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.
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
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.
For someone like you I totally understand. Getting sunburned after 15 minutes of cloudy weather is extreme. My condolences.