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Smog in London in 2015. Credit: David Holt/Flickr via CC-BY 2.0

‘BIG C TRAFFIC SHOCK’ was the Sun’s standout headline, among several others today, claiming women who live in more polluted areas are at a higher risk of breast cancer.

But women should be reassured that the study reported in these stories didn’t draw that same conclusion.

In fact, there is no good evidence linking air pollution with breast cancer.

Here are 3 reasons why today’s reports are misleading:

1. The study behind these headlines didn’t look at breast cancer

Contrary to some of today’s headlines, the study didn’t look at women’s risk of breast cancer. Instead, the researchers took measurements of breast density from mammograms and then looked for a link between this and air pollution. Breast density is a measure of how much of a woman’s breast is made up of glandular or fatty tissue. And while breast density has been linked to breast cancer risk, it’s still unclear what’s behind the link, or how strong this link may be.

2. There was no link between women with the densest breasts and air pollution

The study split the breast density measurements in to 4 categories, from 1 (least dense) to 4 (most dense). When the researchers looked at women with the densest breasts (category 4), there was no link with pollution levels. Specifically, this was looking at air pollution called PM2.5, which includes the tiny particles of air pollution most closely linked with lung cancer. Instead, a significant link between density and air pollution levels was only found for women in category 3 (the second densest). If air pollution was a cause of breast density, you’d expect to see a link for those with the densest breasts. This wasn’t the case.

3. Correlation isn’t causation

And finally, this type of study cannot prove that pollution causes dense breasts (and certainly not breast cancer), which some headlines implied. It’s interesting to see the link between women in group 3 and pollution; further research will help dig a bit deeper in to this. But finding a link between two things doesn’t mean one caused the other, as the researchers themselves point out in a press release. There could be something else playing a role that muddles the results, which the study can’t account for. That’s the challenge of carrying out this type of research. There’s also no clear biological mechanism for how breathing in air pollution could affect breast density, and there’s certainly no good evidence that it’s affecting breast cancer risk.

What is known is that air pollution can cause lung cancer, which we’ve written about recently.

On a positive note, these misleading reports shouldn’t overshadow the fact that today is World Physical Activity Day, and evidence shows that physical activity reduces the risk of breast cancer.

So if you get chance for a walk outside today, don’t let the headlines put you off.

Emma Shields is a health information officer at Cancer Research UK

Reference

Yaghjyan, L., et al. (2017). Association between air pollution and mammographic breast density in the Breast Cancer Surveilance Consortium. Breast Cancer Research, 19 (1) DOI: 10.1186/s13058-017-0828-3