Sodium lauryl sulfate (SLS) is a synthetic cleaning agent found in shampoos, toothpastes, body washes, and household cleaners. It’s the ingredient responsible for the rich lather you associate with these products. SLS works by breaking down oils and grease so they can be rinsed away with water, making it one of the most widely used surfactants in consumer products worldwide.
How SLS Cleans
SLS is an amphiphilic molecule, meaning one end loves water and the other end repels it. The sulfate “head” of the molecule dissolves easily in water, while the 12-carbon hydrocarbon “tail” is attracted to oils and fats. This dual nature is what makes SLS so effective at cleaning.
When you apply a product containing SLS to your skin or hair, the process works in stages. First, SLS lowers the surface tension of water, helping it spread more evenly across the surface rather than beading up. Then the oil-attracting tails of SLS molecules latch onto grease, dirt, and sebum, pulling them off the surface and suspending them in the water. This process is called emulsification. Once surrounded by SLS molecules, the dirt particles can’t reattach to your skin or hair, so they rinse cleanly away.
Above a certain concentration, SLS molecules arrange themselves into tiny spheres called micelles, with the water-loving heads facing outward and the oil-loving tails tucked inside. Each micelle contains 20 to 50 molecules and acts like a microscopic trap for grease and grime. This is the same basic mechanism behind most soaps and detergents, but SLS is particularly efficient at it.
Where You’ll Find It
SLS shows up in a surprisingly wide range of products. In shampoos, it typically makes up 5 to 25% of the formula, with “gentle” or “sensitive” shampoos using concentrations on the lower end. You’ll also find it in liquid hand soaps, body washes, facial cleansers, bubble baths, and toothpastes. Beyond personal care, SLS is used in household cleaning products like dish soaps and floor cleaners.
Its popularity comes down to cost and performance. SLS is inexpensive to produce, creates generous foam, and strips oils effectively. That foaming action doesn’t actually improve cleaning power, but consumers tend to associate lather with cleanliness, which is why manufacturers favor it.
How SLS Affects Your Skin
SLS is a known skin irritant. The same property that makes it good at dissolving oils also means it can strip the protective lipid layer from your skin’s surface. It disrupts cell membrane proteins and removes the natural fats that keep your outermost layer of skin (the stratum corneum) intact as a barrier.
When that barrier breaks down, your skin loses moisture much faster. Researchers measure this using something called transepidermal water loss, which tracks how quickly water escapes through the skin. In one study, skin exposed to an SLS patch saw water loss jump from a baseline of about 5 g/m²/h to nearly 43 g/m²/h the day after the patch was removed. That’s roughly an eightfold increase, reflecting serious disruption to the skin’s protective function.
SLS exposure also shifts the balance of bacteria living on your skin. It reduces populations of fat-loving bacteria that are part of a healthy skin microbiome, likely because the lipids they depend on get washed away. For most people using SLS in rinse-off products like shampoo or body wash, these effects are temporary and mild. But if you have eczema, rosacea, or generally sensitive skin, even brief exposure can trigger dryness, redness, or itching.
The Cosmetic Ingredient Review panel, which evaluates the safety of cosmetic ingredients, considers SLS safe in products designed for brief use followed by thorough rinsing. For products that stay on the skin, the recommended concentration cap is just 1%.
SLS and Cancer: Clearing Up the Rumor
SLS is not classified as a carcinogen. The International Agency for Research on Cancer does not list it among substances that pose a cancer risk to humans. This rumor has circulated online for years, but no credible scientific body has identified a link between SLS and cancer.
Some of the confusion stems from a related chemical, sodium laureth sulfate (SLES), which can be contaminated with a byproduct called 1,4-dioxane during manufacturing. That byproduct is classified as “possibly carcinogenic to humans.” But 1,4-dioxane is not an ingredient in SLS itself, and manufacturers of SLES-containing products can purify the chemical to remove it.
SLS vs. SLES
Sodium laureth sulfate (SLES) is a modified version of SLS. It’s made by putting SLS through a chemical process called ethoxylation, which adds an ether chain to the molecule. This modification makes SLES milder on skin and eyes, which is why many shampoos and body washes have switched to it.
The tradeoff is that the ethoxylation process can introduce trace amounts of 1,4-dioxane as a contaminant. Reputable manufacturers test for and remove this contaminant, but it’s worth checking whether a company specifically states their SLES is purified. SLES can still cause sensitivity with prolonged use, though it’s generally better tolerated than SLS for most people.
Environmental Impact
SLS breaks down quickly in the environment. Bacteria commonly found in water and soil can degrade it with a half-life of about 6 hours, and standardized biodegradation tests show 94 to 97% breakdown within the testing window. It does not bioaccumulate in fish or other organisms.
That said, SLS is toxic to aquatic life at relatively low concentrations before it degrades. It carries an official classification of “toxic to aquatic life” in the short term and “harmful to aquatic life with long-lasting effects” over longer periods. Rainbow trout, for example, show lethal effects at concentrations as low as 4.6 mg/L over 96 hours, and sensitive mollusk species are affected at concentrations below 1 mg/L over several weeks. In practice, the rapid biodegradation and dilution in wastewater systems mean environmental concentrations rarely reach harmful levels, but the acute toxicity is real if SLS enters waterways in concentrated form.
Sulfate-Free Alternatives
If you want to avoid SLS, look for products labeled “sulfate-free” and check the ingredient list for gentler surfactants. Common alternatives include sodium lauryl sulfoacetate (sometimes abbreviated SLSA, and despite the similar name, it’s a different and milder compound), sodium cocoyl glycinate, sodium cocoyl glutamate, decyl glucoside, and lauryl glucoside. Many of these are derived from coconut oil or other plant-based sources.
These alternatives generally produce less foam than SLS, which can feel like the product isn’t working as well. It is. Foam has very little to do with cleaning ability. Sulfate-free formulas tend to be gentler on color-treated hair, sensitive skin, and dry scalps. They’re also less likely to strip natural oils to the point of triggering rebound oiliness, where your skin or scalp overproduces sebum to compensate for what was removed.
One ingredient to watch out for is sodium coco sulfate (SCS), which sometimes appears in “natural” products. It’s a less refined version of SLS derived from whole coconut oil rather than isolated lauric acid, but it contains SLS as a component and can still irritate sensitive skin.