How to Exercise Safely in the Heat

Food is also important for staying hydrated. Eating regular meals and snacks throughout the day-especially foods such as bananas that contain potassium and other vital nutrients (in addition to water)-will help prep your body for a workout in the heat.

'Maintaining good hydration is key to safety in the heat, but it is not all protective,' Bergeron says. Research that Bergeron took part in has shown that even if kids are well hydrated, they can overheat and even experience heat stroke if they exercise too intensely and get hot too quickly. 'You can still overheat and have serious problems when you are well hydrated,' he explains.

Protect your lungs
Dehydration and heat exhaustion aren't the only hazards of exercising on hot days. The stagnant air caused by heat and humidity tends to trap airborne pollutants, such as car exhaust, which react in the presence of sunlight to form ozone, a main ingredient in smog.

Working out in smoggy air can cause lung trouble in people with respiratory conditions such as asthma and chronic obstructive pulmonary disease (COPD). But even in people without lung conditions, hard exercise on days with lots of smog can reduce lung function and create a reaction akin to an asthma attack, says Norman Edelman, MD, chief medical officer of the American Lung Association.

'Ground-level ozone is an irritant. If you breathe in too much, it irritates the nose, throat, and lungs,' Dr. Edelman says. 'It's like you're getting a sunburn in your airways. Ozone promotes inflammation of the airways, and they get red and swollen.'

A partnership of government agencies publishes daily air-quality forecasts for every zip code in the U.S. at AirNow.gov, ranking air quality on a six-point, color-coded scale ranging from green ('good') to dark red ('hazardous').

The green and yellow ('moderate') levels are normal for most of the country during the summer, Dr. Edelman says. The orange level can cause problems in people who are sensitive to ozone (like asthmatics), however, and on red days he recommends that everyone stay indoors to exercise.

'It's important not to macho it,' he says. 'Lots of people say, 'I'm feeling tight in the chest, but I can run through it.' But the more you run, the more bad air you take in, and if you have [sensitive] airways it can be very irritating.'

If you have to exercise outdoors, the best time to do it is early in the morning, Dr. Edelman says. He also suggests avoiding heavily trafficked roads to reduce the pollutants you inhale. And if you have asthma or other chronic lung disease, talk with your doctor about your exercise routine and make sure you have a plan in case you experience breathing problems.