Check the ingredients listed on your shampoo bottle, and see if they have a substance by the name of sodium laureth sulfate, or simply SLS. This substance is found in most shampoos, and the manufacturers use it because it produces a lot of foam and it is cheap.
SLS is used to scrub garage floors, and it is very strong. It is also proven that it can cause cancer.
I went home and checked my shampoo. It doesn't contain it, but many others do. It's even the first ingredient listed (which means it is the single most prevalent ingredient) in certain "herbal" shampoos.
I called one company and told them their product contains a substance that is carcinogenic. They said "Yeah, we knew about it but there is nothing we can do because we need that substance to produce foam".
Michelle Hailey, University of Pennsylvania Health System.