Scientists are reporting a possible link between the use of sunscreen containing a certain ingredient that mimics the effects of the female sex hormone estrogen and an increased risk of being diagnosed with endometriosis, a painful condition in which uterine tissue grows outside the uterus. They describe the report, published in the American Chemical Society’s journal Environmental Science & Technology, as the first to examine whether such a connection may exist.

Kurunthachalam Kannan and colleagues explain that some sunscreens and other personal care products contain benzophenone (BP)-type ingredients that are very effective in blocking potentially harmful ultraviolet rays from the sun. Small amounts of BPs can pass through the skin and be absorbed into the blood, where they mimic the effects of estrogen. Endometriosis, which affects up to 1-in-10 women of reproductive age, needs estrogen to develop. Despite those facts, scientists until now had not checked for a connection between the use of BP sunscreens and the likelihood of being diagnosed with endometriosis.

To fill that knowledge gap, the scientists analyzed BP levels in the urine of 625 women who underwent surgery for endometriosis. They found that high levels of one BP called 2,4OH-BP were associated with an increased risk of an endometriosis diagnosis. Women tended to have higher levels of BPs during the summer months and if they lived in sunny California, further suggesting a link with sunscreens. “Our results invite the speculation that exposure to elevated 2,4OH-BP levels may be associated with endometriosis,” say the researchers.