United States Air Risk Information EPA 450/3-90-023
Environmental Support Center March 1991
Protection Agency Research Triangle Park NC 27711
SEPA Evaluating
Exposures to Toxic
Air Pollutants
A Citizen's Guide
air
AIR RISK INFORMATION SUPPORT CENTER
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Introduction |
What Are Toxic Air Pollutants?
Toxic air pollutants are substances in the air that, if you are
exposed to them, could increase your chances of experiencing
health problems. Toxic air pollutants also can cause ecological
impacts. An example of a toxic air pollutant is the chemical
benzene, which is in gasoline. Inhaling fumes that contain
benzene could increase your chances of getting cancer.
Which Toxic Air
Pollutants Are of Most Concern?
Government agencies are most concerned about substances
that fit one or more of these descriptions:
• Can cause serious health effects, such as cancer, birth
defects, immediate death, or other serious illnesses.
• Are released to the air in large enough amounts to be toxic.
• Reach many people.
What Is Exposure Assessment?
Scientists and government officials use a process called risk
assessment to estimate people's increased risk of health
problems as a result of exposure to a toxic air pollutant An
exposure assessment is one step of that process and is used to
determine how much of the pollutant people are exposed to
and/or how many people are exposed.
Hazard Iden-
tification
Dose-Response
Assessment
Risk
Characterization
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The 4-Step Exposure Assessment |
The exposure assessment is a four-step process. Step 1 entails
identifying pollutants likely to be in the air. In Step 2, the
amounts of these pollutants released from different sources are
estimated. In Step 3, the concentrations of the pollutants are
estimated for the geographic areas of interest. Finally, Step 4
provides estimates of the number of people who breathe air
containing the pollutant at different levels or at some selected
level, such as a regulatory standard or a health benchmark level.
Step 1 — Identify Pollutants Released I
Many chemicals found in factories, consumer goods, sewage
treatment plants, and other sources can be released to the air
as toxic air pollutants. These are some chemicals typically
released in many U.S. communities:
• Perchloroethylene from dry cleaners
• Methylene chloride from degreasers and consumer products,
such as paint strippers
• Benzene from gasoline, released when you fill your tank and
drive your car
• Chromium from metal plating operations
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Step 2 — Estimate Relea
What are the Sources of Pollutants?
Point sources are sources that have a specific location. Point
sources include chemical plants, steel mills, oil refineries, and
hazardous waste
incinerators.
Pollutants can be
released when
equipment leaks,
when material is
transferred from one
area to another, or
when waste is given
off from a facility
through smoke
stacks.
Area sources of
toxic air pollutants
are made up of many
smaller sources
releasing pollutants
to the outdoor air in a
defined area.
Examples include
automobiles,
neighborhood dry
cleaners, small metal
plating operations,
gas stations, and
woodstoves.
Point Sources
Area Sources
What are the Patterns of Releases?
Routine releases, such as those from industry, cars, landfills, or
incinerators, may follow regular patterns and happen continuously
over time. Other releases may be routine but intermittent, such as
when a plant's production is done in batches. Accidental releases
can occur during an explosion, equipment failure, or a transportation
accident. The timing and, often, the amount released during
accidental releases are difficult to predict.
3
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)f Pollutants from Sources
How Much of a Pollutant Is Released?
Monitoring a Routine
To estimate the amount of a routine
release engineers sometimes use a
monitor to sample the pollutant as it is
released. The amount collected in a given
time period is measured in a laboratory.
For example, if 10 pounds of pollutant XYZ
is collected in an average hour and the
facility runs 24 hours a day, 240 pounds of
XYZ per day would be released.
Alternatively, engineers can use an
emission model to estimate the amount of
pollutant released by a particular facility.
An emission model is a set of mathe-
matical equations that represent the
processes that occur when a facility
generates a pollutant Two kinds of
numbers are put into these mathematical
equations: (1) "emission factors," or average
emission measurements that are made by measuring emissions
from a few "typical" facilities, and (2) "depends on" factors, or factors
that are specific to a certain facility and depend on how that facility
operates. This kind of estimation is similar to determining the fuel
efficiency for your car.
