The greatest benefit of America's big air cleanup in the last decade was life itself -- more years to live, less sickness and discomfort.

But measuring those benefits has proved difficult. Consider Mae Karlsven, a Takoma Park resident who died on June 6 at the age of 58 of emphysema -- deteriorated lungs -- whose case was so bad that she lived with a plastic tube in her nose connected to an oxygen tank. When she went outside, she had to take a portable tank on a shopping cart.

In an interview just weeks before her death, Karlsven described what bad air did to her. "The oxygen doesn't do any good when there's air pollution," she said, her voice small and squeaky with the effort of speaking. "Once in a while I've been caught outside when it was polluted, and my heart just goes crazy. . . . If I'm out in it too much, I have to go to the hospital."

Whether air pollution worsened Karlsven's disease or hastened her death are scientific unknowns, nearly impossible to measure. Too many other factors are involved, including cigarette smoking.

The uncertainty lies at the heart of much of the debate over the nation's air pollution laws. While there's plenty of evidence that certain types of air pollution kill people, other types -- including the automobile-caused smog that Karlsven complained about -- may not be so dangerous. Of course, the experts disagree.

Billions of dollars hang on the resolution of disputes such this as Congress reconsiders the Clean Air Act, the nation's most ambitious environmental legislation. Complicated as the debate may be, in the end it comes down to how much Americans are willing to spend on health and comfort.

Dr. Alfred Munzer, a past president of the D.C. Lung Association, says Washington's air is bad for people with respiratory problems, including 30,000 in the area with emphysema, 101,000 with chronic bronchitis, 87,000 with asthma and 211,000 with hay fever. All these conditions, he said, are irritated by summertime smog, Washington's major pollution problem. In addition, 222,000 Washington-area heart patients and 38,000 with anemia can be affected by carbon monoxide, an automobile pollutant that robs the blood of oxygen.

"The summer months [in Washington] are really bad, very difficult for patients with respiratory disease," Munzer said. "They can't go out. If they do go out, they can't breathe. . . . If they have [asthma], it's much more likely that they will have a bronchial spasm or an attack."

As in Mae Karlsven's situation, a key problem in determining pollution's impact is that small quantities work on people over long periods of time. When the clean air legislation was passed a decade ago, Congress dealt with uncertainties like this by ordering national standards set low enough to protect the most sensitive people with "an adequate margin of safety" and without any regard for what this might cost industry and consumers.

Recently that principle was supported by the National Commission on Air Quality, an independent blue-ribbon group that studied the act in preparation for the congressional debate.

"To protect public health," the commission report concluded "government must act to control potentially harmful pollutants despite scientific uncertainty about the precise harm they cause and the levels of exposure that cause that harm."

However, the principle is being challenged by American industry and the Reagan administration, whose economists seek to replace it with a less demanding one that would define a health hazard in terms of "significant risk of adverse effects." The argument is that the air has already been cleaned enough to save most lives that can be saved, so further expenditures should be scrutinized closely.

Further complicating the picture, scientists now believe that for certain dangerous pollutants there is no threshold below which nobody is affected. If this is true, then to meet the letter of the congressional mandate for these pollutants the Environmental Protection Agency would have to virtually shut down American industry.

On the other side of the debate are health organizations such as the American Lung Association, which says it could be a medical disaster to relax pollution standards. Among evidence of air pollution damage:

A review of 38,207 deaths in Nashville over 12 years found more people died of breathing ailments in sections of the city that had the heaviest air pollution.

A 232-day study of Los Angeles hospital admissions showed a correlation between high air-pollution levels and acute upper-respiratory infections, influenza, bronchitis, allergies and heart trouble.

In perhaps the nation's worst air-pollution disaster, 6,000 fell ill and 20 died in industrialized Donora, Pa., in 1948 when stagnant weather held pollution over the city. The elderly and those with heart and lung diseases were hardest hit with coughing, chest constriction, breathlessness, burning eyes and vomiting.

The American Public Health Association estimated in 1969 that polluted air caused 10 to 20 deaths a day in New York City. In Buffalo, the number of children hospitalized with asthma increased significantly when the air was particularly dirty.

Most studies like this concern the killer pollutants -- sulfur oxides and particles of ash and soot that come mostly from factories and power plants. The Washington area now meets federal standards for these. It doesn't for summertime smog, mostly in the form of ozone, a gas that forms in the air when sunlight strikes automobile emissions.

Generally this smog is not considered dangerous. While sulfur oxides and particles will kill you, many experts say, smog may only cause discomfort and aggrevate some respiratory ailments. Others disagree.

"I don't think there is or is going to be any evidence that photochemical smog [ozone] is life shortening in any significant way," said Walter C. Barber Jr., the EPA's deputy assistant administrator for air quality planning and standards. "It's unpleasant. It probably causes adverse effects in sensitive people -- asthmatics, elderly people. They can't go outside. Who wants to live in a world where you can't go outside?"

"I say there is no health evidence indicating that ozone is [a major] problem, and the costs are huge," said Lester B. Lave, coauthor with Eugene P. Seskin of an exhustive 1977 study called "Air Pollution and Human Health." Lave said the federal standard for ozone, which the EPA relaxed 50 percent early in 1979, could be relaxed even further, saving billions without a marked increase in health effects.

