Though existing inhalation exposure modeling systems have evolved considerably in recent years, the air quality models that provide the inputs to these systems can be substantially improved from ' the perspective of providing exposure-relevant estimates of air quality metrics. Deriving from the discussion in the previous sections. Various specific and evolving needs are summarized here: • Ambient photochemical modeling systems are not currently optimized for estimating pollutants at the neighborhood scale. Therefore, practical methods are needed for downscaling regional/ urban modeling estimates to neighborhood and microenvironmental scales, with an emphasis on consistency in linking and coupling models at different scales. • Microenvironmental modeling efforts need to balance mechanistic detail and usability by developing simplified but adequate models that take into account mixing, and local indoor or outdoor chemistry. These models can be developed either directly or as simplifications of detailed CFD methods. For population exposure assessment, there is a need for computationally efficient . methods for modeling air quality dynamics in representative realistic distributions of outdoor and indoor microenvironments, in ways that allow aggregation and statistical extrapolations of results across the range of such microenvironments within a regional/urban model cell. These approaches should utilize high-resolution information in urban and suburban topography, combined with detailed microinventories of local emission sources. • Comprehensive air quality models such as CMAQ can be enhanced through incorporation of, or linking with, additional modules for dynamic soil, water, and other compartments. In the future, exposure assessments can be substantially improved through the development and application of comprehensive multimedia models that address local, regional and global scales and can be coupled with multi-pathway human exposure models. • Also, in light of the synergistic effects of cooccuring pollutants, there is a need for expanding the range of airborne contaminants included in the one-atmosphere approach to include allergens and other biological agents. Modeling frameworks for exposure assessment in the past have typically focused on individual contaminants and on subsets of their pathways and sources of exposure, potentially neglecting significant contributions from remaining pathways and sources. In recent years, the focus of environmental human and ecological health risk analyses, pursued by both the research community and regulatory agencies, has been gradually shifting from considerations of single to multiple contaminants and pathways.' In the future, integrative analyses that link environmental, behavioral, and biological considerations2 will allow increased accountability and more realistic and accurate risk assessments.