Word of the latest “study” only recapitulates the confusion surrounding the issue.
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Earlier this month the prestigious Journal of the National Cancer Institute carried a news item titled “No Clear Link between Passive Smoking and Lung Cancer.” The report was based not on a published scientific paper, but, rather, on a presentation by a medical student at a conference in June of this year.
What was most surprising about this news item was how it was seized on as providing important new information on the health risks posed to non-smokers by exposure to secondhand tobacco smoke. This issue was first raised in 1981 by a study from Japan, and since that time there have been many dozens of studies evaluating the risk of lung cancer and heart disease posed by passive smoking.
What did this new, as-yet-unpublished study really add to what is already known on this question? The answer, which came through clearly in the brief report was – very little.
The report did underscore some important points. First and foremost, like every other study that has examined the association of smoking itself with risk of lung cancer, the study found that smoking was a strong risk factor for lung cancer: the risk was 13 times higher in current smokers and 4 times higher in former smokers, compared to the risk in never smokers.
In contrast, among women who reported never smoking, exposure to passive smoking overall, and to most categories of passive smoking, did not increase the risk. Only those with 30 or more years of exposure showed a slightly increased risk, but this was not statistically significant – meaning that it could be due to chance.
This study involved 76,304 postmenopausal women who were followed for an average of 10.5 years. Over that period of time, 901 women were diagnosed with lung cancer. However, only 152 of the lung cancer cases occurred in never-smokers.
The results of this study are very much in line with those of previous prospective studies, which have examined the effects of exposure to passive smoking. [Prospective studies are superior to the other main type of study, case-control studies, because they assess exposure in a cohort of generally healthy people and then follow the cohort for a period of years to see who develops disease. This avoids the problem of individuals who have developed lung cancer possibly reporting their exposure to passive smoking differently from controls -- a problem that affects case-control studies.] So, it is not as if this new study suddenly overturns the results of previous studies. Smoking accounts for roughly 85% of lung cancer. And it follows from this that lung cancer occurring in those who never smoked is a rare event. In addition, any risk from passive smoking is very small, and this makes it difficult to detect a significant effect.
In fact, the association is weak and inconsistent. The largest U.S. prospective studies, the American Cancer Society’s (ACS) Cancer Prevention Studies (CPS I and II) show either no association or an inconsistent association, respectively. Widely cited meta-analyses show a 25% increased risk of lung cancer among never smokers exposed to passive smoking (mainly through marriage to a smoking spouse). However, other meta-analyses indicate that the excess risk may be lower, more on the order of 10% or less. In any case, this figure needs to be compared to the 2,000-6,000% increase in risk of lung cancer among smokers. The news item makes the important point that we should not overstate the weak and uncertain association with passive smoking, and should be looking for other, larger risk factors for lung cancer occurring in never smokers.
The most careful studies that have evaluated the actual exposure of non-smokers to tobacco smoke in the home, at work, and in other settings indicate that the average exposure of a passively exposed non-smoker is roughly equivalent to smoking about 10 cigarettes PER YEAR. This is approximately one-thousandth the exposure of the average smoker. These studies were conducted in the 1990s, when smoking restrictions were much more limited than they are today. In view of this level of exposure, it is hardly surprising that even the best epidemiologic studies show a weak, inconsistent, or no risk.
Since the early 1990s, the weak and inconsistent results of epidemiologic studies have been used as the basis for social policy and for instituting restrictions on smoking in public and in workplaces. I have never had a problem with restricting smoking, since I strongly believe that cigarette smoke is an unnecessary form of air pollution that no one should be exposed to involuntarily.
Furthermore, even if it is not clear that exposure to passive smoking causes fatal disease, it can exacerbate asthma and other respiratory conditions. Equally important, it is an annoyance and is unpleasant to many people.
The problem is that the weak results from epidemiologic studies were used to create a dogma that passive smoking kills, and that any exposure is a danger. Those who were concerned with restricting smoking, and promoting quitting among smokers and reducing the initiation of smoking among teenagers had no reason to give any thought to the weaknesses of the evidence. Their sole concern was to use the dogma to reduce smoking prevalence.
This led to the mismatch between what the science has to say and how health and regulatory agencies like the EPA, the U.S. Surgeon General, and the International Agency for Research on Cancer presented the risks from passive smoking. Once authoritative agencies have made categorical pronouncements, it’s very hard to dial back the dogma and make the necessary qualifications. Rather than acknowledging the weakness of the evidence, it was easier for health authorities and anti-smoking activists to smear anyone who questioned the dogmatic assertions as being a shill for the tobacco industry. Not surprisingly, this did a lot to confuse the public about the nature of the risk.
What this latest round of reactions to the JNCI news item reveals is how hard it is for both lay persons and many health officials to acknowledge the limits of our ability to identify a potential hazard due to very low-level exposures. This is, of course, especially true, in the face of a strong social trend to roll back tobacco use.
Due to the strong social and political pressures, it became hazardous for scientists to try to do careful, rigorous work on passive smoking. One analytical chemist who wrote the foremost textbook on the composition of, and exposure to, passive smoking used to end his lectures with the words: “If you like to get verbally abused, study ETS [environmental tobacco smoke] and publish politically incorrect scientific findings.”
Geoffrey Kabat is a cancer epidemiologist at the Albert Einstein College of Medicine and the author or Hyping Health Risks: Environmental Hazards in Daily Life and the Science of Epidemiology.
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