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The results of the dispersion modeling indicated the following maximum annual off-site
concentrations for each of the source/pollutant combinations:
Source Compound Max impact Location
Stack 1 Pollutant A .788 /*g/m3 X
Stack 2 Pollutant A .305 ^g/m3 Y
Stack 2 Pollutant B .131/tg/m3 Y
Stack3 PollutantB .172/zg/m3 Z
Stack 4 Pollutant B .976 /tg/m3 Z
It should be noted that the maximum concentrations from each source/receptor
combination were not co-located. The positions of the maximum concentration from each source
are indicated on Figure 1 corresponding to the letters X, Y, and Z in the table above. In
general, the Tier 3 maximum concentration values are 25 to 30% as high as the Tier 2 values.
4.2.2 Cancer Risk Assessment
Concentrations from the ISCLT2 master file inventory are used by the RISK post-
processor to calculate cancer risks at each receptor site in the ISCLT2 receptor array. RISK can
then provide summaries of the calculated risks according to user specifications. Use of the RISK
post-processor requires the following considerations:
1. As stated above, emission rate multipliers for each pollutant from each source should
be provided as inputs to the RISK post-processor such that the product of the base
emission rate input to ISCLT2 and the emission rate multiplier input to RISK equals the
emission rate being modeled.
2. Unit cancer risk factors are provided to RISK either in the RISK post-processor input
file or through an interactive process in TOXLT.
3. The RISK post-processor output options should be exercised to provide the total
cancer risk at each receptor due to all pollutants, as well as individual pollutant or source
contributions to these receptor-specific risks.
If the maximum predicted lifetime cancer risk in the receptor grid is less than the
designated level of concern (e.g., 1 X 10"6), placement of additional receptors in the ISCLT2
receptor array should be considered as a means of ensuring that the simulation is not
underestimating maximum risk. If the maximum cancer risk in the receptor array is greater than
the designated level of concern, additional runs of the RISK post-processor may be performed
using reduced emission rate multipliers to assess the impacts of possible emission control
scenarios. If the analysis shows no cancer risk greater than the designated level of concern and
the receptor array is deemed adequate, the modeled source is considered to be in compliance
28
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with the specified criterion. In the case of non-compliance, it may be desirable on the part of
the modeler to conduct a more refined analysis. Section 5.0 if this document discusses some
of the possibilities for further modeling refinements.
The output of the RISK post-processor for the example plant indicates that the maximum
lifetime cancer risk outside the plant boundary is 4.2 X 10'7, located at point W on Figure 1.
Such a result would indicate that the facility would not cause a significant cancer risk to the
public, according to the cancer risk level specified by the CAAA of 1990.
4.2.3 Chronic Noncancer Risk Assessment
In this assessment, concentrations from the ISCLT2 master file inventory are used by the
RISK post-processor to calculate chronic noncancer hazard index values for a specific noncancer
effect at each receptor site in the ISCLT2 receptor array. RISK can then provide summaries of
the calculated index values according to user specifications. A separate RISK simulation should
be performed for each chronic noncancer effect being considered. Use of the RISK post-
processor requires the following considerations:
1. As stated above, emission rate multipliers for each pollutant from each source should
be provided as inputs to the RISK post-processor such that the product of the emission
rate input to ISCLT2 and the emission rate multiplier input to RISK equals the actual
emission rate being modeled.
2. Chronic threshold concentration values for the specific noncancer effect are provided
to RISK either in the RISK post-processor input file or through an interactive process in
TOXLT.
3. The RISK post-processor output options should be exercised to provide the total
noncancer hazard index at each receptor due to all pollutants, as well as individual
pollutant or source contributions to these receptor-specific hazard indices.
If the maximum hazard index value in the receptor grid exceeds 1.0, emission reduction
scenarios can be performed (again, using reduced emission rate multipliers) to determine how
this hazard index value can be reduced below 1.0. If the maximum hazard index value in the
receptor grid does not exceed 1.0, the source(s) being modeled is considered to be in compliance
with the specified criteria. In the case of non-compliance, it may be desirable on the part of the
modeler to conduct a more refined analysis. Section 5.0 if this document discusses such
possibilities.
