DES has been actively involved in the development and utilization of tools used to gauge the effectiveness of the air toxics control strategies that are implemented in New Hampshire. Several of these tools are outlined below.
US EPA National Air Toxics Assessment
One tool that is being used to evaluate potential human exposure to air toxics is EPA’s National Air Toxics Assessment Project (NATA). EPA’s Office of Air Quality, Planning, and Standards (OAQPS) initiated the NATA in 1996. The goal of the study was to look at the impacts of multiple toxic pollutants and to use the information to evaluate the possible effects on human health. The air toxics component of the NATA is a national modeling study of 33 HAPs (a subset of 32 air toxics on the Clean Air Act’s list of 188 air toxics plus diesel particulate matter (diesel PM)), which estimates 1996 average concentrations for each census tract in the continental United States, including New Hampshire. Given the limited availability of monitoring data for air toxics, these modeled estimates can provide an understanding of the distribution and relative concentrations of these pollutants in outdoor air. The DES Air Resources Division uses this study to help evaluate the pollutants and pollutant sources driving air toxic health risk in the state.
The NATA utilizes emissions data from EPA’s 1996 National Toxics Inventory (NTI). The NTI is a complete national inventory of stationary and mobile sources that emit air toxics and is developed every three years beginning in 1993. EPA compiled the Inventory using five primary sources of data:
- State and local toxic air pollutant inventories (developed by state and local air pollution control agencies).
- Existing databases related to EPA’s air toxics regulatory program.
- EPA’s Toxic Release Inventory (TRI) database.
- Estimates developed using mobile source methodology (developed by EPA’s Office of Transportation and Air Quality).
- Emission estimates generated from emission factors and activity data.
Modeling Methods
The computer simulation model used by EPA to estimate toxic air pollutant concentrations for the NATA is called the Assessment System for Pollutant Exposure Nationwide, or ASPEN. This model is similar to other air dispersion simulation models used by EPA, such as the Industrial Source Complex Long Term model (ISCLT) or the Human Exposure Model (HEM). These models simulate the behavior of the pollutants after they are emitted into the atmosphere. ASPEN uses estimates of toxic air pollutant emissions, and mathematical approximations of what happens to them after they are emitted, to estimate ambient air concentrations. Emissions data used in the ASPEN model are estimates of the quantities of 33 toxic air pollutants emitted in 1996 from point, area, and mobile sources. Point sources include major industrial emissions points such as power plants and manufacturing facilities. Area sources are typically smaller, more numerous sources such as gasoline stations, small dry cleaners, auto body shops, home heating units, and consumer products. Mobile sources include emissions from automobiles, trucks, and buses. Emissions data are represented by data from EPA’s 1996 National Toxics Inventory. For some pollutants, the concentration estimates include a "background" concentration. Background concentrations are the contributions to outdoor air toxics concentrations resulting from natural sources, persistence in the environment of past years’ emissions, and long-range transport.
Health Risk Estimates
In developing the NATA, EPA established screening benchmark concentrations for many of the modeled toxic pollutants. The benchmark concentrations were based on standard toxicological references, and for each pollutant, represented an air toxic level above which a risk of one excess occurrence in a population of one million exposed individuals was predicted for cancer, or, for non-cancer health effects, a concentration above which no adverse effects were predicted in individuals exposed for a lifetime.
Results
The NATA suggests that of the 33 compounds examined by EPA, six toxic air pollutants exhibited elevated risk of cancer or other adverse health effects everywhere in the country including New Hampshire. The six toxic air pollutants were found to exceed the one-in-a-million risk screening benchmark on a nation wide basis include benzene, carbon tetrachloride, ethylene dibromide, ethylene dichloride, formaldehyde, and chloroform. The predicted highest concentrations of eight additional compounds were above the screening threshold in at least one New Hampshire census tract (census tracts are land areas defined by the US Bureau of the Census, and typically contain about 4,000 residents each). The predicted concentrations of nine more compounds exceeded the screened the benchmark in at least one New Hampshire county. For all of these 24 compounds however, the average risk levels in New Hampshire were significantly below the national average (see graph of relative risk for each of these compounds). DES has developed health effects fact sheets for each of the 24 toxic air pollutants, which were predicted to exceed the one-in-a-million screening benchmark concentration in New Hampshire (see graph).
The modeled estimated concentrations of HAPs are evaluated by comparison with monitoring data available from long-term measurement studies conducted in several locations. The NATA utilizes 1996 emissions data and is therefore not representative of current conditions in New Hampshire, especially in light of the fact that New Hampshire’s Toxic Release Inventory (TRI) data indicates an overall reduction in HAP emissions of over 80 percent between 1988 and 1998 (see graph). However, future work in this area will be valuable in gauging the effectiveness of the implemented federal and state air toxics control strategies on ambient air concentrations in New Hampshire.
The following links contain more information.
- FAQ
- EPA’s Health Effects Notebook
- EPA Exposure Assessment
- Links to Health Effects Fact sheets
- List of Contacts
- New Hampshire's Air Toxics Control Program
(Fact Sheet ARD-1)
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