The lowest concentration at which effects are observed depends upon the level of activity, the duration of exposure, and the sensitivity of each individual to ozone. Although this is almost certainly true for all effects of ozone, levels that cause effects have been best characterized for symptom and lung function changes. For example, an adult of average sensitivity exposed for 2 hours while alternating heavy exercise and rest would be expected to experience small lung function and symptom effects following exposure to 120 parts per billion (ppb) ozone. An average adult undergoing moderate exercise might experience similar effects as well as lung injury and inflammation following an 8-hour exposure to 80 ppb ozone. More sensitive individuals may experience such effects at lower concentrations while less sensitive or less heavily exercising individuals may not experience effects at these levels. Children without asthma experience lung function decrements similar to those of adults, but do not report respiratory symptoms at the lowest ozone concentrations. It is not clear whether this is the result of reduced sensitivity with regard to symptoms or whether children are less likely to recognize and report mild symptoms.

U.S. Environmental Protection Agency Website