Everybody knows that smoking is bad for your health. Even so, people keep on doing it. United States government figures show that more than 50 million Americans still smoke.

"I don't have anything to worry about," you may say. "I don't smoke, and I never will." While that's a great attitude to have and will help you live longer, you may still have a problem with cigarette smoke. It's called passive smoking.

Passive smoking is the experience of being forced to breathe in smoke just because it's in the air around you. You don't have any choice about it. You experience passive smoking when someone smokes near you at the ballpark, in a car, in a restaurant, in your home, or wherever you happen to be.

Have you ever been on an elevator with a smoker? Then you know all too well what passive smoking is. When the smoker exhaled, or blew out a cloud of smoke, you couldn't help inhaling it, or breathing some of it in. You also inhaled the smoke that came out of the tip of the smoldering cigarette. Just as you shared the air in the elevator with the smoker, you shared the cigarette smoke, too.

Cigarette smoke is a carcinogen. Carcinogens are substances that can produce changes in the human body that can lead to cancer. There are other carcinogens found in the air people must breathe. These include fumes from cars, trucks and buses and fumes caused by burning coal and wood. But the most common carcinogen found in the air is cigarette smoke -- especially indoors.

Cigarette smoke also contains chemicals that can damage the cells inside the lungs. The chemicals are called free radicals. The damage these free radicals produce can lead to allergies and serious lung diseases like emphysema.

Many medical studies have shown that children exposed to a large amount of cigarette smoke are more likely to have health problems such as ear, nose and throat infections, lung problems, allergies and asthma than children who are protected from passive smoking do.

When someone lights up, the cigarette produces two kinds of smoke. The smoke that comes from mouthpiece of the cigarette is called mainstream smoke. The smoke from the burning tip of the cigarette is called sidestream smoke. Because it isn't filtered, sidestream smoke contains stronger concentrations of harmful substances than the smoke inhaled by the smoker.

"It's not fair," you say. "I don't smoke, but I have to be around smoke anyway."

Many people agree with that feeling. The American Lung Association has come up with some tips designed to improve things for nonsmokers. Here are three of them: :: Let your family and friends know that you DO mind if they smoke. If they ask permission, politely say, "I'd rather you didn't." :: Put "Thank you for not smoking" stickers, buttons and signs in your home, car and school. You can obtain these stickers from your local chapter of the American Lung Association. You could make your own anti-smoking signs, too. :: When your family goes out to eat, ask to be seated in the nonsmoking section. When you travel, use the nonsmoking section of the airplane or train.

You can probably think of more ways to avoid smoke-filled rooms or to convince other people not to light up. Maybe you have even convinced your parents to quit smoking. If so, you improved their health and your own. Children whose parents both smoke are twice as likely to take up the habit later in life than the children of nonsmokers are. If your parents quit, you have a better chance of avoiding the habit.

If you have ideas to help get rid of smoking, or a nonsmoking success story to tell, send them to: Catherine O'Neill, Health Section, The Washington Post, 1150 15th St. NW, Washington, D.C. 20071. Good ideas will be published on this page later this year.Tips for Parents

Involuntary inhalation of cigarette smoke has been linked to both acute and chronic respiratory problems in children. More evidence was added this year with the release of a study from Long Island Jewish Medical Center and the State Universities of New York at Stony Brook and Brooklyn showed that nonsmoking teenage athletes who were exposed to smoking parents or friends for at least two hours of cigarette smoke a week showed increased coughing and decreased lung function. "The frequency of cough was four times greater in the group exposed to passive smoke," the researchers wrote. According to the study, parents who smoked accounted for the majority of the teens' exposure.

Catherine O'Neill is a free-lance writer in Baltimore.