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Information provided for informational purposes onlyNote: EPA no longer updates this information, but it may be useful as a reference or resource.

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)

Confidence in Cancer Risk Estimates

Confidence in Noncancer Risk Estimates

Overall Confidence

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 exposure concentrations.)

Table A: Composite Judgements to Determine Overall Certainty

Confidence in
Emissions Inventory +
ASPEN Ambient
Modeling Results
Confidence in HAPEM Exposure Modeling Results
lower medium higher
lower Arsenic Compounds
Beryllium Compounds
Cadmium Compounds
Mercury Compounds
Nickel Compounds
Chromium Compounds
Manganese Compounds
Lead Compounds
medium Acetaldehyde
Polycyclic Organic matter (POM)
Diesel PM
Ethylene Dibromide
Ethylene Dichloride
higher Coke Oven Emissions
Propylene Dichloride
Vinyl Chloride
Carbon Tetrachloride
Ethylene Oxide
Methylene Chloride

Overall confidence based on the composite judgements listed in Table A. above can be summarized as follows for a typical individual in a census tract:

  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.
See more details about the determination of overall confidence based on the certainty rank associated with emissions, ambient concentration, and exposure estimates.

Confidence in Cancer Risk Estimates

Confidence in estimates of cancer risk for a typical individual in a census tract:


Confidence in Noncancer Risk Estimates

Confidence in estimates of noncancer risk for a typical individual in a census tract:

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 below:

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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