Using the chronic noncancer threshold concentration values for pollutants A and B of
20.0 and 5.0 /tg/m3, respectively, the RISK post-processor was exercised for the example facility
to obtain a maximum hazard index value of 0.27 located at point Z on Figure 1. This result,
which is approximately 30% of the Tier 2 result, would indicate that the facility does not present
a significant chronic noncancer risk in its current configuration.
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4.3 Short-term Modeling
Short-term Tier 3 modeling uses the TOXST modeling system4 to estimate maiximum
hourly concentrations and the receptor-specific expected annual number of exceedances of short-
term concentration thresholds. For multiple pollutant scenarios, this amounts to the number of
times the acute hazard index value exceeds 1.0. The model uses the ISCST2 model to calculate
these hourly concentrations at receptor sites which are specified by the user. Acute hazard index
values are subsequently calculated at each receptor by the TOXX post-processor, in which a
Monte Carlo simulation is performed for intermittent sources to assess the average number of
times per year the acute hazard index value exceeds 1.0 at each receptor.
4.3.1 Maximum Hourly Concentration Estimation
In addition to the information required to perform a Tier 2 analysis, the Tier 3 short-term
analysis requires the following information:
1. five years of meteorological data from the nearest National Weather Service (NWS)
station. These data are for the most recent, readily-available consecutive five year period.
NWS data are available through the electronic bulletin board (see Appendix A).
Alternatively, one or more years of meteorological data from on-site measurements may
be substituted. These data should be obtained and quality-assured using procedures
consistent with the "Guideline on Air Quality Modeling (Revised)"6.
2. plant layout information, including all emission point and fenceline locations;. This
. information should be sufficiently detailed to allow the modeler to specify emission point
and fenceline receptor locations within 2 meters of their actual locations.
3. pollutant-specific data concerning deposition or reactivity, if applicable.
4. source-specific data concerning the annual average number of releases and their
duration for all randomly-scheduled intermittent releases.
Once these data have been obtained, an input file should be prepared for execution of the
ISCST2 model using the guidance available in the ISC2 User's Guide18. The ISCST2 model
should then be executed using the TOXST system. Procedures utilized should also be consistent
with the TOXST User's Guide5 (available through the electronic bulletin board, see Appendix
A). Specific recommendations concerning the development of these inputs include:
1. Maximum hourly emission rates are used for the analysis. The TOXST modeling
system uses "base emission rates" and "emission rate multipliers" to specify the emission
rate for each pollutant/source combination. Thus, for a given pollutant and source the
emission rate equals the base emission rate (specified in the ISCST2 input file) times the
emission rate multiplier for that pollutant/source combination (specified in the TOXX
input file). The input file to the ISCST2 program should contain the same emission rates
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used in previous modeling tiers for each source, and the input file to the TOXX post-
processor should be provided unit emission rate multipliers (1.0). If more than one
pollutant is being emitted from the same source, that source may be included once in the
ISCST2 input file with a unit emission rate (1.0) and the individual pollutant emission
rates may be provided to the TOXX post-processor. (It should be noted that this may
complicate the interpretation of the printed ISCST2 output. Alternatively, multiple
pollutants from the same source may be modeled as individual sources with actual
emission rates in ISCST2 and unit emission rates in TOXX. This may require more
computing time, but may allow direct interpretation of concentration predictions in the
ISCST2 printed output. Regardless of which method is used, the modeler should take
care that the product of the emission rate used in ISCST2 and the emission rate used in
TOXX equals the emission rate of the pollutant and source being modeled.)
2. All continuous sources of the same pollutant should be modeled as one ISCST2
source group. Each intermittent source operating independently from one another should
be modeled as a separate ISCST2 source group. All intermittent sources of the same
pollutant emitting at the same time may be modeled in the same ISCST2 source group.
However, each source of more than one pollutant should be modeled as a source group
by itself.
