Passive smoking, which even the tobacco industry acknowledges is an "active controversy," is under investigation by one of the nation's most prestigious scientific bodies, the National Academy of Sciences.

Also known as involuntary smoking, secondhand smoke, sidestream smoke or environmental tobacco smoke, passive smoking refers to tobacco smoke in the air inhaled by nonsmokers. Its possible health effects are of growing concern to health experts and the public.

About 20 witnesses, representing groups ranging from the American Lung Association to R.J. Reynolds Tobacco Co., testified on the issue last week in a day-long hearing at the NAS, which was chartered by Congress in 1863 to give federal agencies independent scientific advice on technical issues.

The testimony, along with other evidence, will be considered by the NAS' Committee on Passive Smoking, which includes 11 scientists and physicians. The panel's chairwoman, Dr. Barbara Hulka, an epidemiologist at the University of North Carolina at Chapel Hill, said the report would be completed by "late spring or early summer."

The report on passive smoking was requested by the Environmental Protection Agency and the Department of Health and Human Services, which asked the NAS to investigate two main questions: How can indirect exposure to smoke be measured, and what are its effects on health?

An NAS conclusion that environmental tobacco smoke is a serious health hazard would boost efforts already under way around the country to ban or restrict smoking in the workplace and other public areas.

A Gallup poll conducted last year for the American Lung Association found that 75 percent of all adults -- 62 percent of current smokers, 85 percent of nonsmokers and 78 percent of former smokers -- agree that smokers should refrain from smoking in the presence of nonsmokers. And 79 percent of adults, including 76 percent of current smokers, said companies should limit smoking to certain assigned areas.

But the technical and sometimes contradictory nature of much of last week's testimony suggests the difficulty of the NAS panel's challenge.

"Evidence continues to accumulate on the harmful effects of environmental tobacco smoke," said Lawrence Garfinkel, the American Cancer Society's vice president for epidemiology and statistics. "The risks posed by involuntary smoking may be much smaller than those posed by active smoking, but the potential number of affected individuals is much, much greater."

Garfinkel acknowledged in answer to a question from the panel that it is "very difficult to get accurate information on exposure" to environmental tobacco smoke.

Such smoke exists in two main forms: mainstream smoke, which is exhaled by smokers, and sidestream smoke, which enters the environment directly from the burning cigarette. Sidestream smoke accounts for about 85 percent of tobacco smoke in the air.

Because sidestream smoke has not passed through the cigarette and its filter -- nor a smoker's throat and lungs -- it contains many more potentially toxic ingredients than mainstream smoke does. On the other hand, all secondhand smoke is diluted by surrounding air.

A 1984 report by the U.S. surgeon general, "The Health Consequences of Smoking," concluded that cigarette smoke, at typical levels of smoking and ventilation, "can make a significant, measurable contribution to the level of indoor air pollution."

The surgeon general's report also warned that children of smoking parents have an increased prevalence of reported respiratory symptoms and increased frequency of bronchitis and pneumonia early in life.

Nearly 70 percent of non-allergic nonsmokers exposed to environmental tobacco smoke complained of eye irritation, according to a study cited by the Cancer Society's Garfinkel. Other common complaints included nasal discomfort (30 percent), headaches (30 percent) and cough (25 percent). The percentages were higher in people allergic to tobacco smoke.

Studies in Japan and Greece have suggested that nonsmoking wives of smokers have a higher risk of lung cancer.

A study of 134 nonsmoking women with lung cancer, published last September in the National Cancer Institute Journal, found that those exposed to smoking by a spouse at home had an increased risk of lung cancer of between 13 and 31 percent. The risk of lung cancer doubled in nonsmoking women whose husbands smoked 20 or more cigarettes a day at home.

Other studies, including one reported last November at the annual meeting of the American Heart Association here, suggest that involuntary smoking also increases the risk of death from coronary heart disease.

"Our knowledge of the health effects of passive smoking is incomplete . . . but certainly the evidence taken as a whole indicates that passive smoking is hazardous to health," said Dr. Alfred Munzer, a lung specialist at Washington Adventist Hospital and board member of the American Lung Association.

The tobacco industry's position is that passive smoking poses "no proven harm or risk whatsoever," said Scott Stapf, spokesman for the Tobacco Institute, the industry's trade group. In a nationwide advertising campaign last year, R.J. Reynolds Co., the nation's second-leading tobacco company, called passive smoking "an active controversy" that usually receives "sensational" and "one-sided" media coverage based on "rhetoric more than research."

The Tobacco Institute did not testify before the NAS committee. The only testimony from R.J. Reynolds was a highly technical presentation by an analytical chemist about measuring nicotine in the air. He did not address the question of health effects from environmental tobacco smoke.

John Banzhaf III, law professor at George Washington University and executive director of Action on Smoking and Health (ASH), urged the NAS panel to focus on known short-term health effects in healthy people as well as long-term health hazards, which are more difficult to prove. These "real, observable physical symptoms" include sinusitis, asthma, hay fever, allergies, chronic bronchitis and emphysema.

"Virtually any allergist in the country, called on the phone, would be able to tell you about such people," Banzhaf said.

Detailed scientific studies, Banzhaf said, are "hardly necessary to document the physical irritations which many health nonsmokers suffer" -- including headache, cough, wheezing, sore throat, hoarseness and dizziness.

One thing that public health officials and the tobacco industry can agree on is that precise measurement of exposure to environmental tobacco smoke is not simple. It is subject to an array of what scientists call confounding variables, such as the size of the room, the degree of ventilation and exposure to other pollutants.

"Perhaps the most serious, and certainly the most widespread, flaw" in studies of the effects of passive smoking on the lungs, said Dr. Philip Witorsch, is "the absence of appropriately verified estimates or measurements of exposure . . ."

That flaw applies equally to studies that do and do not show health effects of passive smoking, said Witorsch, a lung specialist and clinical professor of medicine at George Washington University Medical Center. Nevertheless, Witorsch concluded that environmental tobacco smoke's contribution to air pollution is "minimal" and "insufficient to produce a pathophysiologic effect."

Witorsch was one of seven witnesses from the Indoor Air Pollution Advisory Group, whose research is funded by the tobacco industry. The industry exerts no control over the interpretation or publication of the research, said Sorrell Schwartz, a pharmacologist at Georgetown University and chairman of the group.

No one claims that passive smoking is healthy, but the NAS panel must assess the degree of risk to nonsmokers. Committee member Dr. A. Sonia Buist, professor of medicine at Oregon Health Sciences University, said a key challenge would be to distinguish actual health effects from esthetic concerns, such as "I don't like the smell."

"What is harm?" Buist asked witness Banzhaf. "Is harm red eyes? Is harm going to the doctor because of a sore throat, or not going to the doctor because of a sore throat?

"It's very difficult for us to know where to draw the line."