Methanediol is a product of the hydration of formaldehyde H2C=O, and predominates in water solution: the equilibrium constant being about 103, and in a 5% by weight solution of formaldehyde in water, 80% is in the methanediol form.
Methanediol is listed as one of the main ingredients of "Brazilian blowout", a hair-straightening formula marketed in the US. The equilibrium with formaldehyde may cause some concern as formaldehyde in hair straighteners is a health hazard.