Encyclopedia · humectant
Glycerin
A cheap water-binding ingredient in nearly every conditioner; no peer-reviewed hair study tests it, and the popular humidity rules are untested.
What it is
A three-carbon sugar alcohol from plant or petroleum sources, used as a humectant and solvent. It is a cosmetic ingredient with no hair-specific restrictions in the US, EU or UK.
How it works
It binds water, so on hair it can exchange water with the air. Whether that helps or hurts depends on humidity, dose, formula and porosity according to brand guides and blogs, but we found no controlled measurement (see humectants and humidity). Hair water content itself rises roughly linearly with humidity from 40 to 85% relative humidity. Gao 2007
What the studies show
- Neither of the two relevant papers tested glycerin. Water content was linear in humidity from 40-85%, and the work proposed a method to rate anti-frizz products (industry study). Gao 2007
- Humidity changed the friction of curly hair disproportionately. Skedung 2021
How it's used in studies
No hair studies with defined glycerin doses were found.
Side effects and safety
Irritation risk is very low at cosmetic levels.
Bottom line
Fine as one ingredient among several in a conditioner or leave-in. Do not buy or avoid products on glycerin alone. If your hair frizzes in humid air and your products are glycerin-heavy, trying a lower-glycerin product is a cheap experiment. The online claim that glycerin only works in a narrow humidity window is unsupported by published tests we located. We are very unsure of any effect.
Studies 2
Gao T. Evaluation of hair humidity resistance/moisturization from hair elasticity. J Cosmet Sci. 2007
Result: Hair frizz was measured by photographing tresses at controlled humidity; naturally curly and kinky hair was the frizziest.
Read the study ↗Skedung L et al. A Curly Q: Is Frizz a Matter of Friction? Perception. 2021
Result: Rising humidity raised perceived frizz in curly hair partly by changing friction.
Read the study ↗Related
- PanthenolA small vitamin B5 alcohol that enters the hair cortex and raised break stress in a lab study; the only humectant here with real hair-fibre data.
- PropanediolA plant-derived glycol used as a humectant and solvent; no hair-specific study was found.
- Sodium PCAA salt of an amino-acid derivative found in skin's natural moisturising factor, used as a humectant; no hair study was found.
- Humectants and humidityHair swells and softens with humidity, but the popular rules about glycerin and humidity have not been tested; panthenol is the humectant with the best fibre data.