Every noon, they pour out of the federal office buildings, the men and women in shorts, the joggers.

Why do they do it? A 32-year-old secretary says, "I know this sounds funny, but my boyfriend wants me to. And I like it." A paunchy man, 36, a health policy analyst, says, "To get rid of this paunch." A paunchy woman, 45: "I started it to lose weight, and I'm losing some. But I also feel more alive, more awake."

One hundred million sweating Americans are currently exercising in some way.

Like no people before us, we are running, swimming, doing sit-ups and push-ups and hitting or tossing a ball as part of a new lifelong passion to keep fit.

This exercise surge is only part of a new fitness revolution, one that has millions of us concerned not just with exercise but with everything else that affects our health.

Most of us are smoking less, fretting about weight and eating strikingly less sugar and fat in an attempt to slim down and protect our hearts. Our patronage has built an estimated $15 billion a year exercise and weight-loss industry, with thousands of fitness, "figure control" and diet centers, and new chains of sports stores selling mountains of exercise shoes, exercise suits, backpacks and headbands.

All the people who are exercising, looking for healthy foods and eating fiber instead of sugar would have been derided as "health nuts" only a decade ago. In the last 10 years, however, our doctors and our government have been telling us: "Your health is up to you. You can do far more for yourself than any doctors or expensive medical technology."

We, the people, have been getting the message. What we are seeing, say professors, is one of the late 20th century's major sociological events.

It is an event born of many elements. A concern with "self" and "controllling our own lives." A concern with body image and "keeping young" -- and sexually active -- into our late years. A concern over persisting environmental pollution that tells us we have to try harder to stay individually healthy. And the hard-headed, practical realization that health is indeed largely up to us.

"Exercise has certainly helped me," said one unusual Washington exerciser one recent day.

Milton Kronheim had just finished punching a bag at the George Washington University gym. At 6 p.m. every day, five or six days a week, this wiry man spends 45 minutes bending, stretching, riding an exercise cycle and punching the bag.

Not so unusual? No, except for the fact that Kronheim -- the District's leading liquor distributor though he doesn't touch the stuff -- is 91. He started punching the bag when he was 10. Until he was 86 and hurt his shoulder, he regularly pitched nine innings of Sunday baseball. A few years ago, he gave up handball. But he keeps punching the bag.

"It's hard work," he tells you. "It gets monotonous. But all this keeps me going. I feel good."

Dr. Ronald Crystal, 38, heads the pulmonary (lung) disease branch at the National Heart, Lung and Blood Institute. He is also a muscular runner, part of the half-naked army in Nikes and shorts who morning, noon and night plod along the nation's sidewalks and paths.

"Four years ago, I could only run around the block," he says. "Now I go out and run 15 miles, and it's absolutely nothing."

But is he healthier? Are the exercising millions truly any healthier, living longer, having fewer heart attacks for all the sweat?

Many medical enthusiasts say yes. "But as a doctor and scientist," says Crystal, "I have to say we honestly aren't sure yet."

The evidence, in strict scientific terms, is "highly suggestive" that exercisers feel better, sleep better, have more energy and tend to lead lifestyles that help prevent disease and lengthen life. And the evidence is impressive. Exercise has profound physical effects on the heart and blood vessels.It strengthens muscles. The heart, a muscle, pumps more blood. The ability of all tissues to use the increased blood flow and oxygen is increased.

Hard exercisers, even those who work strenuously just three times a week for 20 to 40 minutes, have higher levels of HDL -- high density lipoprotein or "good cholesterol" -- in their blood. (High total cholesterol means more chance of heart disease; a high ratio of high to low-density cholesterol seems to mean less.)

Diligent exercisers tend to lose weight. Even more important for long life, they lower their blood pressure. Conditioning also seems to help the blood dissolve clots; clots in the blood cause strokes and heart attacks.

