In 1928 Schönherr proposed that lung cancers among non-smoking women could be caused by inhalation of their husbands' smoke.1 Since then a substantial body of research has appeared, but the impact of environmental tobacco smoke on health remains under dispute.2 The paper by Enstrom and Kabat in this week's BMJ will add to this debate.3

Given the small health risks associated with exposure to environmental tobacco smoke and thus the large study sizes required, meta-analysis has played an important part in establishing the apparent adverse health effects. A controversial issue in this regard relates to an analysis of the American Cancer Society's first cancer prevention study, funded by the tobacco industry.4 This has not generally been included in meta-analyses, although it would contribute the largest number of events to such an analysis. The main argument advanced for not including it in meta-analyses is that the published analysis of the study was not presented in a format that allowed for the combination of equivalent effect estimates across studies.

Enstrom and Kabat have analysed the Californian subsample of the American Cancer Society's first cancer prevention study (ACSI), with considerable additional follow up, and have presented data in a format that allows inclusion in future meta-analyses. They interpret their findings as null, although, inevitably, statistical uncertainty remains. They may overemphasise the negative nature of their findings. With respect to chronic obstructive pulmonary disease—plausibly related to exposure …