Sodium lauryl sulfate

What it is

Sodium lauryl sulfate (SLS; in labs, sodium dodecyl sulfate or SDS) is the sodium salt of sulfated lauryl alcohol: a 12-carbon tail with a small, strongly negative sulfate head. It is made from coconut or palm-kernel fatty alcohols or from petrochemical alcohols. Commercial grades are mixtures of chain lengths, and the share of 12-carbon chains changes how irritating a grade is (Agner 1989). It is a classic primary shampoo surfactant that foams well in hard and soft water and rinses easily (D'Souza 2015).

How it works

Like all surfactants, it lifts oil off hair by roll-up, emulsification and micelle solubilization (see how shampoo works). What sets SLS apart:

• Small, highly charged micelles. SDS micelles (radius about 20 angstroms) are small enough to enter the pores of the skin's outer layer, and SDS penetration into skin rises with concentration above the micelle threshold (Moore 2003).
• Protein and lipid damage. Once in the skin's outer layer, anionic surfactants bind and denature keratin and extract intercellular lipids, and repeated exposure leads to dryness and irritation (Morris 2019). On hair, anionic surfactant solutions pull proteins, cuticle fragments and pigment out of the fibre in proportion to their charge density (in vitro) (Pires-Oliveira 2014).
• Mildness by blending. Adding amphoteric, non-ionic or amino-acid surfactants reduces SLS penetration and irritation (Moore 2003; Morris 2021; Lee 1994).

What the studies show

• In a German patch-test network, 0.25% SLS was used as an irritant control in 43,478 patients. 22.4% reacted, mostly with weak redness, and stronger reactions were linked to male sex, age 40 or over, and occupational and hand dermatitis (Forkel 2026).
• In 15 volunteers with randomized skin sites and 24-hour covered patches, SLS irritation rose with concentration (0.25-1%). 1% sodium lauroyl glutamate was far milder, and mixing it in reduced SLS irritation (Lee 1994).
• In 12 people given a single exposure to 8 surfactants on the forearm and followed for 2 weeks, 2% SLS and other cleansers caused irritation by similar paths: redness, water loss and later scaling (Barany 1999).
• Patch tests of different SLS grades showed irritancy differed between commercial grades because of different 12-carbon content (Agner 1989).
• In a controlled test, acute SLS irritation was similar in atopic and non-atopic volunteers, or slightly lower in the atopic group (Basketter 1998).
• A safety assessment of related alkyl sulfates supported their safety as used, at 0.1-29% and mostly in soaps and shampoos (Fiume 2010).

No randomized trial has compared SLS-based with SLS-free shampoos on hair breakage, hair loss or long-term hair quality.

How it's used in studies

• In shampoo formulas, the primary surfactant is used at up to about 10% (Thompson 2023).
• As a deliberate irritant in dermatology research and patch-test clinics: 0.25% in water in the German baseline series (Forkel 2026) and 0.25-1% under 24-hour covering in irritation studies (Lee 1994).

Side effects and safety

• Irritant, not an allergen. SLS irritation depends on dose and contact time, and it is used as an irritant control, not because allergy is the concern. A shampoo's contact is brief and diluted, which lowers the risk compared with patch tests.
• Who reacts more: people with existing hand or occupational dermatitis, men and people over 40 in the German data (Forkel 2026). Atopic status did not increase acute irritation in a controlled test (Basketter 1998), but people with eczema often have a weaker barrier day to day.
• Eyes: it is a recognised eye irritant, which is why "no tears" baby shampoos use amphoteric surfactants (D'Souza 2015).
• Claims that SLS causes cancer, hair loss or builds up in organs: we found no credible evidence.
• 1,4-dioxane is not relevant to SLS itself, which is not ethoxylated. That issue applies to sodium laureth sulfate.

Bottom line

SLS is an effective, very cheap cleanser. It makes sense in clarifying shampoos and for oily scalps or heavy product users, especially in a well-buffered blend with amphoterics. It is the first ingredient to avoid for people with sensitive, eczema-prone or irritated scalps, and for dry, curly or chemically treated hair, where its stripping power is more than needed. Formulation matters more than the name: a sulfate blended with betaines can be gentler than a poorly balanced "sulfate-free" product. We are fairly sure about its irritancy on skin and much less sure about any harm to hair fibres at real shampoo exposure. Most drugstore shampoos now lead with SLES rather than SLS, and amino-acid or isethionate shampoos cost more.

Fun facts

• Dermatology clinics apply 0.25% SLS on purpose during patch testing as a known-irritant benchmark. In more than 43,000 German patients, about 1 in 5 reacted to it (Forkel 2026).