We all know that outdoor aerobic exercises like running and biking are good for your health. But during the hottest days of summer, it’s not just excess heat we have to worry about, but smog, the concentrated air pollution you can often see hovering over the cityscape. During the hot summer months, smog can become a serious health problem in the Boston area. We are often downwind from the Midwest’s coal-fired power plants as well as the East Coast’s major cities.

Among air pollution’s cast of characters, ground-level ozone gas and the tiniest air pollution particles called PM2.5 play leading roles. While the PM2.5 particles come directly from our car exhaust, factories and distant coal-fired electric plants, irritating ozone gas builds up in the afternoon when sunlight chemically reacts with air pollution on hot days. The recent record-breaking heat waves dramatically increased the levels of ozone, which can cause sunburn-like effects on our delicate lungs.

Here’s how smog affects the human body. A mixture of the small PM2.5 particles get directly into our bloodstream by sneaking through the tiniest air sacs called alveoli. Once in your blood stream, these air pollution particles act like cigarette smoke, triggering inflammation that makes cholesterol more sticky. This in turn promotes cholesterol blockages that build up and can cause heart attacks years later. In fact, people living in the most polluted cities in the U.S. were found to have up to a 10 percent higher chance of having a heart attack compared to those living in less polluted cities.

But there are a number of other negative health impacts as well. The Harvard School of Public Health has been gathering data on this for the past 30 years, and researchers are trying to get the word out to the public at large. However, it’s hard to compete with the loud voice and deep pockets of the air polluting industries. So what has research taught us so far? Children, senior citizens, and people with heart and lung conditions are especially vulnerable, but smog can impact even those of us in good health. Symptoms during high air pollution days range from eye and nose irritation that can amplify allergy and respiratory infection, to increased cardiac and respiratory deaths. It’s not surprising that the Harvard scientists found that children living in cities with the most air pollution have more asthma visits to the emergency room. But they were surprised to find higher rates of premature births in pregnant women and more sudden cardiac death from lethal heart arrhythmias in older adults.

While the best medicine is to decrease pollution, like using energy-efficient vehicles and appliances, there are things you can do personally to protect your lungs from higher levels of air pollution. First — timing is everything. Urban smog is at its lowest early in the morning, so this is the best time for a jog or bike ride. Try to avoid exercising around traffic or anywhere you can smell car exhaust. If you can smell it, the levels are likely too high to be healthy. On the most polluted days, try to exercise indoors. (To see the local pollution levels on any given day, visit www.airnow.gov.) Children as well as adults with asthma, active heart disease, or lung diseases, such as chronic bronchitis or emphysema, should be especially careful. If you are stuck in a traffic jam, try switching your car air to “re-circulate” to avoid breathing in the tailpipe exhaust of the car in front of you. Avoid close proximity to idling vehicles in parking lots — better yet, encourage others to cut their engines if they are not moving within a minute or two.

There is some hope for our children. The Obama administration is about to heed the advice of the Environmental Protection Agency’s scientific advisors and reverse the Bush EPA’s adoption of the lax air pollution standards supported by industry for the past 10 years. This could mean that someday going outside really can be a “breath of fresh air.”

Rick Donahue is a clinical instructor at Harvard Medical School with 20 years of experience delivering complete primary care. His private practice in Back Bay, MA, Personal Health MD is dedicated to providing comprehensive state of the art primary care.

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