|
|
Evaluating Exposures to Toxic Air Pollutants: A Citizen's Guide
Originally published as
EPA 450/3-90-023
March 1991
(**Indicates modified since original publication)
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 four-step 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.
THE 4-STEP EXPOSURE ASSESSMENT
The exposure assessment is also 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
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
Step 2 - Estimate Releases of Pollutants from Sources
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.
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.
How Much of a Pollutant Is Released?
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 mathematical equations that represent the processes that occur
when a facility generates a pollutant. Two kinds of Monitoring a Routine
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.
Step 3 - Estimate Concentration 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 depends 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.
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 tor
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 decision makers, 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 fence line 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.
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, 1200 Pennsylvania Ave, NW, Washington, DC 20460
- Trends in the Quality of the Nation's Air.
- Write to: Public Information Center PM-211 B, U.S. EPA, 1200 Pennsylvania Ave, NW,
SW, Washington, DC 20460
- Air Quality Trends Report
- A summary of the latest annual Trends Report published by the EPA
Office of Air Quality Planning and Standards.
- **Toxic Chemicals: What They Are, How They Affect You.
- **Document no longer available.
- Chemical Risk: A Primer.
- Write to: American Chemical Society, Department of Government Relations
and Science Policy, 1155 16th Street, NW, Washington, DC 20036
Other Health Risk Publications
Risk Assessments for Toxic Air Pollutants:
A Citizen's Guide.
Air Pollution and Health Risk
|