The following is Dr. Epstein's answer regarding the degree of SLS toxicity: 

Regarding  Sodium Lauryl Sulfate, there is no evidence that SLS is carcinogenic.  However, contrary, SLS is a harsh ionic detergent which is a strong irritant to skin, depending on concentration and the duration of exposure. The Cosmetic Ingredient Review (CIR) 2000 Compendium, of the Cosmetic, Toiletry and Fragrance Association, admits that SLS "causes severe epidermal changes--of the skin of mice--(indicating) a need for tumor-enhancing activity assays," (1). The Compendium also admits that SLS containing products are "designed for brief discontinued use, following which they are thoroughly rinsed
from the surface of the skin."

Furthermore, as reported in model studies, published by the Danish Institute of Public Health, a single 24-hour exposure of SLS to human skin damages skin protein and causes prolonged disruption of "the skin barrier integrity of the skin," to allow the penetration of carcinogens such as nickel and chromate (2,3). Thus, skin absorption of the multiple carcinogenic ingredients commonly found in mainstream industry cosmetics and toiletry products (4), including shampoos, could be greatly increased by SLS type detergents.

Clearly, the American Cancer Society is unaware of this information, besides the fact that mainstream industry cosmetics and toiletries contain a wide range of carcinogenic ingredients, precursors, and contaminants, besides ingredients that break down to release carcinogens such as formaldehyde. 

Furthermore, laureths, besides other ethoxylated alcohols, are generally contaminated by the volatile carcinogen 1,4-dioxane, as admitted by the CIR Compendium (1), and also the carcinogen ethylene oxide.

Samuel S. Epstein, M.D.

1. CTFA. Cosmetic Ingredient Review (CIR) Compendium 2002.

2. Nielsen, G.D. et al. Effects of Industrial Detergents on the
Barrier Function of Human Skin. Int. J. Occup. Med. 6(2):138-142, 2000.

3. Nielsen, G.D. Effects of Four Detergents on the In-Vitro
Barrier Function of Human Skin. Int. J. Occup. Med. 6(2):143-147, 2000.

4. Epstein, S.S. and Steinman, D. The Safe Shopper's
Bible. Macmillan/Hungry Minds, Inc., New York, 1995.

5.  Unreasonable Risk, Avoidable Risks of Cancer, 2002.