A consumer advocacy group is requesting retailers like Dollar Tree and Amazon.com to remove a well known brand of crayons that have been found to contain toxic levels of asbestos.

The U.S. Public Interest Research Group (U.S. PIRG) has found that Playskool crayons have tested positive for asbestos, a well known cause of lung cancer and mesothelioma. 36-packs of play school crayons purchased from a Dollar Tree store were tested. The same brand of crayons are also available with retailers like Amazon, eBay and DollarDays.com.

Hasbro, Playskool’s parent company, has reported that it performing a “thorough investigation” into the claims.

Dollar Tree executives, meanwhile, have reported that independent tests show that its crayons do not contain asbestos.

“The safety of our customers and associates is our top priority,” Randy Guiler, VP of Investor Relations at Dollar Tree, wrote in an email. “We are aware of the report and have since re-verified that each of the listed products successfully passed inspection and testing.”

The test results come three years after a report from the Environmental Working Group Action Fund found that certain brands of crayons manufactured in China were found to have dangerous levels of toxic asbestos fibers.

Federal laws regulate the asbestos levels in drinking water, schools and some consumer products, but do not regulate how much asbestos is allowed in children’s products, according to consumer advocacy group Cook-Schultz.

The group also tested five other crayon brands – Crayola; Target’s Up & Up; Cra-Z-Art; Disney Junior Mickey and the Roadster Racers; and Roseart – all were determined to be asbestos-free.