3. Input parameters in the ISCST2 input file should be set in accordance with the
TOXST User's Guide. The regulatory default mode should be used. The ISCST2 output
options should be chosen to provide summary results of the top 50 impacted receptors
for each source group. (As noted earlier, if unit emission rates are being used in
ISCST2, interpretation of the concentration impacts as absolute may be inappropriate.)
4. Meteorological input files for ISCST2 may be created from NWS meteorological data
using the RAMMET program (this program and a description of its use are available on
the electronic bulletin board, see Appendix A).
5. A polar or rectangular receptor grid may be used, but with sufficient detail to
accurately estimate the highest concentrations from each source. The design of the
receptor network should consider the short-term results of the earlier modeling tiers such
that the highest resolution of receptors is in the vicinity of the highest predicted impacts.
Additional receptors may need to be added in sufficient detail to accurately resolve the
highest concentrations.
6. Where appropriate, direction-specific building downwash dimensions should be
included for each radial direction.
7. The ISCST2 model option to create a TOXFILE output for post-processing should
be chosen. The concentration threshold value (called "pcutoff') used to reduce the size
of this binary concentration output file should be chosen appropriately to eliminate
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predicted concentration values below possible concern. Although it may be set higher,
a good rule of thumb for setting this value is:
IACT
where LACT is the lowest acute concentration threshold value in the group of pollutants
being modeled, and Npolj is the number of pollutants emitted from ISCST2 source; group
i.
The printed ISCST2 output will indicate the top 50 impacts for each ISCST2 source
group, and the TOXFILE will contain all of the concentrations above the cutoff value from each
ISCST2 source group at each receptor.
The ISCST2 model was exercised for the example facility. The maximum 1-hour
concentrations for each source/pollutant combination were determined to be as follows:
Source Compound Max impact Location
Stack 1 Pollutant A 34.5 pg/m3 Q
Stack 2 Pollutant A 67.9 jxg/m3 R
Stack 2 Pollutants 29.1/ig/m3 R
Stack 3 Pollutant B 39.2 /zg/m3 S
Stack 4 Pollutant B 47.5 jtg/m3 S
The locations of the predicted maximum 1-hour concentrations are shown in Figure 2. The
maximum impacts from each source were only slightly lower than those from the Tier 2
analysis.
4.3.2 Acute Hazard Index Exceedance Assessment
Concentrations from the ISCST2 master file inventory are used by the TOXX post-
processor to calculate acute hazard index values for each hour of a multiple-year simulation
period at each receptor site in the ISCST2 receptor array. The program then counts the number
of times a hazard index value exceeds 1.0 (an exceedance) and prints out a summary report
which indicates the average number of times per year an exceedance occurs at each receptor.
The use of the TOXX post-processor requires the following considerations:
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Plant Boundary
Figure 2. Schematic of Example Facility with Short-Term Impact Locations
1. As stated above, in most cases unit emission rate multipliers for each pollutant from
each source are used as inputs to the TOXX post-processor.
2. Acute threshold concentration values are provided to TOXX as the health effects
thresholds in the TOXX post-processor input file.
3. The TOXX output option should be chosen to output the exceedances in polar grid
format. Exceedance counts at discrete fenceline receptors will appear at the end of this
table in the order in which discrete receptor locations were input to ISCST2.
4. If only one pollutant is being modeled, the additive exceedance calculation option
should not be chosen. If multiple pollutants are being modeled, the additive exceedance
calculation option should be chosen. The TOXX post-processor should be set to perform
400 or more simulation years (maximum 1000). Unless otherwise specified by EPA
guidance, background concentrations for toxic air pollutants should be set equal to 0.
