CO2 is also generated in smaller amounts by forest clearing and cement production. Nitrogen oxides cause smog, irritate the lungs and lower resistance to respiratory infections such as influenza. Smog is formed when nitrogen oxides, which are emitted by burning fossil fuels at electric power plants and in automobiles, mix with other chemicals in the air, sunlight, and heat. The effects of short-term exposure to nitrogen oxides are still unclear, but continued or frequent exposure to concentrations higher than normal may cause increased incidence of acute respiratory disease in children. Nitrogen oxides are an important precursor to both ozone and acidic acid rain and can affect both land and water ecosystems.
Sulfur dioxide comes from the combustion of fuel containing sulfur, mostly coal and oil.
It is also produced during metal smelting and other industrial processes. The major health concerns associated with exposure to high concentrations of SO2 include effects on breathing, respiratory illness, alterations in the lungs' defenses, and aggravation of existing cardiovascular disease. While everybody is adversely impacted by SO2 to some degree, people that are particularly at risk include asthmatics and individuals with cardiovascular disease or chronic lung disease, as well as children and the elderly. Pollution credits represent emissions units that any company is permitted to produce. In any area at a given time, there is a finite amount of pollution credits available in the marketplace - this represents a current or improved pollution level.