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.
I don’t understand your math, can you elaborate? It is still of course valid to say that you reduce your occurrence of sun-caused cancers in half.
60% * (1/20) = 3%. 3% + 40% = 43% chance to get skin cancer compared to baseline
60% * (1/40) = 1,5%. 1,5% + 40% = 41,5% chance to get skin cancer compared to baseline
I did get the math slightly wrong I’ll admit but the principle is the same. And yes you could say you reduce your risk of sun caused cancers in half. But it’s not a meaningful reduction of total skin cancers. Redo the calculation to every form of cancer and you will have reduced your risk to all cancers by far less than a percent by switching from spf 20 to spf 40.
By staying in a deep mine the whole year I will reduce my risk of getting killed by a falling asteroid by over 99%. Sounds great right? But how big was the original risk really? That’s worth taking into account. You’re better off looking into ways to reduce the risk of other cancers than switching from spf 20 to spf 40.
Thank you, that is clear.
You say “better off”, but that doesn’t really account for how easy it is to switch. It’s not like the effort spent grabbing the highest available SPF is taking away from other anti-cancer efforts. I’m not seeing a downside here, though the benefit isn’t as great as I assumed; probably a figure of speech.