These creams have some chemical that blocks the UV with some capacity, say a factor of 50. Why can’t I apply two layers of this cream to now get a 100 factor equivalent protection?
I asked the chemist at the store and they said it’s not how it works and that the highest protection factor they have is 75 (which was super expensive).
What gives?
Edit: Thank you for those super informative answers.
DNA doesn’t just start getting damaged before the sunburn. It’s instantly from the first millisecond you’re out in the sun. However it’s surprisingly not a linear relationship where 60 minutes or sun carries twice the risk of 30 minutes of sun. The heavy inflammation associated with a sunburn has a cancer risk of itself, independent from the actual UV dose. So while minimizing UV is important, it may be just as important, if not more important, to make sure you are never sunburned. I can provide sources if asked