SLS vs SLES: what one extra letter changes

Two cleansers, one extra step

The EU's CosIng database describes Sodium Lauryl Sulfate as the sodium salt of lauryl sulfate¹ and Sodium Laureth Sulfate as the sodium salt of sulfated, ethoxylated lauryl alcohol.² Ethoxylated means ethylene oxide has been added to the alcohol before it is sulfated. The "eth" in laureth is the mark that step leaves in the name.

US reviewers treat them as different families. SLES is assessed with the salts of sulfated ethoxylated alcohols³, while the panel's data on SLS and ammonium lauryl sulfate were used to assess a group of related alkyl sulfates.⁴ In the bottle they do the same job: CosIng lists both as cleansing surfactants that also foam.¹²

What the irritation studies show

The most direct comparison is a patch-test study that applied both at 0.125% to 2% for 6, 12 or 24 hours. SLS produced a pronounced reaction and SLES a far milder one. Skin exposed to SLS was still losing more water than normal on day 10, at every concentration tested; the SLES reaction was convincingly measurable only up to day 7.⁵

SLS irritates skin so reliably that researchers use it on purpose: the study above calls it the standard irritant⁵, and the European Society of Contact Dermatitis has published guidelines for SLS exposure tests in people.⁶ SLES is not free of irritation either. The US Cosmetic Ingredient Review found it can irritate skin and eyes but is not a sensitiser, and that it should be used only when it can be formulated to be non-irritating.³

Keep the setting in mind. The same study found that the time a detergent stays on skin and its concentration matter most to the reaction.⁵ A patch held for hours is not a shampoo rinsed off after a minute, and a label never gives the concentration, so these results compare two ingredients, not two bottles.

SLS and SLES side by side

That last row needs a word of explanation. The FDA lists names containing "-eth-", "PEG" and similar among the ingredients whose manufacture can leave traces of 1,4-dioxane, a by-product rather than an ingredient.⁷ EU rules do not count impurities in raw materials as ingredients⁸, so no list will show it either way. The FDA has recommended a purification step, vacuum stripping, since the 1980s and reports that changes in manufacturing have cut the levels it finds in products over time.⁷

"Sulfate" is a family name

A shampoo can leave out SLS and SLES and still contain a sulfate cleanser. A 2015 review of hair cosmetics names ammonium lauryl sulfate, sodium myreth sulfate and ammonium laureth sulfate alongside SLS and SLES as anionic surfactants, the negatively charged group that does most of a shampoo's cleaning.⁹ Three you may meet:

• Ammonium Lauryl Sulfate: SLS's ammonium counterpart, which the US panel assessed together with SLS.⁴
• Sodium Coco-Sulfate: the sodium salt of the sulfate ester of coconut alcohol.¹⁰ The US panel put it in the group of alkyl sulfates related to SLS and assessed it on SLS data.⁴ Its coconut origin does not take it out of that group.
• Ammonium Laureth Sulfate: SLES's ammonium counterpart, assessed in the same report.³

Shampoos rarely rely on one cleanser. Secondary surfactants, such as the amphoteric Cocamidopropyl Betaine, are added alongside the anionic ones to limit the frizz and friction those can leave on hair.⁹ So the sulfate may be only part of the cleansing system.

How to spot each on a label

• Read the middle word. Lauryl sulfate is SLS or a close relative; laureth sulfate is the ethoxylated version. On EU packs both appear under their glossary names, Sodium Lauryl Sulfate and Sodium Laureth Sulfate.¹¹
• Note where it sits. EU lists run in descending order of weight down to 1%, and below that any order is allowed⁸, so a sulfate among the first few names is one of the larger ingredients by weight.
• Check every name ending in sulfate. Not all of them are cleansers: what sulfate-free means covers the salts and conditioners that share the word.
• Sort a whole list at once. Paste it into the ingredient checker to see what each name does.

Neither name is a verdict on a shampoo. A list gives no concentrations, and how a product suits your scalp depends on the whole formula. For the rest of the bottle, see how to read a shampoo ingredient list.