Dr. David Khan, an allergist at the University of Texas Southwestern Medical Center in Dallas, recommends exercising indoors on days ozone smog is high. Running on a treadmill or power walking in a shopping mall are two options.

"Ozone smog can cause asthma attacks, coughing, wheezing, shortness of breath and chest pain when inhaled deeply," Khan says in a statement. "People most sensitive to ozone smog are children, the elderly, people with asthma and other lung diseases and healthy people who work or exercise outdoors."

For those who must be outside, Khan suggests avoiding high traffic areas and heading out in the early morning. Levels of ozone smog are lowest in the morning around sunrise.

As traffic emissions and sunlight increase, so do smog levels, Khan says.

Ground-level ozone, the main ingredient of urban smog, is an air pollutant with harmful effects on the respiratory systems of animals.

Daily air quality -- by state -- is available at the government Web site http://airnow.gov.