Formaldehyde is certified as "known to be a human carcinogen" by the U.S. National Toxicology Program (NTP). According to Cancer.org, there have been multiple studies conducted to test the effect of formaldehyde in animals and humans. One study where rats were made to inhale formaldehyde gas found that it had the potential to cause nasal cancer (via Centres for Disease Control and Prevention). Another one where rats were made to drink water mixed with formaldehyde found an increase in intestinal and stomach tumors in the animal, per Cancer.org.

Human studies have been primarily conducted on people who faced prolonged exposure to formaldehyde at their workplace. Some studies have found a link between formaldehyde exposure and cancer of the nasopharynx. Another study found that people who have consistently been exposed to formaldehyde bear a greater risk of developing leukemia — more specifically, myeloid leukemia.

Although these studies look at the risks of long-term exposure to formaldehyde, it's not ideal to inhale it in shorter periods either. According to the National Cancer Institute, inhaling any more than 0.1 ppm of the chemical can cause "watery eyes, burning sensations in the eyes, nose, and throat, coughing, wheezing, nausea, and skin irritation." However, colorist specialist Brenda Kamt told Marie Claire that you can mitigate the risk of exposure during keratin treatment by choosing to get the procedure done only a few times per year in a well-ventilated salon or an open area.