A new study has a big warning to yoga fans out there. Those mats that you’re using may be hazardous to your fertility thanks to a flame retardant chemical that is used in their manufacture, according to a study published in Environmental Health Perspectives from researchers at Harvard University.

They’re called organophosphate flame retardants (PFRs), and they’re a class of chemicals used in a range of things, including yoga mats as well as car seats and sofas. They’re meant to make those products less dangerously flammable, but they may be having significant effects on people’s fertility, the study found.

Researchers determined that people with the highest amount of the chemical in their urine were 10 percent less likely to achieve fertilization, and had a 31 percent lower likelihood of embryo implantation, were 41 percent less likely to achieve a pregnancy, and 38 percent to get a live birth. However, the study was relatively limited in its sample size and more will need to be conducted to confirm these findings.

“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.”