Despite today's stress and a thousand new pollutions, health officials report marked declines in heart and stroke deaths in the last decade.It has been a decade, they point out, of more attention to controlling blood pressure, less smoking, a lower-fat diet and also more exercise.

Students of disease have looked at sedentary vs. active D.C. postal workers, San Francisco dockers, London busmen and British business and professional men. In every case, sedentary workers were the most likely to die of heart disease. In Framingham, Mass., where a federal team watched 1,909 men for five years, those with the most active jobs and lives cut their heart attack risk by 30 percent after age 45.

And they did so because they also followed other good habits. They lowered their blood pressure and blood cholesterol and cut out cigarettes as well as staying active.

Dr. Ralph Paffenbarger questioned 16,936 Harvard male graduates aged 35 to 74 over six to 19 years. He too found that "vigorous" exercisers -- those who ran for an hour three or four times a week or bicycled, played handball or walked with equal intensity -- had fewer heart attacks than their fellows. Those with less strenuous activity got moderate benefit if they did enough, but those with only "light" play (golf, bowling, baseball, dancing, yard work) showed little or no benefit unless they added more strenuous work.

Does this mean exercise has to be hard to do much good?Paffenbarger indeed says "an ideal prescription" for lower heart risk must include "robust exertion" that makes the lungs fill and expand and the heart beat faster. Dr. Kenneth Cooper named this activity "aerobic."

But Dr. Lester Breslow, dean of public health at the University of California at Los Angeles, found exercising even moderately improves life expectancy. He found that those who live longest are those who follow seven health rules: daily breakfast; three meals a day at regular times with no between-meal indulging; seven or eight hours of sleep; no smoking; moderate weight; no or moderate alcohol use, and even moderate regular exercise.

Kronheim, the 91-year-old George Washington University bag puncher, pretty much follows such rules. In his gym shorts and shirt, he looks plainly old. He wouldn't deny it. His legs are spare, his skin dry and wrinkled. But his eyes sparkle, and if he told you he was only 71, you'd believe it.

When Kronheim arrives at the gym daily, and when other men, young and old, arrive for their late afternoon stints, they are at the end of their workdays. Most look tired. When they leave, you notice, almost all look fresh and sparkling. You might say they'd had some kind of fix.

Is it possible that exercise indeed may provide a natural fix, an extra dose of brain chemicals that makes exercisers feel so good they themselves often speak of their "addiction"?

The chemical evidence for this is still new and sometimes puzzling. The brain during a moderate run seems to pour out extra endorphins, "the brain's own opiates," or pain-relievers. This doesn't seem to happen to marathoners.

"Perhaps these chemicals come into play under stress, but at a certain point in training, the body no longer needs them," Dr. David Pickar of the National Institute of Mental Health conjectures. "We do know that if you bicycle very hard on a machine, pedaling as hard as you can for 15 or 20 minutes, you double this opiate activity. It's certainly attractive to think this accounts for the 'high,' the euphoria so many runners report."

The same mental chemistry may explain the fact that exercise seems to help depressed mental patients, and in some cases does more good than drugs. At the University of Wisconsin, starting with long walks linked by short runs, therapists gradually encouraged 28 depressed patients to run several times weekly. This helped many of them as much as conventional psychotherapy, at less cost.

This wouldn't have surprised Dr. Paul Dudley White, the late, famous heart doctor to President Eisenhower. He bicycled, preached fitness and helped start the exercise boom. He was certain intellectual health depends on healthy, fit circulation of blood to the brain.

"I think my running is quite addicting," admits Dr. Crystal of the heart and lung institute. "I think it's a positive addiction, one that gives you psychological and emotional benefit. I don't know whether or not this is a chemical effect, but I do know I feel much better."

San Francisco's Dr. Joan Lamb Ullyot, author of "Women's Running," was "the least athletic person" in her Harvard medical class by her admission. But "at age 30," she says, "I had a growing problem with middle-aged spread . . . Now at 38, I'm also slimmer and fitter than at any other time in my life."

