The evidence that secondhand smoke causes breast cancer keeps piling up. When will Surg Gen, CDC, ACS and others start acting on this evidence?

In 2005 the California EPA found secondhand smoke caused breast cancer in younger women. (Related regulatory documents available here.) Amazingly, as of 2024 — 19 years later — neither the CDC (and the Surgeon General, who follows the CDC) nor the American Cancer Society list smoking or secondhand smoke as risk factors for breast cancer.

Now Irene Possenti and colleagues published “Exposure to second-hand smoke and breast cancer risk in non-smoking women: a comprehensive systematic review and meta-analysis” that further documents the overwhelming case that secondhand smoke causes breast cancer. The literature has grown to 63 original articles (as of October 2022) that they included in a meta-analysis that show secondhand smoke is associated with a 24% increase in the risk of breast cancer among nonsmoking women (relative risk [RR] 1.24; 95% confidence interval, CI, 1.15-1.34).

They also found important dose-response relationships, which further strengthens the conclusion that this risk is real: breast cancer risk increased with increasing duration, intensity and pack-years of SHS exposure.

Possenti and colleagues do a good job of explaining how important this risk is:

… moderate alcohol consumption (i.e., at least 12.5 g per day of alcohol) increased the risk of breast cancer by 23%. For this reason, quitting alcohol drinking or reducing alcohol consumption is today considered one of the main strategies to prevent breast cancer risk. With an excess risk of 24% for women exposed to SHS, our meta-analysis places SHS exposure among the main modifiable lifestyle-related risk factors for the development of breast cancer This is particularly relevant given the relatively high frequency of SHS exposure, affecting globally 35% of women, much higher than the estimate of 25% of women who consume alcoholic beverages. [citations dropped]

Of course, this is not the first study confirming the 2005 CalEPA conclusions. In 2014 — ten years ago — we published a paper concluding that “there are now 12 large cohort studies that consistently demonstrate a dose-response relationship with smoking before first birth and increased breast cancer risk. The Surgeon General’s increasing caution is preventing young women around the world from appreciating the risks that smoking and secondhand smoke pose for developing breast cancer.”

It is long-past time for the Surgeon General, CDC, American Cancer Society and breast cancer advocacy groups to accept and fight to eliminate this major avoidable list for breast cancer.

Here is the abstract:

Through the use of an innovative method to identify original publications, we conducted a meta-analysis of all epidemiological studies evaluating the association between second-hand smoke (SHS) exposure and breast cancer risk among female non-smokers published in English up to October 2022. Pooled relative risks (RR) were obtained through the use of random-effects models. Dose-response relationships were derived using log-linear functions. Out of 73 identified eligible studies, 63 original articles were included in the meta-analysis. The pooled RR for breast cancer for overall exposure to SHS was 1.24 (95% confidence interval, CI, 1.15-1.34, number of articles, n = 52). Regarding the setting of exposure, RRs were 1.17 (95% CI 1.08-1.27, n = 37) for SHS exposure at home, 1.03 (95% CI 0.98-1.08, n = 15) at the workplace, 1.24 (95% CI 1.11-1.37, n = 16) at home or workplace, and 1.45 (95% CI 1.16-1.80, n = 13) for non-specified settings. The risk of breast cancer increased linearly with higher duration (RR 1.29; 95% CI 1.04-1.59 for 40 years of SHS exposure, n = 12), intensity (RR 1.38; 95% CI 1.14-1.67 for 20 cigarettes of SHS exposure per day, n = 6), and pack-years (RR 1.50; 95% CI 0.92-2.45 for 40 SHS pack-years, n = 6) of SHS exposure. This meta-analysis shows a statistically significant excess risk of breast cancer in women exposed to SHS.

The full citation is: Possenti I, Scala M, Carreras G, Bagnardi V, Bosetti C, Gorini G, Maci C, Malevolti MC, Odone A, Smits L, Specchia C, Gallus S, Lugo A. Exposure to second-hand smoke and breast cancer risk in non-smoking women: a comprehensive systematic review and meta-analysis. Br J Cancer. 2024 Jun 28. doi: 10.1038/s41416-024-02732-5. Epub ahead of print. PMID: 38942988. It is available here.