Therefore, the new standard of 0.08 ppm averaged over an 8-hour period became the current threshold limit for acceptable exposure without risking human health. The EPA issued this as a guideline in 1997, with nationwide enforcement beginning sometime in 2001. North Carolina is currently enforcing the 8-hour standard. Parts per million, or ppm, is a ratio that describes how many parts of something you have per one million equally-sized parts of something else. It is safe to do exercise in the morning, because ozone does not usually reach unhealthy levels until the afternoon when temperatures are the hottest.
Morning is a good time for biking, walking, jogging or other types of strenuous outdoor activities.
As always, it is good to know your own limitations. When children breathe, they take in more air per pound of body weight than adults. In addition to this, their lungs are still developing, making them more vulnerable to the adverse effects of ozone exposure. Scientists don't yet know why, but sensitivity to ozone varies from person to person. Individually, our cars, lawnmowers and refueling habits aren't the culprits in ozone pollution.