Is It Safe Yet to Have an Honest Conversation About Secondhand Smoke and Lung Cancer?
Several years ago I was talking to an epidemiologist who is skeptical of the idea that smokers pose a mortal threat to people in their vicinity. Although he supported workplace smoking bans, he was frustrated by the willingness of so many anti-tobacco activists and public health officials to overlook or minimize the weakness of the scientific case that secondhand smoke causes fatal illnesses such as lung cancer and heart disease. He wondered when it would be possible to have a calm, rational discussion of the issue, one in which skeptics would not be automatically dismissed as tools of the tobacco industry. I suggested that such a conversation might take place once smoking bans became ubiquitous, at which point the political stakes would be lower. Judging from a recent article in the Journal of the National Cancer Institute, headlined "No Clear Link Between Passive Smoking and Lung Cancer," that conversation may have begun.
The article describes a large prospective study that "confirmed a strong association between cigarette smoking and lung cancer but found no link between the disease and secondhand smoke." The study tracked more than 76,000 women, 901 of whom eventually developed lung cancer. Although "the incidence of lung cancer was 13 times higher in current smokers and four times higher in former smokers than in never-smokers," says the JNCI article, there was no statistically significant association between reported exposure to secondhand smoke and subsequent development of lung cancer. "We don't want people to conclude that passive smoking has no effect on lung cancer," says one of the researchers, Stanford oncologist Heather Wakelee. "We think the message is, this analysis doesn't tell us what the risk is, or even if there is a risk."
While hardly the last word on the subject, the study has advantages over most of the research commonly cited as evidence that secondhand smoke causes lung cancer. "To our knowledge," the authors say, "this is the first study to examine both active and passive smoking in relation to lung cancer incidence in a complete prospective cohort of US women." The prospective design avoids a weakness of studies that start with lung cancer cases and "match" them to controls. "Many studies that showed the strongest links between secondhand smoke and lung cancer were case-control studies, which can suffer from recall bias," notes the JNCI article, since "people who develop a disease that might be related to passive smoking are more likely to recall being exposed to passive smoking."
Even more revealing than the study's findings are the comments from experts quoted in the article (emphasis added):
Jyoti Patel, MD, of Northwestern University School of Medicine said the findings were not new….
"Passive smoking has many downstream health effects—asthma, upper respiratory infections, other pulmonary diseases, cardiovascular disease—but only borderline increased risk of lung cancer," said Patel. "The strongest reason to avoid passive cigarette smoke is to change societal behavior: to not live in a society where smoking is a norm."
In other words, although the U.S. Centers for Disease Control and Prevention will tell you that "secondhand smoke causes an estimated 3,400 lung cancer deaths among U.S. nonsmokers each year," scientists have long understood that the actual number might be closer to zero. The basic problem is that the doses of carcinogens absorbed by nonsmokers are much lower than the doses absorbed by smokers, so any lung cancer risk would be correspondingly small and therefore hard to detect using the blunt tools of epidemiology. The associations found in studies of secondhand smoke and lung cancer (which generally involve wives of smokers) are weak, meaning it may be impossible to rule out alternative explanations. But none of that really matters, Patel says, because the main goal of smoking bans was "to change societal behavior" by stigmatizing smoking, making it less convenient and less socially acceptable. Indeed, even if you accept every allegation about the hazards of secondhand smoke, it's clear that the real "public health" payoff from smoking bans, in terms of reducing tobacco-related morbidity and mortality, comes from shrinking the number of smokers.
That is not what advocates of smoking bans said, however. Their main rationale was always protecting bystanders, and they never had any patience for the distinction between public and private property, or the notion that people who choose to enter a bar or restaurant where smoking is allowed thereby consent to any risk posed by exposure to secondhand smoke. The warning that "secondhand smoke kills," with lung cancer as the paradigmatic example, played an important role in the debate over government-imposed smoking bans. By raising the stakes, it helped transform a complaint into a right, so that people annoyed by tobacco smoke now felt justified in demanding that it be eliminated everywhere they might want to go, including other people's property. As another expert quoted in the JNCI article puts it, "We've gotten smoking out of bars and restaurants on the basis of the fact that you and I and other nonsmokers don't want to die. The reality is, we probably won't." Now they tell us.
