I wonder why?
A large prospective cohort study of more than 76,000 women confirmed a strong association between cigarette smoking and lung cancer but found no link between the disease and secondhand smoke.
Investigators from Stanford and other research centers looked at data from the Women’s Health Initiative Observational Study (WHI-OS). Among 93,676 women aged 50–79 years at enrollment, the study had complete smoking and covariate data (including passive smoking exposure in childhood, adult home, and work) for 76,304 participants. Of those, 901 developed lung cancer over 10.5 mean years of follow-up.
The incidence of lung cancer was 13 times higher in current smokers and four times higher in former smokers than in never-smokers, and the relationship for both current and former smokers depended on level of exposure. However, among women who had never smoked, exposure to passive smoking overall, and to most categories of passive smoking, did not statistically significantly increase lung cancer risk. The only category of exposure that showed a trend toward increased risk was living in the same house with a smoker for 30 years or more. In that group, the hazard ratio for developing lung cancer was 1.61, but the confidence interval included 1.00, making the finding of only borderline statistical significance.
There's no such thing as borderline statistical significance. It's either significant or its not, and if it includes 1.0, it's not. It will be interesting to see the full results when they are published, but judging by the Journal of the National Cancer Institute report, it appears that this major cohort study did not find any statistically meaningful association between secondhand smoke and lung cancer in the home, in the workplace or in childhood. Even if we disregard statistical significance (and why would we?), it seems that only decades of chronic exposure in the home might, at most, have an extremely modest effect on lung cancer risk. There is nothing to indicate the slightest risk to those who work in or patronise the bars and restaurants which have been forced to ban smoking.
This should not be particularly surprising. Very few passive smoking/lung cancer studies are published these days compared to the glut of the 1980s and 1990s, but the handful that have appeared in recent years continue to support the null hypothesis. For all the campaigners' talk of "overwhelming evidence", the link between secondhand smoke and lung cancer has always been very shaky. It tends to be the smaller, case-control studies which find the associations while the larger, cohort studies do not (and, as the JNCI report notes, case-control studies "can suffer from recall bias: People who develop a disease that might be related to passive smoking are more likely to recall being exposed to passive smoking.")
Taken as a whole, the epidemiological evidence is a mess of conflicting results and weak, non-significant findings that go in both directions. The World Health Organisation's attempt to settle the controversy with a pan-European study of its own, ended in failure in 1998. Attempts to generate significant relationships with meta-analysis have relied on cherry-picking and questionable statistical practice.
The only real surprise is the knowledge that large, prospective studies are still being carried out in a field that has already served its political purpose of justifying smoking bans. And it's interesting to see how sanguine the researchers and others are about these findings. It's almost as if they never believed the hypothesis in the first place and are just happy to have smoking bans.
[Gerard Silvestri, MD, of the Medical University of South Carolina, a member of NCI’s PDQ Screening and Prevention Editorial Board] finds some reassurance in the passive-smoking findings. “We can never predict who is going to develop lung cancer,” he said. “There are other modifiers. But you can say, with regard to passive smoke, it’s only the heaviest exposure that produces the risk. We kind of knew that before, but it’s a little stronger here.”
“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,” said Silvestri. “The reality is, we probably won’t.”
Take that in for a moment. Contrast that little admission with the quackery of Stanton Glantz who has spent the last few days implying that one minute of secondhand smoke 'exposure' could be lethal. Contrast it with the BMA's assertion that "there is overwhelming evidence, built up over decades, that passive smoking causes lung cancer" (a quote made in response to yet another study that showed no such relationship.)
According to Dr Jyoti Patel, of Northwestern University School of Medicine, the real reason for smoking bans isn't saving nonsmokers from lung cancer, but denormalising smoking. Who woulda thunk it?
“The strongest reason to avoid passive cigarette smoke is to change societal behavior: to not live in a society where smoking is a norm. It’s very reassuring that passive smoke in the childhood home doesn’t increase the risk of lung cancer [in this study],” said Patel. “But it doesn’t decrease the need for us to have strong antismoking measures.”
And, as the person who presented these findings says...
“The fact that passive smoking may not be strongly associated with lung cancer points to a need to find other risk factors for the disease [in nonsmokers],” said Ange Wang, the Stanford University medical student who presented the study at the June 2013 meeting of the American Society of Clinical Oncology in Chicago.
Well, yeah. Maybe that's what we should have been doing all along.