Health Effects: Ozone, the main component of smog, is the most widespread air pollutant. It is a powerful respiratory irritant at levels found in much of the nation during warn weather. The U.S. Environmental Protection Agency reports that 107 million Americans live in areas that violate health standards for ozone. Symptoms include shortness of breath, pain during deep breaths, wheezing and coughing.

Ozone exposure has been linked to increased hospital admissions and emergency room visits for asthma and other respiratory problems. It can also reduce the body's resistance to infection. Long-term, repeated exposure to high levels of ozone may lead to large reductions in lung function, inflammation of the lung lining and more frequent and severe respiratory discomfort. The American Lung Association likens ozone exposure to a "sunburn on the lung." Recent studies have also linked ozone to premature death, though research continues on this topic.

Ozone is especially dangerous for children, the elderly, people with chronic lung and heart disease -- even healthy people who exercise outdoors.

Children are at particular risk because their lungs are still growing and developing. They breathe more rapidly and more deeply than adults do, so a greater doze of air pollution may be delivered to their lungs. Children also spend significantly more time outdoors, especially in summer when ozone levels are highest.

Sources: Ozone is not emitted directly into the air, but is formed by the reaction of volatile organic compounds (VOCs) and nitrogen oxides (NOx ) in the presence of heat and sunlight. VOCs are emitted by a variety of sources, including motor vehicles, chemical plants, refineries, and other factories. NOx is emitted by motor vehicles, electric power plants, and other combustion sources. Ozone can be transported into an area from pollution sources found hundreds of miles upwind.