1, 4-DioxaneUpdated a month ago

A contaminant that is not required to be listed on an ingredient label in the US. It is a by-product of the ethoxylation process in which carcinogenic ethylene oxide is reacted with other ingredients to make them less harsh on the skin. The EPA (U.S. Environmental Protection Agency) has classified 1,4-dioxane as likely to be carcinogenic to humans. The FDA studies showed that 1,4-dioxane can penetrate human skin. 1,4-dioxane can be reduced or removed from a product through the vacuum-stripping process. However, independent tests show that many manufacturers fail to do so.  

Sources: 

https://www.fda.gov/Cosmetics/ProductsIngredients/PotentialContaminants/ucm101566.htm
http://www.safecosmetics.org/get-the-facts/chemicals-of-concern/14-dioxane/
https://www.atsdr.cdc.gov/phs/phs.asp?id=953&tid=199
https://www.organicconsumers.org/press/ocas-new-study-finds-greatly-reduced-carcinogens-personal-care-products