What does exercise do for women? The Framingham study detected no protection from heart attacks among more active women. A Dutch study of merely mild exercises and gardeners did. But most studies have not included women at all.

The "problem," researchers explain, is that women get few early heart attacks, so it's harder to establish an exercise effect. The scientists also say, "Men get more heart attacks, so it's more important to study them." The scientists, of course, are almost all men.

Women as well as men do have elevated "good cholesterol" if they are active.

More men than women exercise, but in the last few years more women than men have been taking up exercise.

"I started running in 1971 for the same reasons that motivated most beginning joggers -- health and appearance," Ullyot says. But now, "I have a positive addiction. If I don't run, I miss the health and good feelings."

In October 1978, Maryland representative and marathoner Goodloe Byron, a mere 49, died of a heart attack after running 12 miles. "He had a strong family history of coronary disease," Medical World News reported. "Two exercise tolerance tests, the latest in January, indicated coronary atherosclerosis. But Bryon ran on, spurning warnings."

Exercise may be great medicine, but it is no cure-all. The fittest persons can and do have heart attacks and die, even at early ages.

Some doctors think heedless running can spur a heart attack. Others think running has nothing to do with the occasional disaster. No one knows for sure. But a Stanford study found that of 18 persons who died during or just after running, six had ignored warning signs like pain and difficult breathing.

All authorities agree that sporadic bursts of exercise may do more harm than good. Even regular exercise, overdone, may be harmful.

If running has become a positive addiction for some, it has become a dangerous "negative addiction" for others, the University of Wisconsin's William Morgan warns. These are persons who run almost endlessly while their other relationships deteriorate, who become anxious and irritable when they can't run and, the most telling symptom, ignore pain and injuries that can do lasting harm.

Many doctors, many of them runners, think running has been oversold as the ultimate activity. They think many unknowledgeable runners will end up with bad, arthritic knees and, almost as bad, disillusionment that ends all activity.

It is probably true that only 20 to 60 minutes of continuous hard activity like jogging or swimming can give the heart, lungs and body the most thorough workout. But "for the general public," Dr. Arthur Leon of the University of Minnesota says, "brisk walking five days a week for at least an hour will give you the same metabolic benefits, without as much risk."

Bicycling or rope-jumping or almost anything done without slowing or stopping can achieve about the same goal, and are probably better conditioners than games like handball or double tennis that demands only brief bursts.

Dr. Irving Wright, New York cardiologist, quarrels with the claim that golfers can't get real exercise. "A lot of old people won't go for a walk. But a golf game can get them to walk five miles."

"Vigorous dancing is an activity we often overlook," Dr. Samuel Fox at Georgetown University contributes. Dr. Joseph Fetto, New York orthopedist, says, "A lot of good exercises, like running in place and situps, don't require any special equipment."

The bottom line, it seems, is that there is no "right" exercise for everyone. The "best" exercise is the one that you'll do.

Not everyone exercises. Whites exercise more than minorities, the well-off more than the poor, the educated more than the less educated, the young more than the old. Fifty percent of those who buy jogging suits or sign up at the "Y" soon drop out.

Unlike our forebears, most of us don't walk, pull, lift and labor as a necessary part of life. We ride and we sit, and the number of basically sedentary persons is still "growing with each passing decade" because of the "sloth" created by technology. Dr. William Kannel, co-chief of the government's Framingham study, reports.

"I think becoming sedentary is the most profound change, metabolically, that has happened to man in civilized history," says Dr. Henry Blackburn of the noted Laboratory of Physiological Hygiene at the University of Minnesota. "I think this is responsible for many of our metabolic maladaptations" -- the mechanisms by which we develop many diseases.

Where do we go next, then, in the exercise movement?

Dr. Kannel urges us to consider how to start a second stage: "one to engineer physical activity" into our daily living once more.