Mix in sooty particles, known as particulates, and you have smog. High in the atmosphere, the natural ozone layer is good, because it filters out harmful ultraviolet rays. But it is not meant to be breathed by humans, animals and plants, and it is damaging to all three at ground level. Because it forms in warm sunlight, it is primarily a summertime problem, but can crop up in affected areas anytime between April and October. Ten metro areas, home to 57 million people, are severely polluted, exceeding EPA standards by 50 percent or more.
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.