Polluted air causes an estimated 7 million deaths worldwide each year, according to the World Health Organization. Much of the mortality comes from PM2.5, particulate pollution smaller than 2.5 micrometers in diameter that can enter the lungs and bloodstream and cause respiratory and cardiovascular problems. In addition to particles emitted directly into the atmosphere, ammonia (NH3), nitrogen oxides (NOX), and sulfur dioxide (SO2), which are emitted by factories, ships, cars, and power plants, are all precursors that can contribute to the formation of PM2.5. The effects of particulate pollution are not evenly distributed, however.
