The product contains methylene glycol, which releases formaldehyde when heated. That is where the concern comes from and it is a fair question. The US Food and Drug Administration has proposed banning formaldehyde and methylene glycol in hair-smoothing products, and the safety research consistently points at over-application and poor ventilation as what makes the difference. If you have walked into a salon doing one of these and the smell knocked you back, somebody used too much. In keratin smoothing, less is more. Come in with questions. I would rather answer them than have you sit there wondering.