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:

1. https://www.fda.gov/Cosmetics/ProductsIngredients/PotentialContaminants/ucm101566.htm

2. http://www.safecosmetics.org/get-the-facts/chemicals-of-concern/14-dioxane/

3. https://www.atsdr.cdc.gov/phs/phs.asp?id=953&tid=199

4. https://www.organicconsumers.org/press/ocas-new-study-finds-greatly-reduced-carcinogens-personal-care-products

An antimicrobial preservative that slowly releases carcinogenic formaldehyde into a product over time (see Formaldehyde). People may develop a contact allergy to formaldehyde or to the ingredient itself. The American Society of Contact Dermatitis lists 2-bromo-2-nitropropane-1,3-diol as one of the allergens even in concentrations as low as 0.5%. In the presence of amines (such as triethanolamine, diethanolamine, and morpholine) in a product, 2-bromo-2-nitropropane-1,3-diol can form nitrosamines, which can penetrate the skin and increase the risk of cancer. Longer storage times and higher temperatures increase the amount of formaldehyde released from 2-bromo-2-nitropropane-1,3-diol, which can lead to higher risks of health problems.

Sources:

1. http://www.ewg.org/skindeep/ingredient/700019/2-BROMO-2-NITROPROPANE-1%2C3-DIOL_%28FORMALDEHYDE_RELEASER%29/#

2. http://www.safecosmetics.org/get-the-facts/chemicals-of-concern/formaldehyde/

3. https://online.personalcarecouncil.org/ctfa-static/online/lists/cir-pdfs/pr208.pdf

4. https://www.cancer.org/cancer/cancer-causes/formaldehyde.html

5. https://www.contactderm.org/i4a/pages/index.cfm?pageID=3563

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