Air pollution comes from many different sources, both natural and human-caused.
Natural processes that affect air quality include volcanic activity, which produce sulfur, chlorine, and ash, and wildfires, which produce smoke and carbon monoxide. Cattle and other animals emit methane as part of their digestive process. Even pine trees emit volatile organic compounds (VOCs).
Human-caused air pollution comes from many sources. Industrial plants, combustion-fired power plants and vehicles with internal combustion engines generate nitrogen oxides, VOCs, carbon monoxide, carbon dioxide, sulfur dioxide and particulates. Cars are the primary source of these pollutants in urban areas. Stoves and incinerators, especially ones that are coal or wood-fired, and farmers burning their crop waste produce carbon monoxide, carbon dioxide, as well as particulates. Other human-made sources include aerosol sprays and gases leaking from refrigeration systems, as well as fumes from paint, varnish, and other solvents. Additional pollutants, like ozone, are made in the atmosphere when human-made gases combine chemically.
Air pollution can cause a range of impacts to human health such as irritation to eyes, throat, and lungs. Wheezing, coughing, burning eyes, chest tightness, headaches, and difficulty breathing are all commonly reported when the Air Quality Index is high. Increased doctor visits, hospitalization, and school absences also frequently occur at such times. Not all health impacts are immediate; slow and subtle health effects from long-term air pollution exposure may culminate in life-threatening illnesses such as cancer.
Certain people appear to be more vulnerable to air pollution, namely the elderly, the young, and those with cardiopulmonary disease such as asthma or severe bronchitis. Ozone and air particles appear to be especially harmful to children’s health. This is because their lungs are still growing, they are often outside for long periods, and they are usually quite active. As a result, pound for pound they inhale more polluted outdoor air than adults typically do.