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 get chasing the biggest spf numbers. An spf 20 already blocks 1/20 of UV rays. I personally get a sunburn after approx 1 hour in my climate and with my skin. Logically I would then have to be outside for 20 hours to get a sun burn! That will never happen. Not even counting that the skin will partly self heal during those 20 hours. Even if you’re one of those who burn in 15 minutes a spf of 20 should still get you 5 hours before a burn.
I say those 50+ SPF sunscreens are over the top and extremely few need a strength that high. Maybe if you’re sun allergic I could understand. And for cancer. Reducing my risk by 1/20 is enough for me. That’s a massive reduction and on top of that skin cancer is the world’s easiest treatable cancer with the lowest death rate. Skin aging? I don’t care. I’m not out to look 15 at 50. I wanna look 50 at 50. That’s my take on it all.
It’s 19/20 of course but you’re thinking in terms of avoiding rare but risky events. Sun damage is cumulative. Every minute of UV exposure in your lifetime adds up. That means you get half the cancer over the course of your life if you use 39/40 instead of 19/20.
60% of skin cancer is caused by sun exposure. The rest has other causes. Therefore if you switch from 19/20 to 39/40 you would decrease your risk from 42% of baseline to 41% of baseline. Much less impressive if you put it that way.
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.
Unless it goes metastatic then the death rate is very high.
Sunblock is not about preventing sunburn, DNA gets damaged far before sunburn. I suggest you get better information, because you clearly don’t understand this.
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
As someone with the skin complexion of a jar of mayonnaise, burns after ten minutes on a cloudy day, and got sun burned to the point of sun poisoning and second degree burns once in my youth - yeah, I’m going to chase that bigger number. Skin cancer also runs in my family. Once had a dermatologist tell me that I shouldn’t go out of the house without a minimum of spf 50 on all exposed skin, period. Doesn’t matter the time of year. I don’t tan. I freckle and more freckles mean more checking for the ABCs of skin cancer. Just had an age spot (according to the dermatologist) removed not too long ago because the freckle became raised, rough, and irregularly shaped.
For someone like you I totally understand. Getting sunburned after 15 minutes of cloudy weather is extreme. My condolences.