Does breathing polluted air increase risk of breast cancer?

The quality of air we breathe is known to have detrimental effects on our health. In fact, air pollution is classified as a cancer causing agent by the WHO’s International Agency for Research on Cancer (IARC). But this classification is because of the proven link between air pollution and lung cancer. As to whether air pollution can cause other cancers, including breast cancer remains largely unknown and a matter for scientific inquiry.

Researchers at Harvard University attempted to answer part of this question by investigating whether air pollution could increase one’s risk of developing breast cancer. To answer this question, researchers obtained data on nurses who have been part of on-going studies in the US. This data included; nurses residences, nurses mammography results (a mammogram is an x-ray of the breast), date when the mammogram was taken, age, body mass index, whether they had had children and number of children, use of hormonal therapy among other factors that could also influence a woman’s risk of breast cancer. The researchers then obtained measurements of air quality in areas closest to where the nurses lived.

Air pollution is measured in terms of the size particulate matter (PM) suspended in the air and the distance between nurses’ residences and roadways. Particulate matter is normally categories into three; PM2.5 – which contains particles less than 2.5 micrometers, PM2.5-10 which contains particles sizes between 2.5 and 10 micrometers and PM10 which comprises of particulate matters of size 10 micrometers or less. The smaller the size of the suspended particles the more dangerous they are when inhaled. Sources of particulate matter generally include motor vehicle emissions, tire fragments, road dust, industrial and agricultural combustion, wood burning, pollens and molds, forest fires, volcanic emissions, and sea spray.

The researchers then compared the results from the mammograms with the air pollution measurements. From the mammograms, the researchers obtained what is known as breast density. Simply, this means the percentage of breast that is made of fat tissues. There is some evidence that breast density is related to one’s risk of breast cancer such that; women with more dense breasts have higher risk of breast disease.

The researchers could not find evidence that air pollution as measured using particulate matter and proximity to roadways influenced nurses’ their breast density.

Although this research was generally well conducted, it had some weaknesses. First, the study measured air pollution at nurses’ residences. It is possible that nurses spend less time at their places of residence than other places and therefore air pollution at their residence may not reflect well the actual air quality that nurses were exposed to. Secondly, nurses spend a lot of time in health facilities which usually have well-regulated air quality and therefore research done on nurses may not be applied to the rest of residents in the US.

Original article: DuPre NC et al (2017) Residential particulate matter and distance to roadways in relation to mammographic density: results from the Nurses' Health Studies. Breast Cancer Res. 2017 Nov 23;19(1):124. doi: 10.1186/s13058-017-0915-5.