Validation of a novel air toxic risk model with air monitoring.

Three modeling systems were used to estimate human health risks from air pollution: two versions of MNRiskS (for Minnesota Risk Screening), and the USEPA National Air Toxics Assessment (NATA). MNRiskS is a unique cumulative risk modeling system used to assess risks from multiple air toxics, sources, and pathways on a local to a state-wide scale. In addition, ambient outdoor air monitoring data were available for estimation of risks and comparison with the modeled estimates of air concentrations. Highest air concentrations and estimated risks were generally found in the Minneapolis-St. Paul metropolitan area and lowest risks in undeveloped rural areas. Emissions from mobile and area (nonpoint) sources created greater estimated risks than emissions from point sources. Highest cancer risks were via ingestion pathway exposures to dioxins and related compounds. Diesel particles, acrolein, and formaldehyde created the highest estimated inhalation health impacts. Model-estimated air concentrations were generally highest for NATA and lowest for the AERMOD version of MNRiskS. This validation study showed reasonable agreement between available measurements and model predictions, although results varied among pollutants, and predictions were often lower than measurements. The results increased confidence in identifying pollutants, pathways, geographic areas, sources, and receptors of potential concern, and thus provide a basis for informing pollution reduction strategies and focusing efforts on specific pollutants (diesel particles, acrolein, and formaldehyde), geographic areas (urban centers), and source categories (nonpoint sources). The results heighten concerns about risks from food chain exposures to dioxins and PAHs. Risk estimates were sensitive to variations in methodologies for treating emissions, dispersion, deposition, exposure, and toxicity.

Multipathway screening factors for assessing risks from ingestion exposures to air pollutants.

Regulatory agencies are frequently called upon to assess the potential for significant environmental impacts from air pollution emissions. These assessments often entail air dispersion modeling to estimate air concentrations that can be compared with standards or health benchmarks. Some air pollutants can also impact human health through pathways in media besides air. Risk assessment models are available that consider pollutant deposition, movement, uptake, and other processes on land and water and in biota, but they are typically effort-intensive. A screening-level assessment of potential multipathway effects would be useful. We developed multipathway screening factors (MPSFs) that can be applied to inhalation risk estimates to give screening estimates of risks via ingestion pathways. The MPSFs were generated using a generic multipathway risk assessment, consisting of air dispersion and deposition modeling followed by risk modeling for 42 persistent, bioaccumulative air pollutants. MPSFs are defined as the ratio of ingestion risks to inhalation risks. We report here the results of a sensitivity analysis that evaluates the effects on the MPSF ratio of varying inputs to the air dispersion and deposition modeling analysis. Model input parameters were systematically varied and multipathway risks recalculated. From the sensitivity analysis results, reasonable upper-bound values for the ratio of ingestion risks to inhalation risks for each pollutant were selected. The particle size distribution and the method of calculating particle deposition had the most disproportionate effect on inhalation versus ingestion risks and the greatest effect on MPSFs. Risk calculations are often done at the points of maximum air concentration and maximum deposition. In this study, the MPSFs were usually highest at the location of the maximum inhalation risk.