Vulnerable individuals who are particularly susceptible to the adverse health impacts of traffic-related air pollutants because of personal, environmental, and socioeconomic factors include the young and the elderly individuals with existing cardiovascular or respiratory diseases, such as asthma and individuals living, working, or frequenting locations near roads with high levels of traffic. Exposure to traffic-related air pollutants has been associated with a range of adverse health effects, such as exacerbation of asthma, asthma onset, impaired lung function, cardiovascular morbidity and mortality, adverse birth outcomes, and cognitive decline ( 3– 5). Because vehicle emissions are released at or near ground level and mostly in urban areas, exposure to traffic-related air pollutants is widespread. Vehicles are a major source of air pollutants, including oxides of nitrogen (NO x), carbon monoxide (CO), particulate matter (PM) less than 2.5 μm in diameter (PM 2.5), and volatile organic compounds (VOCs) ( 1, 2). The paper highlights issues with the development and understanding of the significance of traffic-related exposures through the use of dispersion models in urban-scale exposure assessments and epidemiology studies. The temporal and spatial variability in emissions and exposures over the 2.5-year study period and at more than 300 home and school locations was characterized. Air pollutant concentrations were estimated with a hybrid modeling framework that included detailed inventories of mobile and stationary sources on local and regional scales the RLINE, AERMOD, and CMAQ dispersion models and monitored observations of pollutant concentrations. This paper summarizes the modeling system used to estimate exposures in the Near-Roadway Exposure and Urban Air Pollutant Study, an epidemiological study that examined 139 children with asthma or symptoms consistent with asthma, most of whom lived near major roads in Detroit, Michigan. Although dispersion models have been widely used to evaluate air quality strategies and policies and can represent the spatial and temporal variation in environments near roads, the use of these models in health studies to estimate air pollutant exposures has been relatively limited. In addition to emissions from vehicles, ambient concentrations of air pollutants include contributions from stationary sources and background (or regional) sources. Traffic-related air pollutants, including carbon monoxide, nitrogen oxides, particulate matter less than 2.5 μm in diameter, and diesel exhaust emissions, have been associated with adverse human health effects, especially in areas near major roads. A growing number of nonbiologists, including physical oceanographers, find indicator organisms helpful, and sometimes essential, to their re search.Vehicular traffic is a major source of ambient air pollution in urban areas. In particular, indicator organisms are being used to describe local environments and natural or anthropogenic perturbations to them, although there are pitfalls and problems associated with those usages. In the past two decades, indicator organisms have played increasingly important roles in the development and implementation of public policy. The use of indicator organisms, while certainly not new, is the corner stone for much scientific research. For example, how does one distinguish true indicators from biological anomalies? What kinds of organisms can appropriately be associated with conditions and events at various scales in time and space? To what extent does one species represent other species in the same environmental setting? Can the indicator concept be applied to the context of modern sampling and analytical technology? How can anthropogenic perturbations be distinguished from natural phenomena? How can unlike matrices from differing data bases with differing scales best be matched? Such questions are especially pertinent in today's research environment. Ques tions were posed about the appropriate uses of indicator organisms and the 'rules' governing the application of the indicator concept to particular problems. The need for a volume dealing with the concept of indicator organisms became evident during a symposium on the subject, organized by the present editors for the Southern California Academy of Sciences.

Kinsie dispersio