Parabens   
Caution
A group of food grade preservatives such as butylparaben, proplyparaben, methylparaben, and ethylparaben. Research is inconclusive about whether these are harmful. However, it's agreed that it is more harmful to use products with no preservatives in them than products with these in them. For more information, check out: Parabens.

The Parabens are widely used to preserve cosmetics because they cause less irritation than other preservatives. Many of the studies on Parabens found them to possibly cause breast cancer, but many of these tests were on animals, and were given orally or through injections (which is not how humans usually come in contact with them). To keep things in perspective, many plants produce similar chemicals in our bodies (estrogen in high levels may cause breast cancer in some women) that are believed to increase cancer risk such as soy.

Though traces of parabens were found in breast tumor tissues, there haven't been studies to find out if the same levels of parabens are present in healthy breast tissues. No one is yet certain if the presence of parabens in cosmetics is the problem (since they are also used in foods, the parabens found in human tissue may be from parabens that were eaten instead). The European trade association has said that the four main parabens used in cosmetics in Europe have been studied and found safe. [Begoun (Cosmetics) pgs 1132-1133, Winter 7th ed., pgs 392-393].
See also: Preservative
Source(s): Begoun Winter