Is the Sodium Lauryl Sulfate in My Shampoo Safe?

Almost every shampoo you find in the hair-care aisle has sodium lauryl (or laureth) sulfate in it. Considering that these products have to be approved by the U.S. Food and Drug Administration before they end up on the shelf, can it really be that they cause cancer?

As with so many widely forwarded, fear-inducing emails, it's wrong. Sources from the American Cancer Society to The International Agency for Research on Cancer all say the same thing: SLS is noncarcinogenic [source: Bondi et al.]. That SLS is also an ingredient in garage-floor cleaners doesn't mean much. Arsenic, an ingredient in wood preservatives, is also in our drinking water. But in our water, it doesn't kill us — it's all about the level of concentration. The typical shampoo has a SLS concentration of 10-25 percent [source: Schaefer].

A 2015 review of sodium lauryl sulfate found that "the perception that SLS is carcinogenic is often based on studies that use the ingredient to evaluate the carcinogenicity of other agents." SLS is a soluble agent used in toxicology studies to test whether other ingredients cause cancer but that doesn't mean it is toxic. It is also falsely claimed that SLS and formaldehyde together create nitrosamines (which are carcinogenic) as a by-product. But SLS and formaldehyde can't react and form a nitrosamine. Neither ingredient has any nitrogen atoms, while nitrosamines have two nitrogen atoms. "The perception that SLS is a threat to human health is not scientifically supported, and claims made to the contrary should be regarded as false and misleading," conclude the study authors.

Sodium lauryl sulfate can hurt you, but not in the way the email rumor suggests. SLS can irritate skin under certain circumstances. It may seem strange that a cosmetic product like shampoo would contain an irritant, but the only way SLS irritates the skin is if it's left on for an extended period of time. It's entirely legitimate to say that if you like to leave shampoo on your hair all day, you should probably go SLS-free or face a dry, rashy scalp. The worst-case scenario with an SLS-containing shampoo, then, is irritation if you leave it on your head too long.

You may also notice that your toothpaste container warns you not to swallow the stuff inside, and part of the reason for that is that toothpaste contains SLS. Will you get cancer if you swallow too much SLS-containing toothpaste? Doubtful. But you may end up with a case of diarrhea.

No one is sure how the rumor about sodium lauryl sulfate and shampoo got started. Snopes speculated that it might have been due to someone confusing ethanolamine lauryl sulfate (which was used in some shampoos in the 1970s) with sodium lauryl sulfate. Ethanolamine lauryl sulfate caused the formation of nitrosamine but this ingredient is no longer in shampoos.

The SLS-free market has really boomed since the late '90s when that incorrect email made the rounds — so much so, in fact, that many experts suspect the email originated from a natural-foods marketing agency [source: LEDA]. Still, even with the total lack of evidence linking SLS and cancer, some people would rather stay away from a potential irritant. For them, there are lots of SLS-free shampoos offered by natural-cosmetics companies. You can find them in any health-food-type store and all over the internet, and they don't cost any more than most regular shampoos.

Originally Published: Dec 10, 2008