Parents beware: As your kids head back to school and you start to buy school supplies, be leary of this brand of crayons.

The U.S. Public Interest Research Group Education Fund, a consumer advocacy group reports that it tested six brands of crayons at Dollar Tree, according to Fortune. Tremoline, a type of asbestos, was reportedly found in a green Playskool crayon. Playskool is made by Hasbro, and is sold at Dollar Tree stores in New England and the rest of the U.S. These Playskool crayons are also available on eBay and Amazon.

Asbestos is potentially dangerous if ingested or inhaled and can cause serious health problems, as per Fortune. The advocacy group is asking Dollar Tree to remove Playskool crayons from its stores.