5. The frequency of operation for each emission source is specified by providing values
for the probability of the source switching on and the duration of the release. For each
continuous emission, the probability of the source switching on is 1.0, and for each
intermittent emission source, the probability of the source switching on is equal to the
average number of releases per year divided by 8760 (the number of hours in a non-leap
year). The duration of release for each continuous source should be set equal to 1.0, and
the duration of release for each intermittent release should be specified as the nearest
integer hour which is not less than the release duration. (For example, if the average
release duration is less than 1 hour, the duration of release should be set equal to 1; if
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the average release duration is 3.2 hours, the duration of release should be set equal to
4.0)
If the maximum number of acute hazard index exceedances in the receptor grid is less
than some specified value (e.g., 0.1, equivalent to an average of 1 hourly exceedance every 10
years), the modeled source is considered to be in compliance with the acute threshold
concentration criteria. However, resimulation with placement of additional receptors in the
ISCST2 receptor array should be considered as a means of assuring that the simulation is not
underestimating the maximum acute hazard index. If the maximum number of hazard index
exceedances in the receptor array is greater than the specified value, additional runs of the
TOXX post-processor with reduced emission rate multipliers may be performed to assess the
impacts of possible emission control scenarios. In the case of non-compliance, it may be
desirable on the part of the modeler to conduct a more refined analysis. Section 5.0 of this
document discusses such possibilities.
The TOXX post-processor was exercised for the example facility using the results from
the ISCST2 simulation. The frequency of operation for each source ranged from 0.14 to 0.84,
reflecting the actual yearly frequency of "on" time for each source. The output showfjd that
none of the receptors experienced an impact resulting in a hazard index value of 1.0 or greater.
Comparing this result with the Tier 2 result indicates that the hazard index never exceeds 1.0
because in a Tier 3 analysis the maximum impacts are seen not to occur at the same place and
time. This indicates that the facility does not cause a significant health risk from acute
exposures in its current configuration.
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5.0 ADDITIONAL DETAILED ANALYSES
If any Tier 3 analyses indicate non-compliance with any of the user-specified criteria, it
may be desirable to conduct an additional, more refined analysis. This may mean the use of on-
site meteorological data or it may mean that a more appropriate modeling procedure is deemed
applicable for the specific case. The determination of an appropriate alternative modeling
procedure can only be made in a manner consistent with the approach outlined in the "Guideline
on Air Quality Models (Revised)"6.
In some cases, the EPA may allow exposure assessments to incorporate available
information on actual locations of residences, potential residences, businesses, or population
centers for the purpose of establishing the probability of human exposure to the predicted levels
of toxic pollution near the source being modeled. In such cases, use of the Human Exposure
Model (HEM II)19 with the ISCLT dispersion model is preferred. Again, if the use of other
modeling procedures is desired, the approval of a more appropriate alternative modeling
procedure can only be made in a manner consistent with the approach outlined in Section 3.2
of the "Guideline on Air Quality Models (Revised)"6.
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6.0 SUMMARY OF DIFFERENCES BETWEEN MODELING TIERS
To summarize the major differences between the 3 modeling tiers described in this
document, Table 3 below briefly lists the input requirements, output parameters, and
assumptions associated with each tier. This Table may be used to quickly determine whether
a given scenario may be modeled at any particular tier. Within each tier, cancer unit risk
estimates, chronic noncancer concentration thresholds, and acute concentration thresholds are
required to convert concentration predictions into cancer risks, chronic noncancer risks, and
acute noncancer risks, respectively.
Modeling Tier
Input Requirements
Output Parameters
Major Assumptions
Tier 1
emission rate, stack
height, minimum distance
to fenceline
maximum off-site
concentrations, worst-case
cancer risk or worst-case
noncancer hazard index
(short- and long-term)
Worst-case meteorology,
worst-case downwash,
worst-case stack
parameters, short-term
releases occur
simultaneously,
maximum impacts co-
located, cancer risks
additive, noncancer risks
additive
Tier 2
emission rate, stack
height, minimum distance
to fenceline, stack
velocity, stack
temperature, stack
diameter, rural/urban site
classification, building
dimensions for downwash
calculation
maximum offsite
concentrations, worst-case
cancer risk and/or worst-
case noncancer hazard
index (short- and long-
term)
Worst-case meteorology,
short-term releases occur
simultaneously,
maximum impacts cal-
located, cancer risks
additive, noncancer risks
additive
Tier 3
emission rate, stack
height, actual fenceline
and release point
locations, stack velocity,
stack temperature, stack
diameter, rural/urban site
classification, local
meteorological data,
receptor locations for
concentration predictions,
frequency and duration of
short-term (intermittent)
releases
concentrations at each
receptor point, long-term
cancer risk estimates,
chronic noncancer hazard
index estimates at each
receptor point, annual
hazard index exceedance
rate at each receptor
cancer risks additive,
noncancer risks additive
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REFERENCES
1. Environmental Protection Agency, 1988. Glossary of Terms Related to Health, Exposure,
and Risk Assessment. EPA-450/3-88-016. United States Environmental Protection Agency,
Research Triangle Park, NC 27711.
