Air pollution stems from the addition of unnatural compounds and toxins through indirect and direct mechanisms. Direct sources of air pollution include electricity, fuels and transportation, while indirect sources are goods and services that require energy to produce and deliver, such as vehicles and household appliances.
Goods and services for the majority of air pollution, followed by home heating oil and other residential fuels. Electricity, on a residential and commercial scale, contributes the third-largest volume of air pollution, while transportation contributes the least. Common air pollutants include nitrogen oxides, carbon monoxide, volatile organic compounds, sulfur dioxide and particulate matter. Lead and mercury also account for some air pollution, but are primarily associated with water contamination. These toxins are synthetic chemicals and compounds, which means that they do not biodegrade when released into the surrounding air. They cause environmental, health and aesthetic disturbances, and occur primarily in vapor or liquid form. Nitrogen oxides are among the biggest contributors to air pollution; these compounds come from the burning of gasoline, coal, oil and natural gas. Emissions from car and truck exhaust systems are primary sources of nitrogen oxides, which may cause respiratory illness and lung damage to humans. Carbon monoxide is another leading source of air pollution; like nitrogen oxides, carbon monoxide poses human health risks and contributes to acid rain.