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. A company that reduces its emissions will have a surplus of credits that can be banked, sold or retired. Companies that are unable or unwilling to reduce emissions below the required amount can purchase credits from others in the marketplace. The overall level of pollution is not reduced by trading credits. Purchasing and retiring pollution credits removes them from the marketplace and reduces the total amount of emissions possible across the market's region.