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 per cent 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 per cent, 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."