Methylene glycol is an ingredient used in some hair-straightening products and is formed by the reaction of formaldehyde with water.
Formaldehyde is a toxic substance, and since the reaction with water is reversible, products containing methylene glycol can, under some circumstances, release formaldehyde; raising the question of product safety.
“Currently, there are restrictions on the use of formaldehyde is cosmetics, but none of these explicitly mention methylene glycol in hair straightening products,” says the EC’s Scientific Committee on Consumer Safety.
The scientific opinion of the SCCS is that when methylene glycol is used in hair straightening products at a concentration of 0.2 per cent, the amount of formaldehyde vapour released may exceed the World Health Organization (WHO) indoor air quality guideline of 0.1 mg/m3, and pose a health risk.
“Therefore, the Committee concluded that the use of methylene glycol hair straightening products at levels as low as the currently allowed concentration limit of 0.2 per cent for formaldehyde is not considered to be safe,” an SCCS fact sheet adds.
Both compounds methylene glycol and formaldehyde exist in equilibrium in straightening products and are constantly and rapidly transformed into each other, depending on temperature, pH, concentration, and on the presence of other molecules.
Therefore, although chemically they are two different molecules, the SCCS considers that methylene glycol, in a solution, is equivalent to formaldehyde.
Currently, it is used in hair straightening products, with concentrations that may reach up to 9.6 per cent in some cases; much higher than the limit set for formaldehyde, which is 0.2 per cent.
The use of those products also involves heat and application with a straightening iron or a blow dryer, which can release formaldehyde vapours, potentially exposing hair professionals significantly to them.