Ozone is a severe irritant that can cause choking, coughing and stinging eyes. It damages lung tissue, aggravates respiratory disease and makes people more susceptible to respiratory infections. On days when ozone levels are high, emergency room visits for asthma attacks have been shown to increase by as much as 36 percent. Children and the elderly are especially vulnerable to ozone's harmful effects, as are adults with asthma and other lung and heart ailments. People who exercise on high-ozone days may actually be damaging their lungs by breathing ozone.
Ozone also inhibits plant growth and can cause widespread damage to crops and forests.
The standards, which are controversial, were revised as the result of a court decision that required EPA to revise its standards. The new standard is based on the theory that prolonged exposure to lower levels of ozone is more harmful than brief exposure to high levels.