When everyday air pollution combines with warm, sunny weather the result is ozone smog. Ozone at ground-level affects your health by irritating your breathing passages and your eyes. People most at risk from ozone smog have asthma, emphysema and other respiratory diseases. Infants, pregnant women, the elderly and people with heart disease also are susceptible to ozone's effects. And if you work or play hard outdoors, your exposure to air pollution increases.
Ozone also harms plant growth.
While ground-level ozone is harmful, ozone is beneficial in the upper atmosphere where it protects the earth from harmful solar radiation. People don't produce ozone smog directly. Ground-level ozone forms when there's enough heat and sunlight to cause chemical reactions between other air pollutants. As the day wears on, there's more ozone in the air. In the Puget Sound and Portland-Vancouver areas ozone gets to be a problem when high temperatures remain over 80 degrees for more than a day or two.