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United States
Environmental
Protection Agency
Air Risk Information
Support Center
Research Triangle Park NC 27711
EPA 450/3-90-024
March 1991
SEPA Risk Assessment
for Toxic Air
Pollutants
A Citizen's Guide
-AIR RISK INFORMATION SUPPORT CENTER-
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Introduction
What are Toxic Air Pollutants?
Toxic air pollutants are poisonous substances in the air that come
from natural sources (for example, radon gas coming up from the
ground) or from manmade sources (for example, chemical
compounds given off by factory smokestacks) and can harm the
environment or your health. Inhaling (or breathing) toxic air
pollutants can increase your chances of experiencing health
problems. For example, inhaling the benzene fumes that are given
off when you pump gas into your car can increase your chances of
experiencing health effects that have been associated with exposure
to benzene, such as leukemia.
What are Health Risks?
Health risks, put simply, are a measure of the chance that you will
experience health problems. Exposure to toxic air pollutants can
increase your health risks. For example, if you live near a factory
that releases cancer-causing chemicals and inhale contaminated air,
your risk of getting cancer can increase. Breathing air toxics could
also increase your risk of noncancer effects such as emphysema or
reproductive disorders.
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What is Risk Assessment?
Risk assessment is one tool used in risk management. It is the
process that scientists and government officials use to estimate the
increased risk of health problems in people who are exposed to
different amounts of toxic substances.
A risk assessment for a toxic air pollutant combines results of
studies on the health effects of various animal and human
exposures to the pollutant with results of studies that estimate the
level of people's exposures at different distances from the source of
the pollutant.
Hazard
Identification
What health problems
are caused by the
pollutant?
Exposure
Assessment
How much of the pollutant do
people inhale during a specific,
time period? How many
people are exposed?
Dose-Response
Assessment
What are the health
problems at different
exposures?
Rlsk u s
Characterize
What is the extra^
health problems/jR
exposed populate
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While the estimates provided by these risk assessments are far from
perfect, they do help scientists evaluate the risks associated with
emissions of toxic air pollutants. Using risk estimates and other
factors, the government can set regulatory standards to reduce
people's exposures to toxic air pollutants and reduce the risk of
experiencing health problems.
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Hazard Identification
or, What Health Problems Are Caused
by the Toxic Air Pollutant?
Health Problems of Concern
The toxic air pollutants of greatest concern are those that cause
serious health problems or affect many people. Health problems
can include cancer, respiratory irritation, nervous system problems,
and birth defects.
Some health problems occur very soon after a person inhales a
toxic air pollutant. These immediate effects may be minor—such as
watery eyes. Or they may be serious, such as life-threatening lung
damage.
Other health problems may not appear until many months or years
after a person's first exposure to the toxic air pollutant. Cancer is
one example of a delayed health problem.
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Weight of Evidence for Health Problems
of Concern
One good animal study,
no human studies
Probably Causes Cancer
In a hazard identification, scientists Possibly Causes Cancer
evaluate all available information
about the effects of a toxic air
pollutant to estimate the likelihood
that a chemical will cause a certain
effect in humans. The better the
evidence, the more certain scientists
can be that a toxic air pollutant
causes specific health problems.
The amount, type, and quality of
evidence are all important.
The best type of evidence comes
from human studies. This evidence
may be in the form of case reports,
such as physicians' reports of an
unusual number of cases of a
specific illness. Other more formal
studies can be done that compare
the number of cases of a particular
illness in groups of people with
different levels of exposures (for
example, cases of leukemia in
rubber manufacturing workers).
Because human information is very
limited for most toxic air pollutants,
scientists often conduct studies on
laboratory animals, such as rats.
Animal studies are performed under
controlled laboratory conditions.
Scientists can study a variety of
health effects by exposing animals
to pollutants at varied concentra-
tions and for varied time periods.
When relying on animal studies only,
scientists need to be satisfied that
health effects in humans are likely to
be the same as those in the animals tested. Scientists try to use
animal species with body functions that are similar to humans.
Some evidence in
human studies, or two or
more good animal studies
Known to Cause Cancer
Good evidence
in human studies
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Exposure
or, How Much of a Pollutant Do Peop
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nnn n
An exposure assessment estimates how much of a pollutant people
inhale during a specific time period, as well as how many people are
exposed.
There are many sources of toxic air pollutants. For example, they
can come from a factory smokestack or from thousands of
automobiles crossing a busy intersection each day. So the first step
in an exposure assessment is to decide which sources are giving off
the pollutant of concern.
