Table 2

Sources of air pollution

Particulate matterTrafficSummer and winter
Forest fires
Wood smoke
Infiltrates indoors
Ultrafine particulate matter (< 0.1 μm in diameter)Diesel trafficClose to busy roads
OzoneSecondary to aerochemical reaction to nitrogen oxides and volatile organic compoundsSummer afternoons; heat and sunshine
Nitrogen dioxideTrafficClose to busy roads
Carbon monoxideTrafficClose to busy roads
Sulfur dioxideIndustrial plants—combustion and refining of coal, oil, and metal-containing oresClose to industrial sources
Gasoline, although sulfur content has recently been reduced