If you enjoy yoga, there’s a big warning from a study just published in Environmental Health Perspectives. Researchers from Harvard claim that chemicals in yoga mats may be negatively affecting the fertility of women who use them.

It’s because of organophosphate flame retardants (PFRs), which are a class of chemicals used in all sorts of things from yoga mats to car seats to sofas in order to make them less flammable. Scientists found that those who had the highest amount of the chemical in their pee were 10 percent less likely to achieve fertilization, and 31 percent less likely to get a successful embryo implantation in their uterus.

In addition, they were 41 percent less likely to get pregnant, and 38 percent less likely to see a live birth. It’s not a conclusive study, as it is based on one population of women visiting a fertility clinic in Boston, but it does call into question many of the chemicals in our daily lives we take for granted.

“These findings suggest that exposure to PFRs may be one of many risk factors for lower reproductive success,” said first author Courtney Carignan, a research fellow in the Department of Environmental Health at Harvard Chan School. “They also add to the body of evidence indicating a need to reduce the use of these flame retardants and identify safer alternatives.”

The study will be published online August 25, 2017 in Environmental Health Perspectives.