Evidence secondhand smoke causes breast cancer in 2005 stronger than for lung cancer in 1986

Objectives. To compare the strength of evidence from epidemiologic studies of secondhand smoke of the US Surgeon Generalʹs 1986 conclusion that secondhand smoke caused lung cancer with the California Environmental Protection Agencyʹs (CalEPA) similar 2005 conclusion on breast cancer in younger, primarily premenopausal women. Methods We reviewed each report for criteria used to assess causality: numbers of studies, statistically significant increases in risk, and pooled summary risk estimates. Results. Both the Surgeon General and CalEPA used updated Bradford Hill criteria for assessing causality and found that the evidence met those criteria. Six of 13 lung cancer studies (46%) had statistically significant increases (one of three cohort studies). Pooled risk estimates for lung cancer for spousal exposure were 1.53 for 10 combined case–control studies and 1.88 for seven studies with dose–response results. The CalEPA reported 10 of 14 studies (71%) had statistically significant increases in breast cancer risk (two of four cohort studies). Pooled relative risk estimates for younger, primarily premenopausal women were 1.68 (95% CI: 1.33, 2.12) for all exposed women and 2.19 (1.68, 2.84) for five studies with better exposure assessment. Conclusions. The evidence from epidemiologic studies of secondhand smoke in 2005 for breast cancer in younger, primarily premenopausal women was stronger than for lung cancer in 1986.