Playskool brand crayons sold at Dollar Tree, Amazon and other retailers were found to contain traces of asbestos.The findings were released by the U.S. Public Interest Research Group that tested the safety of school supplies. The group also said three-ring binders sold at Dollar Tree contained high levels of phthalates, and dry-erase markers were found to contain benzene, which is considered a potential carcinogen. Carcinogens are substances considered to potentially can cause cancer in living tissue, according to Cancer.org.U.S. PIRG has recommended a voluntary recall of products found to contain harmful chemicals.The advocacy group tested 36 packs of crayons from a Dollar Tree in Chicago. The crayons are for sale on eBay and DollarDays.com as well."There is no reason to be exposing kids to a known carcinogen, especially in crayons," U.S. PIRG toxins director Kara Conok-Schultz said, according to the Washington Post.Hasbro, the parent company of Playskool, said it is conducting a "thorough investigation" into the group's claims. Leap Year Publishing, the company manufacturing the crayons, said it also is looking into lab tests of the crayons.Randy Guiler, vice president of investor relations at Dollar Tree, said, "The safety of our customers and associates is our top priority. We are aware of the report and have since re-verified that each of the listed products successfully passed inspection and testing."U.S. PIRG tested five other crayon brands, including Crayola, RoseArt, Up & Up by Target, Cra-Z-Art, Roadster Racers and Disney Junior Mickey. Those brands did not contain asbestos.It is legal for crayons to contain the carcinogen in the United States.U.S. PIRG annually tests toy safety, and the tests have resulted in more than 150 recalls and regulatory actions in the past 30 years. Last year, Target removed two types of fidget spinners after the group determined that the products contained as much as 330 times the legal limit of lead allowed in children's products.