Where does the pollution come from?

Cars, trucks, ships, and planes are the single biggest source of nitrogen oxides in the country, and a huge chunk of the fine particles floating around most cities. Anywhere there's a lot of traffic, there's a lot of pollution to breathe.

Power plants and heavy factories, especially the ones still burning coal or gas, put out sulfur dioxide, nitrogen oxides, mercury, and tiny particles. A single old plant can affect air quality for miles around.

This one surprises people. Wood-burning stoves, older furnaces, and gas ranges all add pollution to the neighborhoods around them, and to the air inside your own house. It gets noticeably worse in winter.

Not every source is human. Wildfires, dust storms, pollen, and volcanoes can push air quality into the red across whole regions in a matter of days, and climate change is making the big ones more common.