Hair smoothing product can expose to unsafe levels of formaldehyde

According to a recently published study in the Journal of Occupational and
Environmental Hygiene, both salon workers and their customers can be exposed to
potentially dangerous levels of formaldehyde during the application of hair
smoothing products. In particular, the University of California Berkeley
researchers found that "using Brazilian Blowout, without proper engineering
controls like local exhaust ventilation, could expose hairdressers and their
clients to formaldehyde at levels above the short-term occupational exposure

More info - Source: Occupational Safety and Health Administration