Once the identity and location of the source(s) are known, the next
step is to determine the amounts of the toxic air pollutant released in
a specific time period and how it moves away from the source(s).
Engineers use either monitors or computer models to estimate
the amount of pollutant released from the source and the amount of
pollutant at different distances from the source. Monitors are used
to sample the air and measure how much of the pollutant is present.
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sessment
ihale During a Specific Time Period?
Exposed Person's
Concentration, C
Dispersion, D
Wind
Speed,W
[_• D-W
.pot-
Distance, L
Amount
Released, Q
Computer models use mathematical equations that represent the
processes that occur when a facility releases a pollutant and also the
movement of pollutants through the air. Factors such as distance
from the source to exposed persons, wind speed and direction, and
smokestack height (for factories) affect these estimates.
The number of people exposed at different distances from the site of
release can be estimated with computer models that use information
from the census and from maps. Some models can even estimate
exposures for the different places people are each day—including
indoor, automobile, outdoor, and workplace exposures.
The final step in an exposure assessment is to estimate the amounts
each person inhales. To do this, scientists combine estimates of
breathing rates and lifespan of an average person with estimates of
the amount of pollutant in that person's air.
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Dose-Response Assessment
or, What Are the Health
Problems at Different Exposures?
How Toxic Air Pollutants
Move Through the Body
Entering the Body. Toxic air
pollutants get into the body mainly
through breathing. They can also be
ingested (for example, children eating
soil contaminated with lead) or
absorbed through the skin.
Movement and Changes in the
Body. Once a pollutant enters the
body it can stay in the lungs (like
asbestos), be exhaled, or move into
the blood from the lungs (like the
oxygen we breathe) or from the
digestive system or skin. In the blood
it is carried to all parts of the body. As
it moves around the body, a pollutant
can undergo chemical changes,
especially as it passes through the
liver, becoming less, or more, toxic.
Fate. The pollutant can be exhaled, it
can leave the body in urine, bowel
movements, sweat, or breast milk, or
it can be stored in hair, bone, or fat.
How Toxic Air Pollutants
Change the Way the Body Works
Toxic air pollutants can cause health problems by interfering with
normal body functions. Most commonly they change chemical
reactions within individual cells, the building blocks of living things.
These changes can kill cells, impair cell function, or redirect cell
activity. The results can be damaged organs, birth defects when the
cells of an unborn child are damaged, or cancer that develops when
cells begin to grow at an uncontrolled rate.
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Dose-Response Relationships
The dose-response relationship for a specific pollutant describes the
association between exposure and the observed response (health
effect). In other words, it estimates how different levels of exposure
to a pollutant change the likelihood and severity of health effects.
Just as in the hazard identification, scientists use results of animal
and human studies to establish dose-response relationships.
Cancer
Response
Dose
Dose-Response Relationship for
Cancer
Non-
cancer
Response
Dose
Dose-Response Relationship for
Noncancer Effects
Dose-response relation-
ship for cancer. In the
absence of clear evidence to
the contrary, EPA assumes
that there are no exposures
that have "zero risk"—even
a very low exposure to a
cancer-causing pollutant can
increase the risk of cancer
(albeit a small amount).
EPA also assumes that the
relationship between dose
and response is a straight
line—for each unit of
increase in exposure (dose),
there is an increase in
cancer response.
Dose-response relation-
ship for noncancer effects.
A dose may exist below
which no adverse effects
occur. EPA typically
assumes that at low doses
the body's natural protective
mechanisms repair any
damage caused by the
pollutant, so there is no ill
effect at low doses.
However, for some
substances noncancer
effects may occur at low
doses. The dose-response
relationship (the response
occurring with increasing
dose) varies with pollutant,
individual sensitivity, and
type of health effect.
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Risk Characterization
or, What Is the Extra Risk to Health?
Risk information is presented in different ways to illustrate how
individuals or populations may be affected. Some of the most
common risk measures are described here.
Maximum Individual Lifetime Cancer Risks
Combining the results of the exposure assessment and the dose-
response assessment gives an estimate of the increased lifetime
risk of cancer for an individual exposed to the maximum predicted
long-term concentration.
Maximum
Distribution of Individual Risks
Many people may be exposed to less than the maximum level.
Depending on the amount of exposure, an individual's risk of cancer
will vary. The distribution of individual risk is usually expressed as the
number of people estimated to
be at various levels of risk.
