SLS, SLES and What the Labels Mean

Three letters, four letters, a different spelling on every bottle. SLS, SLES, SDS, ALS, sulfate with an f on an imported label and sulphate with a ph on an Australian one. Somewhere in there is a real difference worth knowing about, and a lot of noise that is not worth your time.

So here is the plain version. What the letters stand for, what the one that keeps appearing in headlines actually is, and what a bottle promising to be free of all of it is really telling you.

The letters, decoded

Every name on the list below is a surfactant, a molecule with a water-loving end and an oil-loving end that lets water carry away oil it would otherwise slide straight past.

What the abbreviations on a shampoo label stand for

The SDS and SLS overlap catches people constantly, and it is the source of a lot of circular online argument. They are not two ingredients with different safety profiles. They are one ingredient with a cosmetic name and a laboratory name.

Why one extra syllable changes the molecule

Look at lauryl and laureth. The difference is a process called ethoxylation, which strings ethylene oxide units onto the molecule and makes it physically larger. Larger molecules penetrate skin less readily, which is why laureth sulphate is the milder of the pair.

You can see the consequences in how formulators talk about them. In her review of hair cleansing for the International Journal of Trichology, dermatologist Zoe Draelos describes lauryl sulphates as working well in both hard and soft water, producing rich foam and rinsing easily, then adds that they are excellent cleansers, but hard on the hair. Laureth sulphates get a noticeably warmer write-up, providing excellent cleansing while leaving the hair in good condition, with consumers liking them for their abundant foam. A review in the Indian Journal of Dermatology reaches the same place, describing laureth sulphates as having good cleansing and foaming properties suited to normal-to-dry hair.

Which gives you a useful rule. If a shampoo contains a sulphate and your hair is dry or coloured, laureth is the gentler of the two to find on the label.

Where 1,4-dioxane comes into it

Ethoxylation has one genuine complication, and it is the most legitimate concern in the whole sulphate conversation. The process can leave behind a trace contaminant called 1,4-dioxane.

The United States Food and Drug Administration is clear that it is not an intended ingredient. It forms as a byproduct when certain ingredients are manufactured. The agency flags the name fragments that signal an ethoxylated ingredient, which include PEG, polyethylene glycol, polyoxyethylene, oxynol and the one hiding in plain sight on your shampoo, the syllable eth. That is the eth in laureth.

What regulators actually say about it

Worth getting the proportions right here, because the contaminant is real and the exposure is small.

• The United States National Toxicology Program describes 1,4-dioxane as reasonably anticipated to be a human carcinogen, based on animal studies, with human data inadequate for evaluation. The Environmental Protection Agency calls it likely to be carcinogenic to humans.

• Since the 1980s the FDA has recommended manufacturers use a technique called vacuum stripping to reduce it, and reports that manufacturing changes have produced a significant decline over time.

• In an FDA survey, around 80 per cent of products tested contained none at all, and the highest level found was 11.6 parts per million.

• The European Union Scientific Committee on Consumer Safety has concluded that trace levels at or below 10 parts per million are considered safe for consumers.

So the honest summary is that a hazardous substance exists at trace levels in some ethoxylated ingredients, the amounts measured are at or near the threshold regulators consider safe, and the trend is downward. It is a reason to prefer a formula that avoids ethoxylated sulphates. It is not a reason to panic about a bottle you already own.

The rumour that will not die

You have probably met the claim that sulphates cause cancer, usually in a forwarded email or a caption with too many exclamation marks. It does not hold up.

A 2022 review in the Journal of the American Academy of Dermatology examined the aluminium, paraben and sulphate disclaimers now printed on personal care products and concluded there is limited evidence that any of the three pose a health risk, with the evidence on topical sulphate-containing products weaker still. The authors note that product disclaimers face limited regulation and may be helpful or may mislead the public.

The real, documented issue with sulphates is irritation and stripping, not cancer. That is a question of comfort, colour longevity and how your hair feels, which is a perfectly good basis for a decision, and the one you should actually be making it on.

Sulphate-free is a formulation choice, not a safety badge

The same 2022 review contains a warning that deserves far more attention than it gets. When parabens were removed from products in response to consumer pressure, manufacturers reached for alternative preservatives. Some of those replacements are more allergenic, and the authors describe the resulting rise in allergic contact dermatitis from isothiazolinones as an epidemic.

The lesson generalises. Removing an ingredient class changes what has to do its job instead, and the replacement is not automatically kinder to you.

The ingredient most likely to bother you is not a sulphate

Here is the part the category rarely volunteers. Cocamidopropyl betaine, the mild amphoteric surfactant that appears in a great many sulphate-free and baby shampoos, was named Allergen of the Year by the American Contact Dermatitis Society in 2004. A review in the journal Dermatitis puts contact sensitisation prevalence at between 3.0 and 7.2 per cent, and attributes much of it to manufacturing byproducts called amidoamine and dimethylaminopropylamine rather than the betaine itself.

That is not a reason to avoid it. Most people use it happily for decades, purity grades have improved, and the rate is still low. It is a reason to treat free-from claims as information about a formula rather than a guarantee about your skin. If a product irritates you, the culprit is as likely to be a mild surfactant, a fragrance or a preservative as anything with sulphate in the name.

How to read a label in ten seconds

Ingredients are listed in descending order of concentration, so the cleansing system is almost always in the first three or four names after water.

• Look for the word sulphate or sulfate in those opening names. Both spellings mean the same thing, and either may appear depending on where the product was labelled.

• Check for the syllable eth if ethoxylation matters to you, as in laureth. No eth means no ethoxylation and no 1,4-dioxane question.

• Recognise the common replacements, which include sodium lauroyl methyl isethionate, sodium cocoyl isethionate, cocamidopropyl betaine and the glucosides.

• Ignore the front of the bottle. Claims on the front are marketing. The list on the back is the formula.

A worked example helps. One of our sulphate-free shampoos leads with water, then sodium lauroyl methyl isethionate, then cocamidopropyl betaine. No sulphate, no eth, and still two genuine cleansing agents doing the work.

So why go sulphate-free at all

Because of how hair behaves, which was always the better argument. Gentler surfactants lift less of your natural oil, which suits dry, curly, coloured and heat-styled hair, holds colour longer and tends to sit better with a reactive scalp. You can read how we think about what goes into our formulas, and the trade-off is honest, since less oil removal per wash is the point rather than a flaw.

What we are not going to tell you is that sulphates are dangerous and we have saved you from them. The dermatology literature does not support that, and you are better served by a reason that is actually true. More in our frequently asked questions, and if you are unsure which cleanser suits your hair, the hair quiz is a faster route than reading labels in the aisle.

The short version

• SLS and SDS are the same molecule. Lauryl and dodecyl both mean a twelve-carbon chain.

• SLES is SLS that has been ethoxylated, which makes it larger and milder.

• Ethoxylation is where the 1,4-dioxane question comes from, at trace levels regulators treat as safe and that have been falling for decades.

• The cancer claim is not supported by dermatology reviews. Irritation and stripping are the real effects.

• Free-from labels describe a formula, not your tolerance. Read the first four ingredients and judge by how your scalp responds.

Learn the five names in the table and you will never need a free-from badge to work out what is in a bottle again.