Propylene Glycol
This is a tricky one because PG is very closely related to a couple other chemical compounds that are severely toxic (carcinogenic, even), but PG has been approved by the FDA for use in not only cosmetics, but in food (things like ice cream, microwavable popcorn, etc.). However, it can also be a primary ingredient in antifreeze.
But, just so I'm not only providing negative info on something many companies and researchers believe to be OK, here's a good article on the matter from Tom's.
Mineral Oil
What's highly flammable, used as a laxative, destructive when you get it in your eyes or swallow it, and something people rub all over babies every day? Mineral Oil! This stuff also goes by many aliases, such as "hydraulic oil" and "cable oil." Mineral oil, with added fragrance, is called "baby oil," but it is also commonly prescribed as a laxatives. Basically, when used in cosmetics, mineral oil is used to "cut" the product to make it last longer and go farther (the same basic idea coke dealers use when they cut their product with things like bleach).
And, once again, so that I'm letting you decide what you want, here's a site arguing for the safety and continued use of mineral oil.
Parabens
Currently, the FDA doesn't see a serious carcinogenic cause of concern with parabens, used as preservatives in many cosmetic lines, but the products are still under review. The reason parabens are still being tested is that there have been tests that have shown that bodies may react to the estrogen-esque properties in parabens, which may lead to certain types of cancers (primarily for females, who, incidentally, are the primary users of cosmetic products that contain parabens) even when the products containing parabens were only rubbed into the skin - they did not have to be ingested or injected. Problems in young boys who have exposure to parabens have been noticed as well - for instance sperm abnormalities and testicular cancer - and more attention is being paid to general allergies to parabens. For more information about research and cosmetics, visit ThinkBeforeYouPink.org.
I didn't find any web sites extolling the virtues of parabens.
Sodium Lauryl Sulfate/Sodium Laureth Sulfate
This stuff, abbreviated to SLS and SLES, are commonly used as cleaning agents in shampoos, soaps, bubble baths, garage floor cleaners, and engine degreasers. They are used in products we expect to foam, but it's a cheap product (which is why it's used in general) and it's a severe irritant (think of the searing pain you experience every time you accidentally get soap in your eyes). There are no known carcinogenic effects of SLS, however, which is the biggest argument for the use of SLS or SLES.
