While children may spend only a few hours per day on school buses, the elevated levels of air pollution that can be encountered on-board school buses can add considerably to their daily and annual exposures to PM2.5 and DPM. This is a concern because children in many Ontario communities are already exposed to levels of air pollution that are harmful to their health. The Ontario Medical Association estimates that air pollution contributes to approximately 1,829 premature deaths, 16,907 hospital admissions, and 59,696 emergency room visits in Ontario each year.