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.
