|Limitations in the 1996 National-Scale Air Toxics Assessment: Uncertainty
Uncertainty: With what confidence can these results be
Note that a more refined analysis of uncertainty is underway and should be available here in the summer of 2002.
Confidence (considers all exposure components which include emissions, ambient concentration and exposure estimates)
in Cancer Risk Estimates
Noncancer Risk 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)
||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
See more details
about the determination of overall confidence based on the certainty rank associated with emissions, ambient concentration, and exposure estimates.
||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 Cancer Risk Estimates
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 Noncancer Risk Estimates
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?
of uncertainty did the national-scale assessment include?
How was the
uncertainty analysis conducted?
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Limitations, Variability, and Uncertainty Page?
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