Some products containing SLES contain traces (up to 300 ppm) of 1,4-dioxane, which is formed as a by-product during the ethoxylation step of its production. The U.S. Food and Drug Administration recommends that these levels be monitored.[9] The U.S. Environmental Protection Agency classifies 1,4-dioxane as 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.[10] Under Proposition 65, 1,4-dioxane is classified in the U.S. state of California to cause cancer.[11][12] The FDA encourages manufacturers to remove 1,4-dioxane, though it is not required by federal law.[13]