This margin is intended to compensate for what Congress recognized as the incomplete state of our knowledge about the adverse effects of air pollution and the long-term effects of chronic exposure to air pollutants. This standard is also intended to protect sensitive groups, such as people with asthma or emphysema, children, and elderly people, who are adversely affected by ambient levels of air pollution at lower concentrations than will adversely affect the average healthy adult. Nearly one third of our citizens are among these sensitive groups that react to ozone at low concentrations.
Ozone also affects healthy exercising adults at relatively low concentrations.
Ozone is an invisible gas so toxic at high concentrations that it was once widely used to sterilize laboratory instruments and is still employed by many cities to disinfect drinking water. At levels routinely encountered in most cities, ozone damages the lungs and airways causing them to redden and swell. This can cause coughing and burning sensation and shortness of breath. Prolonged ozone exposure increases susceptibility to bacterial infections. Over time, scars and lesions form in the airways.