While aerobic activity is one of the keys to a healthy lifestyle, air pollution and exercise can be an unhealthy combination. This is especially true if you have asthma, diabetes, or heart or lung conditions. Young children, older people, and people who work or exercise outdoors can be especially susceptible to the effects of outdoor air pollution.

Outdoor air pollution can come from many sources, including:

• Motor vehicle traffic
• Pollen from flowers, trees and shrubbery
• Wind-blown dust
• Burning wood
• Construction
• Agricultural operations, including raising animals and clearing land
• Power plants

Even when you're not exercising, exposure to air pollution can cause health problems. But with the combination of air pollution and exercise, the potential health problems are increased.

One reason for this increased risk may be that during aerobic activity, you usually inhale more air and breathe it more deeply into your lungs. And because you're more likely to breathe deeply through your mouth during exercise, the air you breathe in generally bypasses your nasal passages, which normally filter airborne pollution particles.

Health problems associated with air pollution include:

• Headaches
• Irritation of the eyes, nose and throat
• Damage to airways of the lungs
• Increased risk of asthma development
• Worsening of existing asthma or other lung conditions
• Increased risk of heart attacks and strokes
• Increased risk of death from lung cancer and cardiovascular disease

People living in many areas of the world, especially large cities, are regularly exposed to air pollution levels far beyond the limit recommended by the World Health Organization. What's not clear with air pollution and exercise is how much exposure is a danger, how long you have to be exposed, or which types of outdoor air pollution are the most harmful over time.

However, because exercise has clear health benefits, don't give up on exercise entirely, unless your doctor has instructed you to stop. Research has shown that the long-term benefits of regular exercise outweigh the risks associated with exposure to air pollution.

To stay as healthy as possible while you exercise, focus on ways to minimize your exposure to air pollution. You can limit the effects of air pollution when you exercise in many ways, including:

• Monitoring air pollution levels. Most communities have a system for air pollution alerts. Contact your local or state air pollution control agency, a local hospital, or your doctor for information. Certain websites provide information about air quality from organizations such as the Environmental Protection Agency, and these sites can be accessed at any time. Local radio and television stations as well as newspapers also often report on air quality.

• Timing your workouts carefully. Avoid outdoor physical activity or reduce the intensity and duration of your outdoor exercise when an air quality alert has been issued. Air pollution levels tend to be highest near midday or in the afternoon, so try to avoid outdoor exercise during these times of the day.

  Exercising during rush hour can expose you to higher amounts of pollution. If you can, avoid exercising near roadways where there is heavy traffic.

• Avoiding high-pollution areas. Pollution levels are likely to be highest within 1/4 mile (400 meters) of a road. Urban environments and outdoor smoking areas also have higher pollution levels. If possible, avoid these kinds of areas when exercising.
• Exercising indoors. Vary your routine with occasional indoor activities, especially on poor air quality days. Take a fitness class, check out a local gym or run laps on an indoor track.

If you have asthma, diabetes or another condition, check with your doctor about when it's safe for you to exercise.