For the most part, this air pollution comes in the form of carbon dioxide (CO2) and methane. Common sources of particulate matter (sometimes referred to as "PM 2.5" pollution) include emissions from cars, the burning of coal, secondhand smoke, chemicals from power plants, smoke from cooking, and natural sources like forest fires.
Burning fossil fuels also sends carcinogenic particles into the air, like benzene (a chemical linked to childhood leukemia and blood disorders) and formaldehyde (a known carcinogen).