Secondhand smoke increases bladder cancer risk

We have known since the 1980s that secondhand tobacco smoke causes lung cancer, but there has not been much attention to whether secondhand smoke causes other cancers that active smoking causes.

I recently became aware of a well-done meta-analysis of the association between secondhand smoke and bladder cancer. (Bladder cancer is right behind lung cancer in the level of risk due to active smoking.) Five years ago, in 2018, Huaqing Yan and colleagues in the Department of Urology in the Zhejiang University College of Medicine in China published “Secondhand smoking increases bladder cancer risk in nonsmoking population: a meta-analysis.” This paper, which is based on 14 population studies of the association between secondhand smoke exposure and bladder cancer, found a statistically significant increase in risk, with a 22% increase in risk.

The chart above presents the results of all 14 studies, with the point estimates as dots and the width of the bars showing the 95% confidence intervals (which are commonly called “margin of error”). For an individual study to be statistically significant, the 95% confidence interval needs to exclude 1.0 (which means no change in risk. All but one of the studies’ confidence intervals include 1.0, which means that, considered one at a time, most of the studies do not indicate a risk of bladder cancer higher than one would expect by chance.

At the same time, 12 of the 14 studies reveal point estimates above 1.0, which suggests an elevated risk. (It is highly unlikely to flip a fair coin 14 times and get heads 12 times by chance.)

What a meta-analysis does is pool all 14 of the studies to get a single overall risk estimate. Doing so yields a pooled relative risk of 1.22, with a 95% confidence interval extending from 1.06 to 1.40. Because the pooled confidence interval excludes 1.0, the risk is statistically significantly different from 1.0.

Huaqing and colleagues also did a series of tests to show that the results are reliable.

They also examined the association between childhood exposure and adult bladder cancer. While the overall analysis also showed a positive association, they found that this conclusion was particularly sensitive to one study, so concluded that more research is needed. While this is a scientifically prudent decision, it does raise concerns.

Since this paper is 5 years old, it would be worthwhile updating the searches to see what adding newer data shows. It’s also time for the CDC and others to take a formal look at passive smoking and bladder cancer and update its list of secondhand smoke-caused cancers. Clean air advocates also need to add bladder cancer to the list of reasons for protecting nonsmokers from secondhand smoke.

Here is the abstract:

Background: Tobacco smoking has been widely acknowledged to be the most important risk factor for bladder cancer. However, whether secondhand smoking (SHS) increases the risk of bladder cancer still remains uncertain. We conducted a meta-analysis about the risk of bladder cancer and lifetime SHS and childhood SHS.

Materials and methods: We searched PubMed, EMBASE, Web of Science, and Chinese National Knowledge Infrastructure (CNKI) up to March 12, 2018, and checked references of the retrieved articles and relevant reviews to include 14 studies. Relative risk (RR) and 95% confidence interval (CI) were used to assess this risk.

Results: The pooled RR of 14 eligible studies based on the retrieved articles and relevant reviews illustrated a significantly increased risk of bladder cancer with RR 1.22, 95% CI 1.06-1.4. No heterogeneity or publication bias was found. But we need more evidence to prove a more reliable association between childhood SHS and bladder cancer.

Conclusion: There was a statistically significant 22% increased risk of bladder cancer for lifetime SHS exposure in nonsmoking patients compared with unexposed nonsmoking population. But the association between childhood SHS exposure compared with unexposed nonsmoking population was unclear. Further research should be conducted to confirm our findings and reveal the potential biological mechanisms.

The full citation is: Yan H, Ying Y, Xie H, Li J, Wang X, He L, Jin K, Tang J, Xu X, Zheng X. Secondhand smoking increases bladder cancer risk in nonsmoking population: a meta-analysis. Cancer Manag Res. 2018 Sep 21;10:3781-3791. doi: 10.2147/CMAR.S175062. PMID: 30288109; PMCID: PMC6159806. It is available here.