SPF 50 blocks 25% more UVB than SPF 40 does. Measuring it as percentages makes it non-linear in a way that most people find confusing. Imagine we had one sun cream that blocked 99.9% and another that blocked 99.5%. Sounds like nothing; only an 0.4p difference, but is actually 5 times as effective.

You're right about how long it lasts also being an important factor. UV-A protection is also another very important factor. But as someone with pale skin even by Scottish standards, the difference between SPF 40 and SPF 50 around noon is significant, even through I consistently re-apply every hour. I won't get burnt, but I'll end up with more sun damage - and that lasts until late autumn.

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treis 4 days ago | parent | next [-]

>99.9% and another that blocked 99.5%. Sounds like nothing; only an 0.4p difference, but is actually 5 times as effective.

I disagree. Both effectively stop all damage to the skin. It's like having 10 inches of steel armor for bullet proofing instead of 1. A bullet isn't getting through either so they are equally effective.

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aero_code 4 days ago | parent | next [-]

But sunscreen doesn't stop all damage to skin. I spent weekdays working inside on a computer, then sometimes spent summer weekends outside in the sun. I get sunburned easily, sometimes in like 10 minutes of direct sun. You wouldn't try to deny a light sunburn isn't skin damage? SPF 50 suncreen, blocking 98% of sun, extends the 10 minutes by 50x to 8.3 hours, but that is still not that great. I can still exceed that in two days. And I don't see why having light skin and wanting to spend the weekend outside would be unusual. Blocking 99% of UV and doubling the time over 98% would help quite a bit.

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treis 4 days ago | parent [-]

That isn't how sunscreen works. If you put SPF 50 on and spend 8 hours in the sun you're coming back a lobster.

Say you burn in 5 minutes. SPF 50 means you burn in 250 minutes. But it's more like 100% protection for 245 minutes and then 0% for the last 5. It's not a steady cooking at 2.5% intensity.

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rcxdude 4 days ago | parent [-]

Got any source for that? Everything I can find (and the intuitive explanation of it) points to the opposite: SPF is how much of the UV blocks, not at all how long the sunscreen stays on your skin (which varies wildly with what you're doing).

	▲	treis 4 days ago \| parent [-]
		https://cdnmedia.eurofins.com/apac/media/609921/spf-iso-prot... That's my entire point. The way they generate SPF measures how much of the sun it blocks in the lab shortly after it's applied. That one blocks 97.5% and another 98% is meaningless for the real world.

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istjohn 4 days ago | parent | prev [-]

Your metaphor is not at all apt. No bullet is going through ten inches of steel. Some UV radiation will penetrate the strongest sunscreen.

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actuallyalys 4 days ago | parent | prev | next [-]

It seems situational. Someone with your skin at noon absolutely benefits from a higher SPF. Someone with even slightly darker skin [0] going out a few hours before sunset might still want to wear sunscreen, especially if they’re going to be in direct sun the whole time, but a high SPF doesn’t seem very critical.

That being said, I am not a dermatologist, and it’s easy to underapply sunscreen so erring on the side of higher SPFs probably makes sense.

[0]: Note that even people with a lot of melanin still need sunscreen: https://www.pbs.org/newshour/amp/health/why-its-important-ev...

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s1artibartfast 4 days ago | parent | prev [-]

No, it seems clear that it is 0.4% more effective. It all depends on what you use as your base case hypothetical.