Particles can originate from a variety of mobile, stationary, and natural sources, including power plants, diesel trucks, wood stoves, and dust from roads. The chemical and physical composition of particulates varies widely. Most of the smallest particles result from burning fossil fuels. These small particles can remain aloft for days, even weeks, and can travel thousands of kilometers. For example, researchers at the National Oceanic and Atmospheric Administration's station at Mauna Loa Observatory have tracked carbon particulates to Hawaii from specific smokestacks in Beijing.
Particulates pose a major hazard for human health.
The adverse effects have been traced mainly to small particles with an aerodynamic diameter of 10 microns or less, which can reach deep into the lungs. Smaller particles can go even deeper into the lungs, where those that are still tinier can enter the blood stream. These particles can disturb the lungs physically and chemically. They can directly irritate the lungs, and they can carry toxic heavy metals and other pollutants. The average person breathes several million liters of air a year.