Contributions of diesel truck emissions to indoor elemental carbon concentrations in homes in proximity to Ambassador Bridge

Ambassador Bridge, connecting Detroit, Michigan and Windsor, Ontario, is the busiest international commercial vehicle crossing in North America, with a large percentage of heavy duty diesel trucks. This study seeks to examine the contribution of diesel truck traffic across Ambassador Bridge to indoor exposure patterns of elemental carbon (EC), a common surrogate for diesel exhaust particles, in homes in close proximity to the bridge. We also aim to understand the relative importance of home ventilation characteristics and wind speed. Measurements were collected as part of the Detroit Exposure and Aerosol Research Study (DEARS). Residential indoor and outdoor EC measurements were collected over five consecutive 24 h periods in both the summer and winter at 16 homes in close proximity to Ambassador Bridge. Ambient concentrations and meteorological data were collected at a central-site monitor, and home air exchange rates were estimated using a perfluorocarbon tracer. The contributions of ambient concentrations and Ambassador Bridge, and potential effect modification by wind speed and home ventilation status were quantified with regression analyses. Both ambient concentrations and the percentage of time a home was downwind from the bridge were associated with an increase in indoor concentrations. Ambient concentrations significantly contributed to indoor concentrations regardless of wind speed category but were a greater influence in home experiencing calm winds. The effect of the percent of time downwind variable on indoor levels was only significant in homes where the ventilation status was high. The distance a home was from the bridge tollbooth complex was not significantly associated with indoor concentrations. We conclude that diesel traffic emissions related to Ambassador Bridge may have an impact on indoor EC exposures. Given that people spend the majority of their time indoors, it is important to evaluate the impact of traffic-related pollution in the home environment.