The answer is the common cosmetic preservative called paraben. There are many varieties of parabens, but it seems like all of them can contribute to breast cancer, from the conclusions of this study. The suspicion that these chemical preservatives were dangerous has not kept American cosmetics companies from continuing to use them in their cosmetics formulations. There are many several alternative cosmetic preservatives, but many cosmetic companies choose not to use them because it may cost them a few more cents per batch of cosmetics to use. I find this fact extremely disturbing.
What can you do? Do you have to stop wearing cosmetics to prevent breast cancer? No. There are cosmetics companies that sell cosmetics with no parabens in their formulations. You just need to do a little searching to find these companies. Also, read labels at the drugstore. These formulations are harder to find, but they do exist. Any cosmetic product that is labeled organic or all-natural most likely will not contain these parabens, as well as any brands of mineral cosmetics.