As environmentalists have pointed out, it can be as dangerous to be outdoors behind a city bus -- walking or bicycling -- as it is to be in front of one. All the exhaust and smoke -- even when they have been reduced by clean-air technology -- can damage a person's health. The dangers of urban air pollution are of special concern to those who exercise by running, bicycling or skating. These individuals, while trying to help their bodies through exercise, should take care that they do not harm themselves through exposure to air pollution.

Dr. Ronald Crystal, chief of pulmonary and critical care medicine at NewYork-Presbyterian Hospital/Weill Cornell Medical Center, says that air pollution is definitely a problem for those who work out in the city. The main culprits are ozone, fine particulate matter, and carbon monoxide, he says. These pollutants irritate the lungs and respiratory system, and can exacerbate the problems of individuals with underlying disease -- such as asthma, bronchitis, emphysema or cardiopulmonary maladies.

If you have heart or lung disease, Dr. Crystal says, in summer, you should, if possible, exercise indoors, preferably in an air-conditioned room. If you must go outdoors, the early morning or evening is best. It will be cooler, the sun is not at its peak, and the ozone levels will be at their lowest.

Dr. Crystal says that epidemiologic studies have linked air pollutants to harmful effects on the heart and lungs, to emergency hospital admissions, and to deaths. The pollutants affect the lungs by causing inflammation or irritation of the airway lining. More mucus and phlegm is produced, he says, and small muscles surrounding the airway respond by squeezing down. The work of breathing increases and it becomes more difficult to get oxygen into the body.

In addition to fine particulates -- which are emitted by the diesel engines of trucks and buses -- the two most significant environmental culprits are carbon monoxide and ozone, Dr. Crystal says. Carbon monoxide arises from cigarette smoke and automobile exhaust. It has a tremendous ability to force oxygen out of our circulatory system -- it combines with hemoglobin 200 times faster than oxygen. Overexposure may lead to headache, dizziness, confusion, and dangerous increases in body temperature.

Ozone, which is a large component of the smog found in cities like Los Angeles and New York, results from the interaction of sunlight and chemicals found in car exhaust. Ozone adversely affects a person's breathing pattern and causes the airways in the lungs to become smaller and more resistant to oxygen exchange. Because of ozone, a person working out has difficulty taking deep breaths, and has to breathe faster. As a result, the exercise becomes more stressful and difficult.

Dr. Crystal offers these simple tips:

- Do not run on or near roads where there is heavy truck or bus traffic.
- Work out in the early morning or later in the evening.
- Exercise indoors if possible.
- If you experience any difficulty breathing, stop your exercise immediately and see your doctor.

By taking a few simple precautions, Dr. Crystal says, you can make your exercise a wholly good thing, and keep air pollution out of your body.

NewYork-Presbyterian Hospital/Weill Cornell Medical Center