2. Environmental Protection Agency, 1987. The Risk Assessment Guidelines of 1986. EPA-
600/8-87-045. United States Environmental Protection Agency, Washington, DC 20460.
3. Environmental Protection Agency, 1992. The SCREEN Model User's Guide. EPA-450/4-
92-006. United States Environmental Protection Agency, Research Triangle Park, NC 27711
in preparation).
4. Environmental Protection Agency, 1992. Toxic Modeling System Short-Term (TOXST)
User's Guide. EPA-450/4-92-002. United States Environmental Protection Agency, Research
Triangle Park, NC 27711 (in preparation).
5. Environmental Protection Agency, 1992. Toxic Modeling System Long-Term (TOXLT)
User's Guide. EPA-450/4-92-003. United States Environmental Protection Agency, Research
Triangle Park, NC 27711 (in preparation).
6. Environmental Protection Agency, 1988. Guideline on Air Quality Models (Revised). EPA-
450/2-78-027R. United States Environmental Protection Agency, Research Triangle Park, NC
27711.
7. Environmental Protection Agency, 1990. User's Guide to TSCREEN: A Model for
Screening Toxic Air Pollutant Concentrations. EPA-450/4-90-013. United States Environmental
Protection Agency, Research Triangle Park, NC 27711.
8. Environmental Protection Agency, 1991. Guidance on the Application of Refined Dispersion
Models for Air Toxic Releases. EPA-450/4-91-007. United States Environmental Protection
Agency, Research Triangle Park, NC 27711.
9. Catalano, J.A., D.B. Turner, and J.H. Novak, 1987. User's Guide for RAM -- Second
Edition. United States Environmental Protection Agency, Research Triangle Park, NC 27711.
10. Irwin, J.S., T. Chico, and J.A. Catalano. CDM 2.0 - Climatological Dispersion Model -
- User's Guide. United States Environmental Protection Agency, Research Triangle Park, NC
27711.
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11. Environmental Protection Agency, 1991. Procedures for Establishing Emissions for Early
Reduction Compliance Extensions. Draft. EPA-450/3-91-012a. United States Environmental
Protection Agency, Research Triangle Park, NC 27711.
12. Environmental Protection Agency, 1978. Control of Volatile Organic Emissions from
Manufacturers of Synthesized Pharmaceutical Products. EPA-450/2-78-029. United States
Environmental Protection Agency, Research Triangle Park, NC 27711.
13. Environmental Protection Agency, 1980. Organic Chemical Manufacturing Volumes 1-10.
EPA-450/3-80-023 through 028e. United States Environmental Protection Agency, Research
Triangle Park, NC 27711.
14. Environmental Protection Agency, 1980. VOC Fugitive Emissions in Synthetic Organic
Chemicals Manufacturing Industry - Background Information for Proposed Standards. EPA-
450/3-80-033a. United States Environmental Protection Agency, Research Triangle Park, NC
27711.
15. Environmental Protection Agency, 1990. Protocol for the Field Validation of Emission
Concentrations from Stationary Sources. EPA-450/4-90-015. United States Environmental
Protection Agency, Research Triangle Park, NC 27711.
