Science Selection March 2018 | Volume 126 | Issue 3
Air Pollution and Breast Cancer in Postmenopausal Women: Evidence across Cohorts
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Published: 27 March 2018
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Related EHP Article
Long-Term Exposure to Ambient Air Pollution and Incidence of Postmenopausal Breast Cancer in 15 European Cohorts within the ESCAPE Project
Air pollution contains complex mixtures of potentially toxic gases and particles. In 2013, the World Health Organization’s International Agency for Research on Cancer classified outdoor air pollution as a whole, and particulate matter specifically, as carcinogenic to humans, largely on the basis of studies linking it with lung and bladder tumors.1 Some researchers suspect that air pollution contributes to breast cancer as well, but the supporting evidence is mixed and weaker by comparison.2,3,4,5 Now a European meta-analysis published in Environmental Health Perspectives concludes that certain constituents of air pollution may in fact elevate the risk of breast cancer in postmenopausal women.6
“The evidence is moderate but suggestive of an association,” says Zorana J. Andersen, an associate professor at the University of Copenhagen and the study’s corresponding author. “For each unit increase in exposure we see a corresponding increase in breast cancer risk.”
For their meta-analysis, Andersen and more than 50 coauthors pooled data for 74,750 postmenopausal women. The women were part of 15 established cohorts in nine countries, each of which participates in the long-term European Study of Cohorts for Air Pollution Effects (ESCAPE). This study was launched by the European Union in 2008 and focuses on many other health indicators in addition to cancer.
At multiple locations within cities and regions, ESCAPE investigators sampled for criteria air pollutants—these included nitrogen dioxide (NO2), nitrogen oxides (NOx), and particulate matter ≤2.5 μm and ≤10 μm in diameter (PM2.5 and PM10, respectively). Then they estimated the women’s exposures to those pollutants at the home where they lived at baseline.
Combining results from all 15 cohorts based on a fully adjusted model indicated positive but statistically nonsignificant associations between postmenopausal breast cancer and exposure to PM10, PM2.5, PMcoarse (the fraction of particulate matter between 2.5 μm and 10 μm in diameter), and NO2. The researchers did find a statistically significant positive association with NOx, with each 20-μg/m3 increase in estimated NOx exposures associated with a 4% increase in breast cancer risk across cohorts. Vehicular traffic is the main source of NOx.7
The new meta-analysis accounted for shortcomings that probably led to inconsistent findings in earlier research, according to Dan Lawson Crouse, a research associate at the University of New Brunswick who was not involved in the meta-analysis. For example, the earlier studies focused on limited populations in defined locations, Crouse says, even though the chemical makeup and carcinogenicity of air pollution varies from place to place depending on the nature of local sources. Moreover, Andersen adds, because researchers tend to model air pollution exposures in different ways, it can be difficult to combine disparate data sets to generate stronger conclusions.
Robert Hiatt, a professor in the Department of Epidemiology and Biostatistics at the University of California, San Francisco, applauds the study’s size and meta-analytical framework. He adds, “While the results are not hugely strong in terms of risk, they are significant for NOx, which is worth paying attention to.” Hiatt was not involved with the current meta-analysis.
Andersen acknowledges that the study has some limitations, especially the fact that it did not account for exposures at younger ages. Mounting evidence, Andersen says, suggests that teenage girls are especially vulnerable to pollution effects that might contribute to breast cancer later in life. “What we really need now are studies with early exposure,” she says. “We might find that’s when air pollution exposures matter most.”
Charles W. Schmidt, MS, an award-winning science writer from Portland, Maine, writes for Scientific American, Science, various Nature publications, and many other magazines, research journals, and websites.
1. IARC (International Agency for Research on Cancer). 2016. “Outdoor Air Pollution.” IARC Monogr Eval Carcinog Risk Hum 109. http://monographs.iarc.fr/ENG/Monographs/vol109/mono109.pdf [accessed 12 March 2018].
2. Crouse DL, Goldberg MS, Ross NA, Chen H, Labrèche F. 2010. Postmenopausal breast cancer is associated with exposure to traffic-related air pollution in Montreal, Canada: a case–control study. Environ Health Perspect 118(11):1578–1583, PMID: 20923746, 10.1289/ehp.1002221.
3. Hystad P, Villeneuve PJ, Goldberg MS, Crouse DL, Johnson K, Canadian Cancer Registries Epidemiology Research Group. 2015. Exposure to traffic-related air pollution and the risk of developing breast cancer among women in eight Canadian provinces: a case–control study. Environ Int 74:240–248, PMID: 25454241, 10.1016/j.envint.2014.09.004.
4. Hart JE, Bertrand KA, DuPre N, James P, Vieira VM, Tamimi RM, et al. 2016. Long-term particulate matter exposures during adulthood and risk of breast cancer incidence in the Nurses Health Study II prospective cohort. Cancer Epidemiol Biomarkers Prev 25(8):1274–1276, PMID: 27257091, 10.1158/1055-9965.EPI-16-0246.
5. Reding KW, Young MT, Szpiro AA, Han CJ, DeRoo LA, Weinberg C, et al. 2015. Breast cancer risk in relation to ambient air pollution exposure at residences in the Sister Study cohort. Cancer Epidemiol Biomarkers Prev 24(12):1907–1909, PMID: 26464427, 10.1158/1055-9965.EPI-15-0787.
6. Andersen ZJ, Stafoggia M, Weinmayr G, Pedersen M, Galassi C, Jørgensen JT, et al. 2017. Long-term exposure to ambient air pollution and incidence of postmenopausal breast cancer in 15 European cohorts within the ESCAPE project. Environ Health Perspect 125(10):107005, PMID: 29033383, 10.1289/EHP1742.
7. Weichenthal S, Bai L, Hatzopoulou M, Van Ryswyk K, Kwong JC, Jerrett M, et al. 2017. Long-term exposure to ambient ultrafine particles and respiratory disease incidence in Toronto, Canada: a cohort study. Environ Health 16(1):64, PMID: 28629362, 10.1186/s12940-017-0276-7.