My annotated list of these studies can be downloaded here and most of the full papers can be read online, so I'll leave the interested reader to make up their own mind, but one question that has been raised is why so many of them are from the '80s and '90s and why are so few of them from the last five years?
The answer, quite simply, is that with SHS popularly recognised as lethal, there is little incentive to carry out further research. Researchers follow research grants and today—with private homes now the focus of anti-smoking campaigns—the big money is to be made in thirdhand smoke, childhood exposure and maternal smoking research.
Studies that produce epidemiological evidence for SHS exposure to adults in normal settings are now few and far between. They tend to include SHS as one possible hazard amongst many and/or receive minimal press attention.
Amongst the first category we might include the very obscure Neuberger study of 2006 ('Risk Factors for Lung Cancer in Iowa Women: Implications for Prevention', Cancer Detection and Prevention) which found that passive smokers were 73% less likely to develop lung cancer than the unexposed group—a finding that the researchers understandably chose not to dwell on.
Amongst the latter category, we could include last year's Tse study ('Environmental Tobacco Smoke and Lung Cancer Among Chinese Nonsmoking Males: Might Adenocarcinoma Be the Culprit?', American Journal of Epidemiology) which found no statistically significant increase in lung cancer risk for passive smokers in either the home or the workplace. As with Neuberger, this 'unhelpful' finding was brushed over in the text of the study by authors, and while they gamely tried to find a stronger (but still nonsignificant) association with adenocarcinoma (which is the type of lung cancer least associated with smoking), this was not a study that received a big press release.
And now—well-spotted by Dick Puddlecote, since it received no press attention—comes another null study. This one (D. Brenner, 'Lung cancer risk in never-smokers: a population-based case-control study of epidemiologic risk factors', BMC Cancer, 2010) found precisely zero increased risk of lung cancer from either childhood or adult exposure to SHS (1.0 95% CI: 0.6-1.8, and 1.0 95% CI: 0.5-2.0 respectively) and no statistically significant increase for workplace exposure (1.2 95% CI: 0.7-2.0). It did, however, find a near-trebling of risk for exposure to paints and solvents (2.8 95% CI: 1.6-5.0) and for exposure to smoke-soot and exhaust (2.8 95% CI: 1.4-5.3). The authors conclude:
Our results support the concept that exposure to exhaust fumes and or soot/smoke (from non-tobacco sources) is a source of carcinogenic exposure.
This study is—or should be—of particular interest since its sample group is unusually large, comprising some 445 lung cancer cases. This puts it high in the rankings of large, well-conducted studies (size being very important when it comes to accurately quantifying risk).
Instead, this study—like most others that fail to support the passive smoking theory—has gone largely unnoticed because, as we all know, 'the debate is over'.