When you take your workout outdoors this summer, aside from slathering on sunscreen, you may want to check the ozone report.  Exercising when ozone levels are moderate to high has been deemed unhealthy, and even linked to an increase in deaths, but until now, there’s been little known about how or why it’s harmful.

A new study published in the journal Circulation found that exposure to ozone is associated with unhealthy cardiovascular changes. Though the changes are temporary, they may increase the risk of heart attacks.

Ground level ozone (different from ozone in the upper atmosphere) builds up when pollutants react to sunlight, making it more common in the summer.  Studies looking at the health effects of ozone have largely focused on its effects it has on our lungs.

The new study, conducted by Robert B. Devlin, senior scientist at the Environmental Protection Agency's (EPA), tested changes to the cardiovascular system, not the lungs. It also isolated ozone from the other pollutants that contribute to health problems. Past studies have not teased out the effects of the pollutants in the air from the effects of the ozone.

The researchers exposed 23 volunteers between the ages of 19 and 33 to ozone for two hours. During each exposure, participants alternated riding a stationary bicycle for 15-minutes and resting for 15 minutes.

For the control arm of the study, on a different day, they exposed the same subjects to clean air for two hours and asked them to perform the same exercise regimen.

None of the participants reported any symptoms after the exposure to the ozone.  However, tests showed significant cardiovascular changes after the exposure as well as the following morning compared to the ozone-free exposure. These changes included an increase in a marker for inflammation that appears to play a key role in heart disease, a reduction in clot-dissolving molecules, and a change in heart rhythm.

The authors suggested that these changes could potentially put a susceptible individual at risk for a heart attack or stroke.  The study did not look at long term effects of exercising when ozone levels are high.

If ozone levels are high (air quality index is 101-150), everyone should limit outdoors exercise. People who may be sensitive to ozone, including those with lung disease, children, older adults and people who exercise or work vigorously outdoors, may experience symptoms at lower levels.

Check the EPA’s AirNow website for air quality alerts and link to their ozone brochure for an explanation of what the alert levels mean.

Here’s some advice from the EPA on lowering your exposure to ozone when levels get high:

• Try to exercise in the morning or evening, when ozone levels are usually at the lowest.

• Do a less intensive exercise when ozone levels are high. The more intense you exercise, the greater your exposure to ozone. Lower intensity exercises include walking, light jogging or an easy hike or bike ride. Higher intensity activities include playing basketball, soccer, and vigorous running, cycling or hiking.

• Slash some time off your outdoor exercise routine to reduce exposure.