Some products containing SLES have been found to also contain traces (up to 279 ppm) of 1,4-dioxane. The U.S. Food and Drug Administration recommends that these levels be monitored. The U.S. Environmental Protection Agency classifies 1,4-dioxane to be a probable human carcinogen (not observed in epidemiological studies of workers using the compound, but resulting in more cancer cases in controlled animal studies), and a known irritant with a no-observed-adverse-effects level of 400 milligrams per cubic meter at concentrations significantly higher than those found in commercial products.Under Proposition 65, 1,4-dioxane is classified in the U.S. state of California to cause cancer.The FDA encourages manufacturers to remove 1,4-dioxane, though it is not required by federal law.