Sources of air pollution
| POLLUTANT | SOURCE | CONDITIONS |
|---|---|---|
| Particulate matter | Traffic | Summer and winter |
| Forest fires | ||
| Wood smoke | ||
| Infiltrates indoors | ||
| Ultrafine particulate matter (< 0.1 μm in diameter) | Diesel traffic | Close to busy roads |
| Ozone | Secondary to aerochemical reaction to nitrogen oxides and volatile organic compounds | Summer afternoons; heat and sunshine |
| Nitrogen dioxide | Traffic | Close to busy roads |
| Carbon monoxide | Traffic | Close to busy roads |
| Sulfur dioxide | Industrial plants—combustion and refining of coal, oil, and metal-containing ores | Close to industrial sources |
| Gasoline, although sulfur content has recently been reduced |