Air Toxics in New England

Important Background Information on EPA's 2002 National Scale Assessment

The 2002 National Scale Air Toxics Assessment (NATA) is an estimate of people’s exposure to air toxics. Exposure is based on a computer modeling estimate of the concentrations of air toxics in the air around us using air toxics emissions estimates from different sources including cars, trucks, factories and waste incinerators. The assessment includes four steps described below and evaluates 180 air toxics, a subset of the Clean Air Act's list of 187 air toxics, plus diesel particulate matter (diesel PM).

The assessment is a useful tool that can be used to identify those air toxics compounds which are likely to present the greatest risks to the largest number of people in the largest number of areas. We believe that the results are most meaningfully interpreted when viewed over large geographic areas, such as national or State levels. However, the results can also be used both at a national and more local level to help prioritize toxic compounds for risk reduction efforts and to identify areas and pollutants for which additional investigation (in the form of monitoring, emissions inventory, risk analysis, etc.) is needed. It is important to note, as well, that there are limitations to the assessment for a number of reasons which are described below.

EPA has completed previous National Scale Assessments and continues to update the assessments with improved methodology, improvements to the emission inventory, and changes in air toxic pollutants and risk characterization. Therefore, results from previous assessments can not be compared. Such comparisons are not meaningful since changes in emissions, ambient concentrations, or risks may be due to improvements in methodology rather than changes in emissions.

A review of available air toxics monitoring data in New England, however, shows that we are seeing a downward trend or levels have remained relatively stable, depending on the compound and location, from 1996 to 2008 in ambient air concentrations at various monitoring locations for (1, 3-butadiene (PDF) (1 p., 16 KB), acetaldehyde (PDF) (1 p., 17 KB), acrolein (PDF) (1 p., 16 KB), arsenic (PDF) (1 p., 17 KB), benzene (PDF) (1 p., 18 KB), and carbon tetrachloride (PDF) (1 p., 18 KB). EPA reviewed the available monitoring data from the Air Quality System (AQS) data base for the following compounds: 1, 3-butadiene, carbon tetrachloride, chromium VI, benzene, acetaldehyde, acrolein, naphthalene, benzo(a)pyrene and arsenic. To determine if there were sufficient data to show a trend for these compounds, the following criteria were used: five years or more of data, sampling was performed either on a one-in-three day, a one-in-six day, or a one-in-twelve day frequency, data capture of 75% or greater for the year, each data point needed to represent a 24-hour average concentration, and reported values that were below or one half of the minimum detection level (MDL) were not included in the trend analysis.

Four steps of the National Scale Air Toxics Assessment

1. Emissions Inventories: EPA compiles a national emissions inventory (NEI) with significant assistance and review from the states. The inventory estimates toxics emissions from all outdoor sources of air pollution every three years. The 2002 NEI was prepared with significant assistance and review from the states. The types of emission sources in the inventory include major stationary sources (e.g., large waste incinerators and factories), area sources (e.g., dry cleaners, small manufacturers, and consumer products), and on-road and off-road mobile sources (e.g., cars, trucks, and boats). EPA and the states are now working on development of an improved air toxics emission inventory for the year 2005.

2. Modeling to Estimate Ambient Air Toxics Concentrations: Using the 2002 inventory as input to air dispersion models, EPA estimated ambient (or outdoor) air toxics concentrations throughout the continental United States, Puerto Rico and the Virgin Islands using the Assessment System for Population Exposure Nationwide (ASPEN) computer model or EPA’s Human Exposure Model (HEM). The results of this modeling effort are available to the public at http://www.epa.gov/ttn/atw/natamain/.

3. Modeling to Estimate Population Exposures: Using the estimated ambient air toxics concentrations modeled by ASPEN or HEM, EPA then used a screening-level inhalation exposure model, known as HAPEM5, to estimate population exposures to air toxics. This model provides estimates of population exposure to air toxics from outdoor emission sources by accounting for people’s activity, such as time spent outside, inside, and commuting to and from work. The results of this model are available at http://www.epa.gov/ttn/atw/natamain/.

4. Characterizing Public Health Risks: Using the estimated population exposures, EPA has characterized potential public health risks due to inhalation of air toxics. This includes both cancer and noncancer effects based on available air toxics health effects, current agency risk assessment and risk characterization guidelines. This information is available at http://www.epa.gov/ttn/atw/natamain/.

Limitations of the Assessment

Some of the general limitations follow.

1. Limitations in the national emissions inventory. The national emissions inventory is a compilation of data for air toxics emitted into the atmosphere (through smokestacks, tailpipes, vents, etc.). The accuracy of emission estimation techniques vary with pollutants and source categories. In some cases, an estimate may be based on a few or only one emission measurement at a similar source. The techniques used and quality of the estimates will vary between source categories (e.g., some have been better studied than others) and between major, area, and mobile source sectors. We also know that there are data gaps where we don’t have emission factors developed for certain pollutants from certain types of sources. In addition, emission estimates for many activities were based on activity data from each county such as, gasoline consumption or solvent use data. Although considerable effort was given to develop the inventory for this project, generalizations used to estimate county scale emissions, as well as census tract scale emission estimates, also create uncertainty.

2. Limitations of models. The models predict air toxics concentrations and exposures based on a number of assumptions and mathematical representations. Because of these assumptions, there is uncertainty in the models’ predictions. Where the ambient air modeling data was compared to limited monitoring data, the comparisons showed the model tends to underestimate air toxics concentrations. http://www.epa.gov/ttn/atw/nata1999/99compare.html

3. Limited to outdoor sources. This assessment is limited to exposure of emissions from outdoor sources. While we recognize that indoor sources of air toxics likely contribute substantially to the total exposures that people experience for a number of toxic air pollutants, these emissions were not included in this initial assessment.

4. Limited to inhalation exposure. For some of the toxic air pollutants, other routes of exposure such as ingestion are generally known to be the main route of exposure.

5. Limited summation of risks. Only health end points related to cancer, respiratory and the neurological system were tabulated in the risks.

6. The model used 2002 emissions data. Therefore, the results do not reflect emission reductions we have seen due to the Federal, State and local control programs implemented after 2002.

For more detailed information on the limitations of the assessment visit link here.

Despite these limitations, the assessment represents an important step in characterizing air toxics risks nationwide. Continued research will enable future assessment activities, both at the national level and at more local refined levels, to yield improved assessments of cumulative air toxics risks.