Keratin hair treatments are controversial because some contain/contained formaldehyde, or more correctly, a chemical called formalin (or methylene glycol) that "when heated to 450 degrees can turn back into formaldehyde, Formaldehyde is Reasonably Anticipated to Be Human Carcinogens and has been linked to certain cancers like leukemia. So the levels of formaldehyde found in these treatments has to be checked to be in safe range especially when proper protection and ventilation are used, and this is in regards to the stylists who do many of these treatments weekly.