16. Pierce, T.E., Turner, D.B., Catalano, J.A., Hale, F.V., 1982. "PTPLU: A Single Source
Gaussian Dispersion Algorithm." EPA-600/8-82-014. United States Environmental Protection
Agency, Washington, DC 20460.
17. California Air Pollution Control Officers Association (CAPCOA), 1987. Toxic Air
Pollutant Source Assessment Manual for California Air Pollution Control District and
Applications for Air pollution Control District Permits, Volumes 1 and 2. CAPCOA,
Sacramento, CA.
18. Environmental Protection Agency, 1987. User's Guide for the Industrial Source Complex
(ISC2) Dispersion Models, Volumes 1, 2 and 3. EPA-450/4-92-008a, b, and c. United States
Environmental Protection Agency, Research Triangle Park, NC 27711.
19. Environmental Protection Agency, 1991. Human Exposure Model (HEM-n) User's; Guide.
EPA-450/4-91-010. United States Environmental Protection Agency, Research Triangle Park,
NC 27711.
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APPENDIX A
ELECTRONIC BULLETIN BOARD ACCESS INFORMATION
The Office of Air Quality Planning and Standards (OAQPS) of the EPA has developed
an electronic bulletin board network to facilitate the exchange of information and technology
associated with air pollution control. This network, entitled the OAQPS Technology Transfer
Network (TTN), is comprised of individual bulletin boards that provide information on OAQPS
organization, emission measurement methods, regulatory air quality models, emission estimation
methods, Clean Air Act Amendments, training courses, and control technology methods.
Additional bulletin boards will be implemented in the future.
The TTN service is free, except for the cost of the phone call, and may be accessed from
any computer through the use of a modem and communications software. Anyone in the world
wanting to exchange information about air pollution control can access the system, register as
a system user, and obtain full access to all information areas on the network after a 1 day
approval process. The system allows all users to peruse through information documents,
download computer codes and user's guides, leave questions for others to answer, communicate
with other users, leave requests for technical support from the OAQPS, or upload files for other
users to access. The system is available 24 hours a day, 7 days a week, except for Monday, 8-
12 a.m. EST, when the system is down for maintenance and backup.
The model codes and user's guides referred to in this document, in addition to the
document itself, are all available on the TTN in the bulletin board entitled SCRAM, short for
Support Center for Regulatory Air Models. Procedures for downloading these codes and
documents are also detailed in the SCRAM bulletin board.
Documentation on EPA-approved emission test methods is available on the TTN in the
bulletin board entitled EMTTC, short for the Emission Measurement Testing Information Center.
Procedures for reading or downloading these documents are also detailed in the EMTIC bulletin
board.
The TTN may be accessed at the phone number (919)-541-5742, for users with 1200 or
2400 bps modems, or at the phone number (919)-541-1447, for users with a 9600 bps modem.
The communications software should be configured with the following-parameter settings: 8 data
bits; 1 stop bit; and no (N) parity. Users will be asked to create their own case sensitive
password, which they must remember to be able to access the network on future occasions. The
entire network is menu-driven and extremely user-friendly, but any users requiring assistance
may call the systems operator at (919)-541-5384 during normal business hours EST.
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42
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APPENDIX B
REGIONAL METEOROLOGISTS/MODELING CONTACTS
Ian Cohen
EPA Region I (ATS-2311)
J.F.K. Federal Building
Boston, MA 02203-2211
FTS: 835-3229
Com: (617)565-3225
E-mail: EPA9136
FAX: FTS 835-4939
Robert Kelly
EPA Region II
26 Federal Plaza
New York, NY 10278
FTS: 264-2517
Com: (212)264-2517
E-mail: EPA9261
Fax: FTS 264-7613
Alan J. Cimorelli
EPA Region III (3AM12)
