Are the Sodium Lauryl Sulfates in Shampoo Safe?

One of the more common lather-producing agents is sodium lauryl sulfate (SLS), the less expensive and more abrasive cousin to sodium laureth sulfate (SLES). The molecules are very similar, though.

SLS is a detergent, a crystalline salt of sulfated lauryl alcohol. It's good at general cleaning because it's a surfactant — a substance that breaks up surface tension, the bonds between molecules in the outer layer of a compound. This is how SLS produces lather.

It eases those bonds, allowing two separate entities — say, shampoo and your hair — to interact more effectively. Without this loosening of surface bonds, shampoo wouldn't be as good at removing dirt and oil from your hair. The ingredient works particularly well on oily hair but could be too harsh on fine hair.

SLS is a common ingredient in all sorts of household cleaning products. You'll also find it in toothpastes, mouthwashes, hand creams, sunscreens and, as noted with horror in the anonymous e-mail, garage-floor cleaners.

The most common reaction at this point is: Really? The stuff in our shampoo is also in garage-floor cleaners? How can that be safe?

It's a bold claim, and the e-mail had the desired effect. Lots of people switched to "all-natural," sulfate-free shampoos that proudly assert they don't include SLS, presumably to avoid shampoo-related cancer risk.

But are these people really less likely to develop cancer than those of us who use regular old SLS-containing hair cosmetics? As it turns out, there are certain risks associated with sodium lauryl sulfate, but not the one you're thinking of.

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

As with so many widely forwarded, fear-inducing e-mails, 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 to 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.

Some have also falsely claimed that SLS and formaldehyde together create nitrosamines (which are carcinogenic) as a byproduct. 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 e-mail rumor suggests. SLS can cause skin irritation 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.

Other Reasons to Avoid Sulfate Shampoos

While SLS can lead to perfectly healthy hair in one person, it might not work for another. Not all hair types, such as curly hair because it can strip oil and leave the scalp dry, respond well to the ingredient.

No one is sure how the rumor about sodium lauryl sulfate and shampoo got started. Snopes speculated that someone might have confused ethanolamine lauryl sulfate (an ingredient 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 e-mail made the rounds — so much so, in fact, that many experts suspect the e-mail 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.