Humans are widely exposed to estrogenically-active phthalates, parabens, and phenols, raising concerns about potential effects on breast tissue and breast cancer risk. We sought to determine the association of circulating serum levels of these chemicals (reflecting recent exposure) with mammographic breast density (a marker of breast cancer risk).

We recruited postmenopausal women aged 55-70 years from mammography clinics in Madison, Wisconsin (N=264). Subjects completed a questionnaire and provided a blood sample that was analyzed for mono-ethyl phthalate, mono-butyl phthalate, mono-benzyl phthalate, butyl paraben, propyl paraben, octylphenol, nonylphenol, and bisphenol A (BPA). Percent breast density was measured from mammograms using a computer-assisted thresholding method.

Serum BPA was positively associated with mammographic breast density after adjusting for age, body mass index, and other potentially confounding factors. Mean percent density was 12.6% (95% CI: 11.4, 14.0) among the 193 women with non-detectable BPA levels, 13.7% (95% CI: 10.7, 17.1) among the 35 women with detectable levels below the median (<0.55 ng/mL) and 17.6% (95% CI: 14.1, 21.5) among the 34 women with detectable levels above the median (>0.55 ng/mL; Ptrend=0.01). Percent breast density was also elevated (18.2%; 95% CI: 13.4, 23.7) among the 18 women with serum mono-ethyl phthalate above the median detected level (>3.77 ng/mL) compared to women with non-detectable BPA levels (13.1%; 95% CI: 11.9, 14.3; Ptrend=0.07). No other chemicals demonstrated associations with percent breast density.

Postmenopausal women with high serum levels of BPA and mono-ethyl phthalate had elevated breast density. Further investigation of the impact of BPA and mono-ethyl phthalate on breast cancer risk using repeated serum measurements or other markers of xenoestrogen exposure are needed.