Is Sodium Lauryl Sulfate in Shampoo Dangerous?

Shampoo users are familiar with the rich, foamy lather it creates when washing hair. This lather is often a key part of the shampooing ritual. Now, imagine learning that this foam might be dangerous.

In 1998, an anonymous e-mail circulated on the internet, claiming that sodium lauryl sulfate in shampoos might be linked to cancer. This chemical is the one responsible for the lather in your shampoo.

Even today, a simple search for 'does shampoo cause cancer' still leads to a variety of sketchy websites with unreliable 'data,' suggesting that the same ingredient, along with many others, may be carcinogenic. Before you switch to sulfate-free shampoos, take a moment to explore more about this common ingredient in hair care products.

Sulfate Shampoos

One widely used lathering agent is sodium lauryl sulfate (SLS), a more affordable and harsher version of sodium laureth sulfate (SLES). Though the two are chemically similar, their effects differ.

SLS is a detergent, a crystalline salt derived from sulfated lauryl alcohol. Its effectiveness in cleaning comes from being a surfactant, which reduces surface tension, or the bonds that hold molecules together in the outer layers of a substance. This action is what creates the lather in SLS.

By loosening these bonds, it allows substances like shampoo and your hair to interact more efficiently. Without this ability to break the bonds, shampoo wouldn't be able to effectively cleanse dirt and oils from your hair. This ingredient is especially effective for oily hair, though it might be too harsh for finer hair types.

SLS is a common ingredient found in a variety of household cleaning products. It’s also included in toothpastes, mouthwashes, hand creams, sunscreens, and, as shocked readers learned from the anonymous e-mail, even garage-floor cleaners.

The typical reaction to this discovery is: Wait, the same ingredient in our shampoo is also used in garage-floor cleaners? How can that possibly be safe?

It’s a daring statement, and the e-mail achieved its goal. Many switched to 'all-natural,' sulfate-free shampoos, claiming they don’t contain SLS, presumably to avoid the cancer risks implied in the email.

But are these people actually at a lower cancer risk than those who continue using regular shampoos with SLS? It turns out there are some risks tied to sodium lauryl sulfate, but they aren’t the ones you might expect.

Sodium Lauryl Sulfate and Cancer

Nearly every shampoo available in the hair-care section contains sodium lauryl (or laureth) sulfate. Given that the U.S. Food and Drug Administration must approve these products before they hit the shelves, can they really cause cancer?

As with many widely circulated, fear-mongering e-mails, this claim is inaccurate. Both the American Cancer Society and The International Agency for Research on Cancer agree: SLS is not carcinogenic [source: Bondi et al.].

The fact that SLS is also found in garage floor cleaners doesn't really matter. Arsenic, a component in wood preservatives, is also present in our drinking water. But in that form, it doesn’t harm us — it’s all about the concentration. The typical shampoo contains between 10 to 25 percent SLS [source: Schaefer].

A 2015 review of sodium lauryl sulfate concluded that 'the belief that SLS is carcinogenic is often based on studies where SLS is used to test other substances for cancer-causing properties.' While SLS is a soluble agent used in toxicology research to test other compounds, this does not imply that SLS itself is harmful.

Some have incorrectly claimed that when SLS combines with formaldehyde, it produces nitrosamines (a carcinogenic byproduct). However, this reaction is impossible because SLS and formaldehyde cannot interact to form nitrosamines — neither contains nitrogen atoms, while nitrosamines require two nitrogen atoms to form.

"The idea that SLS poses a threat to human health lacks scientific backing, and any claims to the contrary should be considered false and misleading," conclude the study authors.

Sensitive Skin and SLS

Sodium lauryl sulfate can be harmful, but not in the way the e-mail rumor suggests. It may cause skin irritation in certain situations. While it may seem odd that a cosmetic product like shampoo contains an irritant, SLS only causes irritation if it remains on the skin for too long.

It's perfectly reasonable to say that if you tend to leave shampoo on your hair for long periods, you might want to avoid SLS or risk a dry, irritated scalp. The worst-case scenario with SLS shampoo is irritation from prolonged contact.

You might also notice warnings on your toothpaste not to swallow it, partly because toothpaste contains SLS. Will swallowing too much toothpaste with SLS cause cancer? Highly unlikely. However, it could lead to a bout of diarrhea.

Other Reasons to Steer Clear of Sulfate Shampoos

While SLS may be beneficial for some people’s hair, it may not be suitable for others. Hair types like curly hair, which are more prone to dryness, can suffer from the oil-stripping effects of SLS, leaving the scalp feeling dry.

Where Did the Rumor Come From?

The origin of the rumor linking sodium lauryl sulfate to shampoo safety remains unclear. Snopes suggested that it might have stemmed from confusion between ethanolamine lauryl sulfate (used in some 1970s shampoos) and sodium lauryl sulfate. Ethanolamine lauryl sulfate was known to cause nitrosamine formation, but it has since been phased out of shampoos.

The SLS-free market experienced substantial growth after the viral e-mail in the late '90s — so much that many experts believe the e-mail might have come from a natural-foods marketing agency [source: LEDA].

Despite the complete lack of evidence connecting SLS to cancer, some individuals prefer to avoid potential irritants. For these people, there are plenty of SLS-free shampoos available from natural cosmetics brands. These can be found in health food stores and online, often at prices comparable to regular shampoos.