Limitations in the 1996 National-Scale Air Toxics Assessment: Uncertainty
Uncertainty: With what confidence can these results be accepted?
Note that a more refined analysis of uncertainty is underway and should be available here in the summer of 2002.
Overall Confidence (considers all exposure components which include emissions, ambient concentration and exposure estimates)
To determine overall
confidence, EPA considered the sources of uncertainty associated with the first three steps of the assessment (estimates of emissions, ambient concentrations, and
Table A: Composite Judgements to Determine Overall Certainty
Emissions Inventory +
|Confidence in HAPEM Exposure Modeling Results|
Polycyclic Organic matter (POM)
|higher||Coke Oven Emissions
Overall confidence based on the composite judgements listed in Table A. above
can be summarized as follows for a typical individual in a
|Lower (orange in Table A.) for Arsenic Compounds; Beryllium Compounds; Cadmium Compounds; Mercury
Compounds; Nickel Compounds, Chromium Compounds; Hexachlorobenzene(HCB) ; Manganese Compounds; PCBs; Lead Compounds; Acetaldehyde; Acrolein; Hydrazine; Polycyclic Organic Matter (POM); Quinoline; 7-PAH; and 1,3-Dichloropropene.|
|Medium (yellow in Table A.) for Chloroform; Diesel PM; Ethylene Dibromide; Ethylene Dichloride; 1,1,2,2-Tetrachloroethane; Formaldehyde; Perchloroethylene; 1,3-Butadiene; Coke Oven Emissions; Propylene
Dichloride; and Vinyl Chloride.|
|Higher (green in Table A.) for Acrylonitrile; Carbon Tetrachloride; Ethylene Oxide; Methylene Chloride; Trichloroethylene; and Benzene.|
Confidence in estimates of cancer risk for a typical individual in a
- Lower for Acetaldehyde; Arsenic Compounds; Beryllium Compounds; Cadmium
Compounds; Carbon Tetrachloride; Chloroform; Chromium Compounds; Ethylene Dibromide; Ethylene Dichloride; Ethylene Oxide; Hexachlorobenzene(HCB); Hydrazine; Lead Compounds; Methylene Chloride; Nickel Compounds; Perchloroethylene; PCBs; Polycyclic Organic Matter(POM); 7-PAH; Propylene
Dichloride; Quinoline; Trichloroethylene; 1,1,2,2-Tetrachloroethane;
- Medium for Acrylonitrile; Coke Oven Emissions;
Formaldehyde; Vinyl Chloride; and 1,3 Butadiene.
- Higher for Benzene.
Confidence in estimates of noncancer risk for a typical individual in a
- Lower for Acetaldehyde; Acrolein; Arsenic Compounds; Beryllium Compounds; Cadmium Compounds; Carbon Tetrachloride; Chromium Compounds; Ethylene Dichloride; Ethylene Oxide; Hexachlorobenzene(HCB); Hydrazine; Lead Compounds; Manganese Compounds; Mercury Compounds; Methylene Chloride; Nickel Compounds, 1,3-Butadiene; and 1,3-Dichloropropene.
- Medium for Acrylonitrile; Benzene; Chloroform; Ethylene Dibromide; Formaldehyde; Perchloroethylene; Propylene Dichloride; Vinyl Chloride; and 1,2-Dichloropropene.
- Higher for Trichloroethylene.
It is important to bear two things in mind. First, the above judgments refer to the relative confidence between two air toxics compounds. A judgment of "Higher" means the confidence is higher for this compound than for compounds assigned a "Medium" or "Lower." Second, the confidence depends on the geographic scale considered. As larger geographic areas are considered, and the exposure or risk is averaged over census tracts in that region, the confidence in estimates of these averages generally will increase. The above confidence ratings apply to the nationwide estimates and not to smaller scales (e.g., State or county-level).
In addition, EPA conducted an uncertainty analysis to determine the uncertainty in the risk to an individual living in a census tract. That analysis indicated that the uncertainty is at least of a factor of 5 (if 68% confidence intervals are considered) or 25 (if 95% confidence intervals are considered). This means the risk to an individual could be a factor of 5 to 25 above or below the values reported in the national-scale assessment for a census tract. This uncertainty, however, will vary between parts of a county. For example, estimates of exposure and risk will have a larger uncertainty in parts of a county with complicated terrain (e.g. hills) than in parts of the county with flat terrain, since the dispersion models are more accurate in flat terrain.
To learn more about these results, please select from any of the items
More Details About the "Overall Confidence" Rankings
What are the components of uncertainty?
Which components of uncertainty did the national-scale assessment include?
How was the uncertainty analysis conducted?
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