Some keratin straightening treatments have been found to contain unsafe levels of formaldehyde, a known carcinogen, or chemicals that release formaldehyde with heating. Side effects of formaldehyde exposure, especially for salon workers, include eye irritation, rash, headaches, breathing problems, nausea, and vomiting. When buying straightening products, read the label and avoid these ingredients: formaldehyde, formol, formalin, methanal, morbidic acid, formic aldehyde, methyl aldehyde, oxymethylene, glutaraldehyde, (glyoxal) ethanedial n-octyl aldehyde, aldehyde C-8, or caprylaldehyde.