They consumed nearly 900 million tons of coal and more than 125 million barrels of oil. Older, dirtier power plants are often exempted from meeting air pollution standards set under the Clean Air Act, as it was expected that they would be soon put out of service and replaced with cleaner, more efficient plants. Instead, however, their lives are being extented, often without any significant clean-up of emissions. Sulfur dioxide is a major factor in the formation of acid rain, which damages forests, lakes, and wildlife habitat. Acid rain also corrodes infrastructure and buildings.
Nitrogen oxide is a major factor in the formation of smog as well as acid rain.
Build-up of these gases, however, is now believed by the world's scientists to be a key factor in the recorded rise in average temperature around the globe. While summer heat waves result from a combination of factors, global warming increases the likelihood of their occurrence and intensity. High levels of smog are formed when heat reacts with air pollutants such as nitrogen oxide to generate high levels of ground-level ozone. Particulate matter is easily airborne far downwind from the source of emission. It is an important factor aggravating respiratory diseases such as asthma.