Population
Cancer Risks
Distributions of individual risk
are used to calculate population
risk. The population cancer risk
is usually expressed as the
expected increased incidence
of cancer (that is, the number of
new cases each year) for all
people exposed to the pollutant.
For example, the estimated
population cancer risk may be
the number of new cancer
cases per year expected
among residents within 30
miles of a certain large source.
Distribution of Individual Risk
High risk
Moderate risk
Low risk
Predicted
Cancer Cases
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Noncancer Risks
Health reference levels refer to exposure levels that will not cause
significant risks of noncancer health effects. Long-term exposure to
levels below these levels are assumed to produce no ill effects.
Health reference levels are an example of one index that govern-
ment agencies use in characterizing noncancer health risks. These
levels are generally developed from exposure levels that do not
produce ill effects in experimental animals. These exposure levels
are adjusted to account for animal-human differences (such as
breathing rate) and for underlying uncertainties (such as the
difference in sensitivity between healthy adults and more sensitive
people like children and the elderly).
Risk analysts then compare the health reference levels with the
exposure estimates to determine how many people are exposed to
concentrations higher than the health reference level. Some of these
people might experience ill effects.
Uncertainty in Risk Estimates
Although scientists can estimate risks caused by toxic air pollutants
in animals experimentally or in humans who have unusual
exposures, converting these estimates to those expected in people
under a wide range of conditions is difficult, and can be misleading.
By their nature, risk estimates
cannot be completely
accurate. The main problem
is that scientists don't have
enough information on actual
exposure and on how toxic
air pollutants harm human cells. The exposure assessment often
relies on computer models when the amount of pollutant getting from
the source(s) to people can't be easily measured. Dose-response
relationships often rely on assumptions about the effects of
pollutants on cells for converting results of animal experiments at
high doses to human exposures at low doses.
When information is missing or uncertain, risk analysts generally
make assumptions that tend to prevent them from underestimating
the potential risk — that is^these assumptions provide a margin.-of
safety in the protection OTtrunW Health.
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Summing It All Up
• Public health agencies concerned with air quality perform risk
assessments to determine the increased risk of illness from a
specific human exposure to a toxic air pollutant.
• Risk assessment is a four-step process: (1) hazard identification,
(2) exposure assessment, (3) dose-response assessment, and
(4) risk characterization.
• Hazard identification describes the illnesses caused by a toxic
air pollutant and the amount of evidence for those illnesses.
• The size of the increased health risks depends on the exposure
level and duration, as well as the number of people exposed.
These are estimated as part of the exposure assessment.
• The dose-response assessment estimates the dose-response
relationship, which mathematically shows the change in the
likelihood of health effects with changes in the levels of exposure
to a toxic air pollutant.
• The risk characterization uses the above assessments to
describe the type and size of any increased risk expected as a
result of exposure to the air pollutant. It also includes a discussion
of the uncertainties associated with the risk estimates.
For More Information—
Chemical Exposures: Effects on
Health. Fact Sheet CC.
Write to: Dr. Maria Pavlova, U.S.
EPA, 26 Federal Plaza, Rm. 737,
New York, NY 10278.
Elements of Toxicology and Risk
Assessment. Write to: Environ
Corporation, 1000 Potomac St.,
NW, Washington, DC, 20007.
The Risk Assessment Manual: A
Guide to Understanding and Using
Health and Environmental
Assessments, by B. Brockband, J.
Cohrsson, and V.T. Covello. Pub-
lished by the Council on Environ-
mental Quality, NTIS N8 PB89-
137772KNK. Write to: National
Technical Information Service, 5285
Port Royal Rd., Springfield, VA
22161. $17.50 charge.
Toxicology for the Citizen, by Alice
E. Marczewski and Michael Kamrin.
Write to: Center for Environmental
Toxicology, Michigan State
University, C231 Holden Hall, East
Lansing, Ml 48824. $1.00 charge.
Air Pollution. Fact Sheet LL
Write to: Dr. Maria Pavlova, U.S.
EPA, 26 Federal Plaza, Rm. 737,
New York, NY 10278.
The Process of Risk Assessment
and Risk Management. Fact Sheet
BB. Write to- Dr. Maria Pavlova,
U.S. EPA, 26 Federal Plaza, Rm.
737, New York, NY 10278. Free.
Glossary of Terms Related to
Health Exposure and Risk
Assessment. EPA Air RISC.
Call (919) 541-0888.
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•&U S. GOVERNMENT PRINTING OFFICE: I9'9I - 527-090/27009
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