SLS (sodium lauryl sulfate) and SLES (sodium laureth sulfate) are two of the most common cleansing and foaming agents in personal care products. They’re the ingredients responsible for the lather in your shampoo, toothpaste, body wash, and dish soap. While their names sound almost identical, they differ in molecular structure, and that difference matters for how they interact with your skin.
How SLS and SLES Differ
SLS is a relatively small molecule made from lauryl alcohol bonded to a sulfate group. It’s an effective surfactant, meaning it reduces the surface tension of water so it can mix with oil and dirt, lifting them away from your skin or hair. SLES starts as SLS but goes through an additional manufacturing step called ethoxylation, which adds ethylene oxide units to the molecule. This creates a larger, more complex structure.
That size difference has a practical consequence: SLES is milder. The bigger molecule doesn’t penetrate the skin barrier as aggressively, which is why SLES has largely replaced SLS in many mainstream shampoos and body washes over the past couple of decades.
Where You’ll Find Them
Both ingredients show up in a wide range of products. SLS is common in toothpaste, where its strong foaming action helps distribute fluoride, and in household cleaners, where its potency is an advantage. SLES dominates in rinse-off personal care products like shampoos, shower gels, and hand soaps, where manufacturers want good foam without excessive irritation. If you flip over a bottle and read the ingredients list, one or both will appear in most products that lather.
SLS and Skin Irritation
SLS is one of the best-studied irritants in dermatology. Researchers routinely use it as a positive control in patch testing, meaning it’s the standard reference point when they want to confirm that a skin irritation test is working properly. A concentration as low as 0.5% applied to the forearm for 48 hours produces a measurable reaction in most people.
The way SLS irritates skin is straightforward: it strips away natural oils and disrupts the skin’s moisture barrier. Researchers measure this disruption through transepidermal water loss (TEWL), which tracks how quickly moisture escapes through the skin. In one study, a single 24-hour exposure to SLS nearly tripled the rate of moisture loss from the skin, jumping from a baseline of about 6 g/m²h to nearly 21 g/m²h by the second day after exposure. The skin didn’t fully recover for over a week.
This is why the Cosmetic Ingredient Review panel, the independent body that evaluates cosmetic ingredient safety in the U.S., draws a clear line for SLS. In products designed for brief use followed by rinsing, like shampoo or body wash, SLS is considered safe. But in leave-on products that stay on the skin for extended periods, concentrations should not exceed 1%.
Why SLES Is Considered Milder
SLES still causes some irritation. It can contribute to scalp dryness, hair follicle damage, and eye irritation at high concentrations. But in head-to-head comparisons, it consistently performs as a milder detergent than SLS. The Cosmetic Ingredient Review panel concluded that SLES is safe as currently used in cosmetic products, without the same concentration restrictions it placed on SLS for leave-on formulations.
If you have sensitive skin, eczema, or a condition that compromises your skin barrier, the difference between the two matters. Switching from SLS-based products to SLES-based ones, or to sulfate-free alternatives, can reduce the daily irritation load on your skin.
The 1,4-Dioxane Question
The ethoxylation process that makes SLES milder also introduces a potential contaminant: 1,4-dioxane, a byproduct that forms during manufacturing. This is the concern most often raised about SLES, and it’s worth understanding in proportion.
1,4-Dioxane isn’t an ingredient that manufacturers add intentionally. It’s a trace contaminant. The FDA has recommended since the 1980s that manufacturers use a technique called vacuum stripping to reduce 1,4-dioxane levels, and most major manufacturers comply. The EU’s Scientific Committee on Consumer Safety has concluded that trace levels at or below 10 parts per million in finished cosmetic products are safe for consumers. To put that in perspective, 10 ppm means 10 milligrams per kilogram of product.
You won’t see 1,4-dioxane on an ingredient label because it’s a manufacturing byproduct, not an added ingredient. If this concerns you, look for products from brands that certify low or undetectable levels, or choose sulfate-free formulations that skip the ethoxylation process entirely.
Environmental Considerations
SLS biodegrades quickly. More than 99% of it breaks down into nontoxic components under standard testing conditions, and it degrades under both oxygen-rich and oxygen-poor environments. However, as a raw material before it breaks down, SLS is moderately toxic to aquatic life, with lethal concentrations for fish reported between 1 and 13.9 mg/L over 96 hours. In practice, the concentrations reaching waterways after going through wastewater treatment are far lower than these thresholds.
SLES is also considered biodegradable, though specific environmental toxicity data for it is less extensively published than for SLS. Both ingredients pass through municipal water treatment systems and break down relatively quickly compared to many other synthetic chemicals.
Choosing Between SLS, SLES, and Sulfate-Free
For most people, both SLS and SLES in rinse-off products are safe and effective. The lather they produce isn’t just cosmetic: it helps distribute the product evenly and lifts away oil and debris efficiently. If your skin and hair feel fine with your current products, there’s no compelling reason to switch based on ingredient labels alone.
Where it gets more personal is if you’re dealing with dry skin, color-treated hair, a sensitive scalp, or conditions like rosacea or eczema. SLS is the harsher of the two and worth avoiding in those cases. SLES is a reasonable middle ground. Sulfate-free surfactants, often derived from coconut or glucose, are the gentlest option but produce less foam and sometimes require more product to clean effectively.
For leave-on products like lotions, serums, or styling creams, check whether SLS appears in the ingredient list. If it does, the concentration should be very low. Most well-formulated leave-on products have already moved away from SLS for this reason.