Sodium Lauryl Sulfate
In plain terms
A strong cleaning and foaming agent found in shampoos, body washes, and toothpaste. It is one of the most common surfactants, the ingredients that lift away oil and dirt so they rinse off with water.
What the evidence shows
Sodium lauryl sulfate is the reference irritant in skin science: dermatologists use it as the benchmark they measure other irritants against, which is a blunt signal of how reliably it strips and disrupts the skin barrier. The effect climbs with concentration and contact time, so a quick-rinse shampoo is a lighter exposure than a leave-on product, but it still bothers many people, and anyone with eczema, rosacea, or a sensitive barrier tends to react even to brief contact. The case for skipping it is not a hidden toxin: it is not a carcinogen, despite a long-running rumor, and it is not an allergen. It is that a reliable, avoidable irritant sits in the formula when gentler surfactants clean just as well with less barrier disruption. One distinction worth keeping straight: SLS is not the same as sodium laureth sulfate (SLES); SLES is ethoxylated and can carry trace 1,4-dioxane, while SLS skips that step. The trade-off is that SLS is the harsher of the two on skin.
On the label
In a foaming product like shampoo or body wash, it is usually near the top of the list, since it is one of the main cleansers.