Important Background Information on EPA's 2011 National Scale Assessment
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The 2011 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 referenced 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 should 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 for most of the pollutants we are seeing a downward trend or levels have remained relatively stable, depending on the compound and location, from 1996 to 2014 in ambient air concentrations at various monitoring locations for 1, 3-butadiene (PDF) (1 pg, 12 K), acetaldehyde (PDF) (1 pg, 12 K), acrolein (PDF) (1 pg, 11 K), arsenic (PDF) (1 pg, 9 K), benzene (PDF) (1 pg, 13 K), carbon tetrachloride (PDF) (1 pg, 13 K), chromium (PDF) (1 pg, 8 K), formaldehyde (PDF) (1 pg, 12 K), benzo(a)pyrene (PDF) (1 pg, 9 K) and naphthalene (PDF) (1 pg, 10 K). 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, formaldehyde 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 all reported values in AQS were used to calculate the annual mean (this includes zero's/non-detects and values below the minimal detection limit).
Four steps of the National Scale Air Toxics Assessment
The assessment includes four steps that focus on the 2011 emissions year:
- Compiling a national emissions inventory of air toxics emissions from outdoor sources
- Estimating ambient and exposure concentrations of air toxics across the United States
- Estimating population exposures across the United States
- Characterizing potential public health risk due to inhalation of air toxics including both cancer and noncancer effects
Limitations of the Assessment
Despite the 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.