841 Chestnut Building
Philadelphia, PA 19107
FTS: 597-6563
Com: (215)597-6563
E-mail: EPA9358
Fax: FTS 597-7906
Lewis Nagler
EPA Region IV
345 Courtland Street, N.E.
Atlanta, GA 30365
FTS: 257-3864
Com: (404)347-2864
E-mail: EPA9470
Fax: FTS 257-5207
James W. Yarbrough
EPA Region VI (6T-AP)
1445 Ross Avenue
Dallas, TX 75202-2733
FTS: 255-7214
Com: (214)255-7214
E-mail: EPA9663
Fax: FTS 255-2164
Richard L. Daye
EPA Region VII
726 Minnesota Avenue
Kansas City, KS 66101
FTS: 276-7619
Com: (913)551-7619
E-mail: EPA9762
Fax: FTS 276-7065
Kevin Golden
EPA Region VIII (8ART-TO)
999 18th Street
Denver Place ~ Suite 500
Denver, CO 80202-2405
FTS: 776-0952
Com: (303)293-0952
E-mail: EPA9853
Fax: FTS 330-7559
Carol Bohnenkamp
EPA Region IX (A-2-1)
75 Hawthorne Street
San Francisco, CA 94105
FTS: 484-1238
Com: (415)744-1238
E-mail: EPA9930
Fax: FTS 484-1076
Rebecca Calby
EPA Region V(5AR-18J)
77 W. Jackson
Chicago, IL 60604
FTS: 886-6061
Com: (312)886-6061
E-mail: EPA9553
Fax: FTS 886-5824
Robert Wilson
EPA Region X (ES-098)
1200 Sixth Avenue
Seattle, WA 98101
FTS: 399-1530
Com: (206)442-1530
E-mail: EPA9051
Fax: 399-0119
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TECHNICAL REPORT DATA
(Please read Instructions on reverse before completing)
1. REPORT NO. 2,
EPA-450/4-92-001
4. TITLE AND SUBTITLE
A Tiered Modeling Approach for Assessing the Risks Due to
Sources of Hazardous Air Pollutants
7. AUTHORS)
David E. Guinnup
9. PERFORMING ORGANIZATION NAME AND ADDRESS
U.S. Environmental Protection Agency
Office of Air Quality Planning and Standards
Technical Support Division
Research Triangle Park, NC 2771 1
12. SPONSORING AGENCY NAME AND ADDRESS
3. RECIPIENT'S ACCESSION NO.
6. REPORT DATE
8. PERFORMING ORGANIZATION CODE
8. PERFORMING ORGANIZATION REPORT NO.
10. PROGRAM ELEMENT NO.
11. CONTRACT/GRANT NO.
13. TYPE OF REPORT AND PERIOD COVERED
14. SPONSORING AGENCY CODE
16. SUPPLEMENTARY NOTES
16. ABSTRACT
This document provides modeling guidance to support risk assessments as applied to stationary sources of
hazardous air pollutants. The guidance focuses on procedures which may be used in support of the petition
processes described in Title III of the Clean Air Act Amendments of 1990. The analysis approach described
herein is a tiered one, in which each subsequent modeling tier requires additional site-specific information to
produce a less conservative estimate of the risk associated with a given stationary source (or group of sources).
The modeling approach begins with Tier 1 screening tables which require only source emission rates, stack
heights, and nearest fenceline distances to estimate maximum cancer and/or noncancer risks. Tier 2 utilizes
additional source parameters (including stack diameter, exit gas temperature and velocity, and nearby building
dimensions) with the SCREEN computer program to develop more refined estimates of maximum risks. Tier 3
utilizes site-specific meteorological data, plant layout information, and release frequency data with the TOXST
and TOXLT computer models to provide additional refinement to these assessments.
HEY WORDS AND DOCUMENT ANALYSIS
«. DESCRIPTORS
Air Pollution
Atmospheric Dispersion Modeling
Risk Assessment
18. DISTRIBUTION STATEMENT
Release Unlimited
b. IDENTIFIERS/OPEN ENDED TERMS
19. SECURITY CLASS /Report)
Unclassified
20. SECURITY CLASS lP»gtl
Unclassified
c. COSATI Field/ Group
21. NO. OF PAGES
22. PRICE
EPA Form 2220-1 (Rev. 4-77) PREVIOUS EDITION IS OBSOLETE
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