Here was an interesting article published on Boing Boing about sunscreens.
The basic premise is that we do not know whether sunscreens actually prevent cancer or not. The reason, most sunscreens are created to stop sun burns not necessarily to stop the UVA light that causes skin cancer.
There are not yet long-term studies to demonstrate that using sunscreens can actually prevent skin cancer mainly because sunscreens haven’t been in use long enough to know. It takes a long time to develop skin cancer.
In the future, regulations in the US will require manufacturers to note whether their products protect from both UVA and UVB radiation. SPF ratings are only indicative of UVB radiation protection.
As a cosmetic chemist, it behooves you to keep up on these things. And in the future, you’re going to have to develop your sunscreens to block the full spectrum of UV radiation. The days of SPF and sun burn blocking only will be gone.