Its all bout doses. I am sure the FDA or any government agency have a daily reference dose for SLS. Meaning that it cannot exceed a certain concentration in the shampoo. Plus, this concentration is usually ALOT weaker then the known concentration of the first adverse effects seen in the labs. Im pretty sure the concentation that is found in your shampoo is alot smaller then the SLS concentration found in engine degreasers =). Many commun compunds are known carcinogens, just look at ethanol. Even coffee is a know mutagenic in AMES tests but its all about the concentration in which you use it
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