The US American Occupational Safety and Health Administration (OSHA) is warning
barber salons for exposing workers to dangerous levels of formaldehyde from
hair-smoothing products.

Many keratin-based hair smoothing products contain formaldehyde dissolved (and
chemically reacted) in water and other ingredients in the product. Because of
the way the formaldehyde reacts in these products, some manufacturers,
importers, or distributors might list other names for formaldehyde on product
information or might claim that the product is "formaldehyde-free."
Formaldehyde might be listed as methylene glycol, formalin, methylene oxide,
paraform, formic aldehyde, methanal, oxomethane, or oxymethylene. All of these
are names for formaldehyde under OSHA's Formaldehyde standard. The bottom line
is that formaldehyde can be released from hair smoothing products that list any
of these names on the label and workers can breathe it in or absorb it through
their skin. Workers can be exposed to formaldehyde during the entire hair
straightening process, especially when heat is applied (e.g. blow-drying, flat
ironing).

More info

AplusA-online.de - Source: Occupational Safety and Health Administration