Burning fossil fuels releases carbon that has been stored underground for millions of years into the atmosphere. The carbon in these fossil fuels is transformed into carbon dioxide, the predominant gas contributing to the greenhouse effect, during the combustion process. Nonetheless, increases in emissions of carbon dioxide and other gases, such as methane, due to fossil fuel burning and other human endeavors, accelerate heat-trapping processes in the atmosphere, gradually raising average worldwide temperatures. While carbon dioxide is absorbed and released at nearly equal rates by natural processes on the earth, this equilibrium is disrupted when large amounts of carbon dioxide are released to the atmosphere by human activities, such as the burning of fossil fuels.
High concentrations of sulfur dioxide affect breathing and may aggravate existing respiratory and cardiovascular disease. Sensitive populations include asthmatics, individuals with bronchitis or emphysema, children, and the elderly. Sulfur dioxide is also a primary contributor to acid rain, which causes acidification of lakes and streams and can damage trees, crops, historic buildings, and statues. In addition, sulfur compounds in the air contribute to visibility impairment in large parts of the country. This is especially noticeable in national parks. Sulfur dioxide is released primarily from burning fuels that contain sulfur (such as coal, oil, and diesel fuel). Stationary sources such as coal- and oil-fired power plants, steel mills, refineries, pulp and paper mills, and nonferrous smelters are the largest releases of these gases.
Nitrogen oxide include various nitrogen compounds like nitrogen dioxide and nitric oxide. These compounds play an important role in the atmospheric reactions that create ground-level ozone and acid rain. Ozone is formed when volatile organic compounds (VOCs) react in the presence of heat and sunlight. Nitrogen oxides form when fuels are burned at high temperatures. The two major sources of nitrogen oxide are transportation vehicles and stationary combustion sources, such as electric utility and industrial boilers