Sulfur dioxide: Sulfur dioxide can cause premature deaths and asthma attacks and is a health threat when it forms "particulate matter" (see below). Children, the elderly and anyone with emphysema or bronchitis are particularly at risk. The majority of airborne sulfur dioxide nationwide comes from electric utilities, especially those that burn coal. Sulfur dioxide contributes to the formation of acid rain, which can damage trees, crops and historic buildings, and can turn soil, lakes and streams acidic -- leaving them unable to support life. Sulfur dioxide gases contribute to haze, dramatically reducing visibility. Mountaintop views in Great Smoky Mountains National Park have been reduced by as much as 80 percent by haze.

Nitrogen oxides: When Atlanta has a smog alert day, nitrogen oxides are partially to blame. Ground level ozone, or smog, is formed when nitrogen oxides, hydrocarbons and sunlight combine. Cars are the top source, followed by power plants. Children and people with lung diseases such as asthma and people who work outside can find it difficult to breathe because of these pollutants. Lung damage can result. Nitrogen oxides also contribute to acid rain formation.

Particulate matter: Tiny particles released in the air or formed by gases are of great concern to public health officials because they can kill. Pollutants released by power plants can turn into particles after reacting with sunlight and water vapor. When inhaled, fine particles can cause breathing problems and death by affecting the cardiovascular system.

Mercury: Coal-fired power plants are the nation's largest source of mercury released into the air. When mercury enters lakes and streams and gets into the food chain, it can be highly toxic. People are usually exposed to mercury by eating contaminated fish. Exposure can cause neurological problems and slow child development.

Carbon dioxide: Carbon dioxide is formed by burning coal, oil, natural gas and wood. It can increase the heat in the atmosphere by acting like the glass in a greenhouse, causing global warming.

Source: Environmental Protection Agency; staff research