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Motorists in the United States spend more than four times as many hours stuck in traffic as they did 20 years ago, according to the U.S. Department of Transportation. Not only does this try your patience, new research suggests it can also do serious harm to your heart.

In a study published last fall in the New England Journal of Medicine, researchers in Germany analyzed the records of almost 700 men and women who experienced a heart attack. Heart attacks were roughly three times as likely to occur within an hour of sitting in heavy traffic, compared with other situations. Most of those who had heart attacks following traffic jams were driving their own cars, although some were caught in traffic while using public transportation.

It's not clear whether the heart attacks were triggered by air pollution or other factors, such as stress. But previous research has established both as important heart risks.

Living, working or exercising in polluted air has been linked to an increased risk of a heart attack in a number of studies. Pollution particles may trigger ischemia, a potentially catastrophic shortage of oxygen to the heart muscle. Carbon dioxide emitted from vehicles' exhaust pipes can displace oxygen in the blood, threatening the heart.

A December 2004 study tied the specific type of pollution generated by stop-and-go traffic to markers of cardiovascular disease in nine healthy, nonsmoking highway troopers in North Carolina.

In addition, a wealth of research has linked anger and other negative emotions to heightened heart attack risk. These emotions can cause a surge in stress-related hormones, such as adrenaline and cortisol, which prime the body to face an emergency. That surge causes physiological changes, such as elevated blood pressure, which can trigger a heart attack or stroke, especially in people whose arteries are already clogged.

You can help protect your heart from these dangers by minimizing your exposure to pollutants and learning to control the way in which you respond to stress behind the wheel. Traveling off-peak (when you can) may lessen the threat, and planning for delays by allowing more time to reach your destination can reduce the anxiety of being late. But if you find yourself in a jam, consider the following strategies:

Keep windows closed in traffic. This is especially important on hot or hazy days, when pollution peaks. It may help to temporarily turn the air-conditioning system to "Recirculate" or "Maximum AC." This recycles the air inside the car rather than bringing in outside air.

Stay off the phone if traffic is moving at all. Even hands-free phones can slow your reaction times, according to a study published in 2003 in the Journal of Experimental Psychology: Applied.

Don't vent. Ask yourself, "Will it really make a difference if I slam on the horn, cut in front of the truck or curse at that person who's trying to cut me off?" Chances are it won't, and research shows you're liable to feel even angrier afterward.

Use calming techniques. Find a good radio station or listen to tapes or CDs with favorite songs, comedy routines or book readings. Sing or hum a soothing tune. Take slow, deep, steady breaths. Consciously tense and then relax each muscle.

How you equip your car may also help mitigate heavy traffic's harmful effects. If you're buying a new vehicle, consider asking the dealer about models that come with a cabin filter (also called a pollen filter or microfilter) in the heating, ventilation and air-conditioning system. Manufacturers say that the filters - which are available in certain foreign and domestic models - prevent pollutant particles from entering the cabin. We, however, have not tested the validity of those claims.

By the editors of Consumer Reports at