Playschool crayons sold at Dollar Tree stores across the country have been the source of a controversial topic this week as parents begin back-to-school shopping.

Playskool crayons tested positive for trace amounts of asbestos. Tremoline, a type of asbestos, was found specifically in the green-colored crayon of the $1 packs.

The US Public Interest Research Group (US PIRG) released an annual report about the safety of school supplies. The group is asking that Dollar Tree remove its Playskool crayons from shelves and issue a nationwide recall.

Asbestos is a carcinogen that can cause serious health complications, including cancer of the lungs, larynx and ovaires. It is also linked to many cases of mesothelioma, a lung infection that takes years to manifest.

Dollar Tree has yet to respond to the demands from the watchdog group but did issue a statement that the crayons “passed all legal and safety requirements.”

US PIRG conducts its annual report and tests products from a variety of stores, both online and in-store. Samples of the supplies were sent to the STAT Analysis Corporation in Chicago. The crayons that tested positive for asbestos were purchased at a Dollar Tree in Chicago.

Kara Cook-Schultz, a member of US PIRG, said in a statement to Business Insider, “We recommend that children not be exposed to asbestos. This level of asbestos is low. But because children are a vulnerable population, we think it is an unnecessary risk to expose children to a known carcinogen like asbestos.”

In 2000, asbestos was found in Crayola and Rose Art crayons. The U.S. Consumer Product Safety Commission said the risk of children inhaling the fibers were extremely low but asked the manufacturers to voluntarily recall the products anyway.

US PIRG informed the CPSC about its investigation and asked the agency issue a voluntary recall and reformulate the controversial crayons.