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.