NCDENR - Division of Air Quality - Exposure Assessment Guidance



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Exposure Assessment Guidance for Site-Specific/Inhabitability Demonstrations
This document presents guidelines for use by the Division of Air Quality in evaluating site-specific risk assessments submitted by industries seeking to demonstrate compliance with the North Carolina Air Toxics rules. These guidelines received input from the Scientific Advisory Board on Toxic Air Pollutants and outline criteria for evaluation of site-specific demonstrations submitted in accordance with 15A NCAC 2H.0610(c)(2)(A). The first portion of the demonstration is a detailed modeling exercise in accordance with North Carolina modeling guidelines and procedures. A detailed presentation of the modeling results must be submitted and approved by the Air Quality Analysis Unit of the Air Quality Section. A graphic representation of the modeled results for each subject toxic air pollutant should be presented on a USGS topographic map or equivalent (with sufficient detail to depict local features) and should include: 1) isopleths of concentration at and above the AAL in areas beyond the property line described as uninhabitable, sufficient to characterize concentration gradients, 2) isopleths of highest modeled concentration in habitable areas, sufficient to characterize concentration gradients, and 3) points of highest modeled concentration beyond the property line. These detailed maps will be used as a basis for the exposure assessment. Toxic Air Pollutants (TAPs) with 15-minute or 1-hour averaging times will not be considered for site-specific exposure assessments. Only carcinogens, and in some cases chronic toxicants with 24-hour averaging times, will be considered. Exposure Assessment Fenceline AAL exceedances may not 1) impact inhabitable areas at greater than 100% of the AAL for the duration of the averaging time or 2) create unacceptable risks for individuals impacted through exposures in uninhabitable and habitable areas. Current and potential future activity patterns and development must be considered. Elements of an exposure assessment should include: Documentation of "uninhabitable" finding: factors that preclude habitation or continuous occupancy for the duration of the averaging time in the areas where appropriate dispersion models indicate the AAL will be exceeded. This might include physical or regulatory constraints, etc. Full assessment of all anticipated human activities in areas where the AAL will be exceeded. Special consideration should be given to the longest expected duration of exposure and the highest modeled off-site ambient air concentrations in habitable and uninhabitable areas. This information will be used to estimate the extent of exposure to the TAP in question. Emissions patterns should be carefully documented. Periodic release of substantial amounts of the compound of concern could result in significant exposures, especially if those emissions coincide with expected activity patterns in exceedance areas. In some cases, short-term models may be needed to more accurately assess the extent of exposure during short-term emissions events. Exposures will be deemed unacceptable in the following situations: 1) the time-weighted average exposure exceeds the exposure anticipated through compliance with the AAL; and/or 2) a compound which causes both acute and chronic health effects but is only regulated by a long-term (24-hr or annual averaging time) AAL is creating an exposure scenario that involves unacceptable acute risks. Inhalation is currently the only relevant route of exposure for the North Carolina air toxics program. All exposure assessments need only account for this route of exposure, but special consideration of expected breathing rates should be documented (i.e. light activity, heavy activity, etc.). EPA's exposure factors handbook is the preferred source for breathing rates.

Exposure Assessment Guidance for Site-Specific/Inhabitability Demonstrations

This document presents guidelines for use by the Division of Air Quality in evaluating site-specific risk assessments submitted by industries seeking to demonstrate compliance with the North Carolina Air Toxics rules. These guidelines received input from the Scientific Advisory Board on Toxic Air Pollutants and outline criteria for evaluation of site-specific demonstrations submitted in accordance with 15A NCAC 2H.0610(c)(2)(A).

The first portion of the demonstration is a detailed modeling exercise in accordance with North Carolina modeling guidelines and procedures. A detailed presentation of the modeling results must be submitted and approved by the Air Quality Analysis Unit of the Air Quality Section. A graphic representation of the modeled results for each subject toxic air pollutant should be presented on a USGS topographic map or equivalent (with sufficient detail to depict local features) and should include: 1) isopleths of concentration at and above the AAL in areas beyond the property line described as uninhabitable, sufficient to characterize concentration gradients, 2) isopleths of highest modeled concentration in habitable areas, sufficient to characterize concentration gradients, and 3) points of highest modeled concentration beyond the property line. These detailed maps will be used as a basis for the exposure assessment. Toxic Air Pollutants (TAPs) with 15-minute or 1-hour averaging times will not be considered for site-specific exposure assessments. Only carcinogens, and in some cases chronic toxicants with 24-hour averaging times, will be considered.

Exposure Assessment
Fenceline AAL exceedances may not 1) impact inhabitable areas at greater than 100% of the AAL for the duration of the averaging time or 2) create unacceptable risks for individuals impacted through exposures in uninhabitable and habitable areas. Current and potential future activity patterns and development must be considered. Elements of an exposure assessment should include:

Documentation of "uninhabitable" finding: factors that preclude habitation or continuous occupancy for the duration of the averaging time in the areas where appropriate dispersion models indicate the AAL will be exceeded. This might include physical or regulatory constraints, etc.
Full assessment of all anticipated human activities in areas where the AAL will be exceeded. Special consideration should be given to the longest expected duration of exposure and the highest modeled off-site ambient air concentrations in habitable and uninhabitable areas. This information will be used to estimate the extent of exposure to the TAP in question.
Emissions patterns should be carefully documented. Periodic release of substantial amounts of the compound of concern could result in significant exposures, especially if those emissions coincide with expected activity patterns in exceedance areas. In some cases, short-term models may be needed to more accurately assess the extent of exposure during short-term emissions events. Exposures will be deemed unacceptable in the following situations: 1) the time-weighted average exposure exceeds the exposure anticipated through compliance with the AAL; and/or 2) a compound which causes both acute and chronic health effects but is only regulated by a long-term (24-hr or annual averaging time) AAL is creating an exposure scenario that involves unacceptable acute risks.
Inhalation is currently the only relevant route of exposure for the North Carolina air toxics program. All exposure assessments need only account for this route of exposure, but special consideration of expected breathing rates should be documented (i.e. light activity, heavy activity, etc.). EPA's exposure factors handbook is the preferred source for breathing rates.