Sodium Laureth Sulfate
In plain terms
A foaming cleanser made by adding ethylene oxide to a harsher detergent to soften it. The "eth" in the name marks that ethylene-oxide step.
What the evidence shows
Sodium laureth sulfate is the foaming agent in most shampoos and body washes. It is made by taking a harsher detergent and adding ethylene oxide to soften it, which is the step the "eth" in laureth refers to. As a cleanser it is mild, gentler on skin than its non-ethoxylated cousin sodium lauryl sulfate. The reason to skip it is the byproduct, not the foam: that ethoxylation step can leave a trace of 1,4-dioxane, which the cancer research agency rates a possible human carcinogen. It can be stripped out during manufacturing, and some makers test for it, but here is the catch a label cannot resolve. 1,4-dioxane is a contaminant, not an ingredient, so it never appears on the package, which means there is no way to tell from the label whether a given product was purified or not. When the downside is an invisible possible carcinogen and the alternative is simple, the cleaner call is to skip it: cleansers that avoid ethoxylation, including sulfate-free formulas, do not form 1,4-dioxane at all. The same tell shows up on other ethoxylated names ending in -eth and on PEG compounds.
On the label
Listed as sodium laureth sulfate; the "eth" is the tell that it was ethoxylated, the same marker found in other -eth and PEG ingredients. Any 1,4-dioxane it carries is a byproduct, so it never appears on the label itself.