Living in an area of high pollution may increase a women's chance of a risk factor for breast cancer, a new study suggests.
Women who live in areas with a high level of fine particles from air pollution may have an increased chance of having dense breasts.
Those who are deemed to have ''high breast density'' are at a significantly higher risk of developing breast cancer than others.
The study, published in the journal Breast Cancer Research, examined data from nearly 280,000 women in the US with an average age of 57.
Participants all had a mammogram at facilities taking part in the Breast Cancer Surveillance Consortium from 2001 to 2009.
Six in 10 lived in urban areas and the rest lived in the countryside.
Researchers found women with dense breasts were 19% more likely to have been exposed to higher concentrations of fine particle matter (PM2.5)
For every one unit increase in PM2.5, a woman's chance of having dense breasts was increased by 4%, they said.
Dr Lusine Yaghjyan, lead author from the University of Florida, 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."
Baroness Delyth Morgan, chief executive at the charity Breast Cancer Now, said: "This is a thought-provoking finding, however the direct association between air pollutants and breast cancer risk remains unclear.
"The evidence in this study linking air pollution to the highest levels of breast density was inconclusive and so further research is now needed.
"What these findings clearly demonstrate is the need for us to continue investing in studies like these which aim to pinpoint the contributing causes of breast cancer.
"Breast density is an important risk factor in the development breast cancer.
"While research is yet to determine how women might reduce their breast density, we do know that there are simple steps all women can take to reduce their risk of breast cancer, such as being physically active, maintaining a healthy weight and reducing alcohol intake."
Catherine Priestley, clinical nurse specialist at the charity Breast Cancer Care, added: "Having dense breasts is a known risk factor for breast cancer, so new insight into how this might be influenced by external causes such as air pollution is welcome.
"However, we cannot look at this in isolation. Breast cancer is a complex disease, and it is not possible to pinpoint any one cause.
"Women call our Helpline every week with anxieties about getting breast cancer and what they can do to avoid it.
"So it's important to stress that, while studies like this are interesting, the main risk factors for breast cancer remain outside our control – being female, getting older and, for some, a significant family history of the disease."