New research suggests that toxic flame retardant chemicals may be prevalent in children’s car seats.

A team from Indiana University tested 18 popular children’s car seats, finding new or traditional hazardous flame retardant chemicals in 15 of them.

“New replacement flame retardants, often marketed as safer alternatives, are lurking in children’s products without rigorous safety testing and may pose risks for children’s health,” Marta Venier, associate scientist at IU’s School of Public and Environmental Affairs and principal investigator on the study, said in a statement. “The abundance of emerging flame retardant chemicals in children’s car seats and the key role these products play as potential sources of chemical exposure is a cause for concern.”

The researchers purchased the car seats, all of which were manufactured between January 2017 and February 2018 in China, Canada or the U.S., used for the study from the Ecology Center, an independent nonprofit organization in Ann Arbor, Michigan. A total of 36 foam samples and different fabrics were tested for the study.

The researchers measured for the first time two cyclic phosphonate esters (PMMMPS) at high levels in North America in 34 of the 36 samples, suggesting that their use as a replacement flame retardant for compounds that are known to be harmful.

A pair of other emerging flame retardants—tris(2,4-di-t-butylphenyl) phosphate (TDTBPP) and resorcinol bis(diphenyl phosphate) (RDP)—were also measured in baby products for the first time ever.

Polybrominated diphenyl ethers (PBDEs) were observed in 75 percent of the samples tested, despite being phased out of use in the U.S. in 2013 over health concerns, but the researchers believe that because the PBDE levels observed were low it is likely that they were not intentionally added.

However, decabromodiphenyl ethane (DBDPE), a brominated flame retardant known to cause oxidative stress, hormone disruption and thyroid problems, was detected in four samples at high levels, suggesting that it was intentionally used.

Under U.S. law, children’s car seats are required to meet the flammability standards for car interiors outlined in the Federal Motor Vehicle Safety Standard 302.

Flame retardants are routinely used as a cost-effective way to meet this standard. However, flame retardants have been linked to a variety of negative health effects, including hormone disruption, impaired brain development, liver damage and cancer. Children are more susceptible to these effects than adults because of their smaller size and their tendency to put their hands and objects in their mouths.

Children can be exposed to flame retardants in car seats by breathing in chemicals that leach into the air out of fabrics and foam. Children can also be exposed to flame retardants by ingesting the dust, which accumulates inside the vehicle, through skin contact or by chewing on their car seats.

“We found that car seat manufacturers are intentionally moving away from certain toxic chemicals compounds that they know to be harmful, which is good news,” Yan Wu, a postdoctoral researcher at Indiana University and the lead author of the study, said in a statement. “However, we know very little about the replacement chemicals they’re using. Car seats are vital for protecting children during a vehicle crash, but more research is needed to ensure that those seats are chemically safe as well.”

The study was published in Environmental Science & Technology Letters.