Despite label claims of being “formaldehyde-free”, many keratin-based hair straighteners, when tested, were found to contain formaldehyde, a known carcinogen. The levels of formaldehyde found were fairly low, and shouldn’t pose a hazard if you’re only straightening your hair a few times a year, but more often than that isn’t a great idea. And stylists, who use the products on their customers regularly, are at risk.