Another chemical to watch out for in the ingredients of a variety of personal care products is diethanolamine or DEA. It’s most commonly added to soaps, shower gels, shampoos, bubble baths and facial cleansers as a foaming agent in the form of Cocamide DEA . Diethanolamine has been subject to numerous studies with evidence of carcinogenic activity related to liver and kidney function. It can also form nitrosamines (potent carcinogen) when mixed with other chemicals in beauty products. Opened products have a much higher likelihood of forming this type of carcinogen. The European Cosmetic Directive specifies that this type of contamination must not be present in health and beauty care products or cosmetics.
Sources:
http://ntp.niehs.nih.gov/index.cfm?objectid=133AAB02-B224-E476-BAEDB5542AADA220