Following the 1979 EPA action, the American Petroleum Institute sued the EPA to have the standard relaxed further. The suit is still pending. Lave and Seskind -- independent scholars not connected with API -- supported the institute's position, writing in a newspaper article: "To put these standards in perspective, even a pessimistic reading of the health evidence implies that just over one person per thousand would experience a few additional hours' discomfort per year as a result of ozone concentrations of 0.16 parts per million." The current standard for ozone, a pungent, slightly bluish gas, is .12 parts per million.

But Munzer, Mae Karlsven's doctor, said ozone "is a very irritating gas. It causes constriction of the bronchi, which are the major air passages, particularly in more sensitive people."

He said ozone also interfers with the alveolar macrophonage, a type of cell in the lung that protects it from infection. "Ozone interfers with [this cell's] ability to seek out foreign particles to reject them and to kill bacteria. This translates into increased respiratory infection when the ozone level is high.

"The standard for ozone is set at a level where normal people feel eye irritation. There is the argument that it is not a significant health hazard, but I compare that to the pain you feel on touching a burning stove. It's a warning signal."

Another emphysema patient of Munzer's, Irving T.A. Markentor, said the air in Washington is the worst he ever breathed. "I thought Los Angeles was the worst, but even [there] i was never bothered going to the doctor as I had to when I came here," said the 52-year-old engineer who came here three years ago and has been in and out of the hospital ever since.

"I don't know what it is in this area," he said, "but as soon as I go out it's like a burning sensation . . . in my throat, like acid."

Munzer said humidity here may act on pollutants to cause this effect, a problem that less humid Los Angeles may not have. This is speculation; scientists don't know. It is known that people adapt to ozone: a Los Angeles study comparing longtime residents with new residents showed the former reacted less strongly to it.

The national commission recommended no relaxation of the ozone standard. Its report said that "while epidemiological evidence of adverse health effects from ozone has been sparse," research showed that "chronic exposure . . . can produce . . . changes involving the small airways and adjacent air spaces in animal lungs that are analogous to the changes caused by aging. . . ."

As a practical matter, some doctors say it is hard to spot the effects of ozone pollution on patients.

Washington allergist Dr. Robert E. Pumphrey said this is because it is accompanied by heat, pollen and other factors: "People see pollution, they do get some congestion, they say the air is tight. . . . A considerable amount of the stress felt by these people is caused by a feeling of closeness, claustrophobia. . . . This has a significant psychic impact."

After a decade of experience, researchers are now able to make estimates of the number of lives saved by the control of the most dangerous air pollutants, sulfur oxides and particles.

Lave and Seskin, for example, concluded that emission restrictions and requirements to burn low-sulfur coal imposed on power plants and factories cut the U.S. mortality rate 7 percent -- increasing the life of the average American by a year. "Clearly, stringent regulations on [sources of the dangerous pollutants] represent a good buy for society . . . though they may prove costly to particular plants," they wrote.

Nevertheless, industry argues that the EPA's sulfur dioxide standard is too stringent. An American Iron and Steel Institute study says there are only limited ill health effects from low levels of sulfur oxide emmissions.

These emissions also combine with moisture in the air to form sulfuric acid, which is even more dangerous to health than sulfur oxides alone and which, blown over long distances by wind, is deposited as "acid rain" in lakes where it kills fish.

The national commission found the standard for particles inadequate because there is now evidence that only the tiniest ones, those that penetrate deep into the lungs, are dangerous. The standard as now written measures particles of all sizes, including large ones that are trapped in the nose and cause no damage.

In addition to the seven widespread pollutions for which it has standards, the EPA has identified a group of cancer-causing pollutants that it labels "hazardous": asbestos, mercury, beryllium, vinyl chloride, benzene, radio-nuclides arsenic.

These are emitted in small quantities, but the potential adverse health effects from them are "likely to be far more severe than those of the widespread pollutants," according to the commission report. The EPA is studying them and has set emmission limits for some. The commission recommended expanding controls over these kinds of pollutants.

Concern about carcinogenic pollutants also is beginning to focus on the growing popularity of diesal automobile engines because, while they emit less carbon monoxide and hydrocarbons, they emit more nitrogen oxides and 30 to 100 times more particles, which contain some carcinogenic compounds.

However, the National Academy of Sciences reported last year that there is no conclusive evidence that breathing diesel exhaust causes cancer, birth defects or lung disease, even though diesel materials painted on or fed to rats may cause cancer in them. Studies of workers regularly breathing diesel exhaust found no excess cancers.

Indoor air pollution is another problem that is just beginning to be assessed as buildings are being sealed tighter to conserve energy, thereby sealing in dangerous pollutants.

John H. O'Hara, a government engineer and Maryland air-pollution activist, said that tests he performed in a restaurant showed that the pollution level inside with just one person smoking a cigarette was three times what it was outside. Yet while the outside level is regulated, the inside level is not.

The clean-air benefit most difficult to quantify is esthetic -- you could call it a mental health benefit.

"We spend millions . . . on various esthetic items," said the EPA's Barber. ". . . We try to do land use planning, we do all kinds of things to try and improve the ambience that we live in and air quality is one element of that. . . . You get a smoggy, hot summer day, it's unpleasant. It may not be unhealthful, but it certainly is unpleasant. That just happens to be a very hard thing to quantify. . . ."

During her lifetime, Karlsven was not involved in America's clean-air debate. But she lived the realities that it dealt with, and in a sense she was its subject.

"In summer when it's hot and humid [and] we have the pollution, I have to stay inside in an air-conditioned building," she said a few weeks before she died. She recalled that, at such times, she tried to "stay just as still as possible, not hardly move, not use any energy that might cause me to need more oxygen. . . ."