Here's how pollution can be bad for your breasts

Turns out, higher the pollution level, higher is your breast cancer risk.

A study of nearly 280,000 women in the United States has found that living in areas with a high level of fine particles from air pollution may increase a woman's chance of having dense breasts - a well-established risk factor for breast cancer.

The University of Florida study found that for every one unit increase in fine particle concentration (PM2.5), a woman's chance of having dense breasts was increased by 4 percent. Women with dense breasts were around 20 percent more likely to have been exposed to higher concentrations of PM2.5. Conversely, women with less dense, or breasts with more fatty tissue, were 12 percent less likely to have been exposed to high concentrations of PM2.5.

Breast density is measured by mammography and reflects the relative amounts of different tissue types in the breast. Breasts with a majority of fatty tissue are easier to interpret by mammography and so abnormalities become easier to spot. Dense breasts contain a higher percentage of fibroglandular tissue which obscures mammography and makes it difficult to identity abnormalities such as a breast tumour.

Lead author Lusine Yaghjyan said: "Our findings suggest that previously reported geographic variation in breast density could, in part, be explained by different air pollution patterns in urban and rural areas. Breast density is a well-established and strong breast cancer risk factor so future studies are warranted to determine if the observed associations are causal, which if confirmed may have implications for risk prevention."

The researchers also looked at the association between ozone exposure and breast density among the 280,000 women included in the study. They found that every one unit increase in ozone concentration was associated with a 3 percent lower chance of having dense breasts.

Yaghjyan explained: "We found a positive association between fine particle concentration exposure and breast density but an inverse association between ozone exposure and breast density. This is an intriguing result that warrants further investigation to unpick any possible biological mechanism that might cause ozone exposure to reduce a woman's chance of having dense breasts."

The study is published in the journal Breast Cancer Research.