What is it?
A conditioning ingredient used mainly in hair conditioners and other hair products, and also in some body lotions and moisturizers. It is a quaternary ammonium compound, or “quat”: a long, waxy 22-carbon behenyl chain attached to a positively charged nitrogen atom. That positive charge helps it cling to hair, where it reduces static and makes hair smoother and easier to comb. It can also help emulsify creams.
Key information
- The CIR Expert Panel concluded in 2012 that it is safe as used in cosmetics when formulated to be non-irritating.
- In the CIR's use data it appeared in 273 products at 0.1 to 10 percent, most often in hair conditioners.
- Despite its name, it is not a sulfate cleanser like sodium lauryl sulfate: methosulfate is the small methyl sulfate ion that balances the quat's positive charge.
- In concentrated raw form it irritates skin, can seriously damage the eyes and is very toxic to aquatic life.
More information
Behentrimonium methosulfate is made by reacting a long-chain fatty amine with dimethyl sulfate, which adds a methyl group to the nitrogen and leaves methyl sulfate (methosulfate) as the balancing ion. Dimethyl sulfate is a hazardous chemical that the International Agency for Research on Cancer classifies as probably carcinogenic to humans (Group 2A). The Cosmetic Ingredient Review (CIR) report does not include measurements of leftover dimethyl sulfate, but dimethyl sulfate breaks down quickly in water, with a half-life of a little over an hour at neutral pH.
The CIR Expert Panel assessed it with 51 other trimonium ingredients and concluded in 2012 that they are safe as used when formulated to be non-irritating. The Panel judged that these ingredients are not expected to penetrate the skin and noted that the medical literature lacked clinical reports of irritation or allergy from them. In a patch test of a hair product containing 0.125 percent behentrimonium methosulfate with two related quats, 1 of 212 volunteers had a mild reaction and none reacted at challenge. At higher concentrations, trimoniums can irritate skin and eyes.
Undiluted, it is a harsher chemical. In notifications to the EU chemicals agency, summarized by PubChem, most companies classify the raw material as a skin irritant that can cause serious eye damage and as very toxic to aquatic life.
Reality check: the word “sulfate” in its name does not make it a harsh sulfate detergent, and at the levels used in conditioners it is considered low risk. Its main hazards belong to the concentrated raw material, which is why the CIR requires finished products to be formulated to be non-irritating.
Sources: International Journal of Toxicology (2012): Safety assessment of trimoniums as used in cosmetics (Cosmetic Ingredient Review); PubChem: Behentrimonium methosulfate (GHS classification); PubChem: Dimethyl sulfate
