Exhaust particulate matter. Coloured scanning electron micrograph (SEM) of particulate matter (PM) collected from the exhaust of a car. These are grains of unburnt fuel and partially burnt soot caused by the incomplete combustion of the fuel in the car's engine. Motor vehicles are a major source of atmospheric pollutants. Exposure to pollutants from vehicle exhausts can cause respiratory conditions, such as asthma, and lung cancer.