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 | |