The manufacturer
provides an average
miles per gallon rating
when you buy a car.
Depending on many
factors such as how
you drive your car, your
actual fuel efficiency
may differ. In a similar
fashion, engineers use
various factors to adjust
for differences between
a "typical" facility and
the facility in question.
Accidental Releases
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Step 3 — Estimate Concentrations
in Air at Different Locations
What Affects the Concentration of a Pollutant?
The concentration of a pollutant decreases as it travels from the
site of release because the pollutant spreads out. The amount
of this dilution, or
dispersion, in the air
depends on weather
— especially wind
direction and speed.
Dispersion also de-
pends on the terrain,
whether it is on flat or
mountainous land or
in a valley.
Other factors can
affect the concentration, or level of a pollutant at a given
location. The amount of a pollutant at any one location can vary
over time depending on the pattern of releases. For example,
industrial processes can release some pollutants only at certain
times and other pollutants continuously. The location of the
release affects the concentration—a pollutant can be released
from smoke stacks high in the air or can leak from equipment or
storage tanks near the ground. The ground-level concentration
near a facility is generally lower when a pollutant is released
from high stacks because the pollutant is more diluted when it
reaches the ground. Other factors that affect concentration
include the temperature and speed of the gas released through
the smoke stack and the location of places in the facility where it
is released.
What Is the Concentration of a Pollutant at
Different Distances from the Source?
Using a technique called dispersion modeling, engineers can
estimate the concentration of a pollutant at different distances
and directions from the source. The computer model is used to
calculate these estimates from information about the amount of
pollutant released, the weather and terrain around the source,
and other factors that affect the concentration of the pollutant.
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Step 4 — Estimate the
Number of People Exposed
For a point source, researchers estimate the number of people
living in various areas surrounding the site of release with a
computer model that uses census information for wider and
wider rings around the point source. For an area source, the
computer model uses census information to estimate the
population living in the area of interest. Where warranted,
census estimates can be adjusted to reflect daily and seasonal
population movements.
Using dispersion and population information in models, agencies
can estimate the number of people exposed to varied
concentrations of a chemical. To aid decisionmakers, these
models can compare exposures to some selected benchmark,
such as a state pollution standard or a level with a known health
effect. For example (see figure below), someone standing at the
northeast fenceline of a factory's property might be exposed to
10 times the state standard while someone living a little further
from the factory might be exposed to 2 times the standard.
Someone living to the southwest may be exposed to very low
levels below the state standard.
U.S.
Region 5, Li':
Agency
= population
unit
= area with
ground-
level con-
centration
above the
standard
= area with
ground-
level con-
centration
10x or more
above the
standard
/ / west Jack
Chicago, II
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Summing It All Up
Assessing people's exposure to pollutants is one step of a
risk assessment.
Exposure assessment has four steps:
- Identify pollutants that may be released.
- Estimate the amount of pollutants released from all
sources, or the source of particular concern, using air
samples or emission models.
- Estimate concentrations of pollutants in air in the
geographic area of concern by using dispersion models
with information about emissions, source locations,
weather, and other factors.
- Estimate the number of people exposed to different
concentrations of the pollutant at different geographic
locations.
For More information—
Chemicals in Your Community: A Guide to the Emergency
Planning and Community Right-to-Know Act.
Write to: Emergency Planning and Community Right-to-Know
Information, U.S. EPA, OS-120, 401 M Street, SW, Washington,
DC 20460
Trends in the Quality of the Nation's Air.
Write to: Public Information Center PM-211B, U.S. EPA, 401 M
Street, SW, Washington, DC 20460
Toxic Chemicals: What They Are, How They Affect You.
Write to: Dr. Maria Pavlova, U.S. EPA, 26 Federal Plaza, Room
737, New York, NY 10278
Chemical Risk: A Primer.
Write to: American Chemical Society, Department of
Government Relations and Science Policy, 1155 16th Street,
NW, Washington, DC 20036
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ifV.S. GOVERNMENT PRINTING OFFICE: 1991 - 527-090/27008
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