No Safety Standard for Cosmetics
Synthetic ingredients used in cosmetics are generally considered safe. The Food and Drug Administration requires that cosmetics makers make sure that their products are safe.
Actually, it's probably true that most people consider cosmetic ingredients - synthetic or otherwise - to be safe, but that's likely because they're also assuming that FDA requires safety testing - and they don't. In fact, they can't. And the only thing FDA could do, which is set a safety standard, so at least all the self-policing companies have something similar to shoot for in their non-required, private tests, they refused to do in September.
You can see EWG's petition, FDA's refusal and related docs, and find out more about the "generally considered safe" ingredients in your favorite products, at our Skin Deep site.