Hacker News Reader

Comment by treis | Hacker News Reader

I feel like this is mostly bullshit because high SPFs are mostly bullshit. A promised SPF of 50 and a tested value of 40 means it blocks 97.5% instead of 98% of the sun.

Anything higher than 30 or even 15 isn't really meaningful. At that point how long it lasts and how resistant it is to water is far more important.

View on HN

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.

>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.

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

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.

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.

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).

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.

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

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...

I had to look it up. SPF is a reciprocal value 1/SPF=amount of sun that gets through. So 1/50 allows 2% of UV through, and the difference between say SPF 2 and 3 is enormous but 49 and 50 tiny.

The point is you sell a product, it better be what it claims to be.

I didn't buy SPF30, I bought SPF50. When I made that choice, I expect at least SPF50.

But you are also dismissing a 25% difference in total transmitted UV - and that's before degradation in the field due to usage and practical concerns, which is why we want SPF50 in the first place.

Aside from what others have said, it does matter because few people apply the proper amount. If you're applying only 1/3 the "proper" amount of a 60 SPF product you still at least get 20 SPF of protection as it scales rather linearly.

And as someone paler than most makeup brands go, for many of us it absolutely does make a difference even when using the proper amount.

I ride or die LRP uvmune 400, few protect as well as it does.

SPF isn't bullshit, it just measure one specific thing, not everything. AFAIK most sunscreens also list expected hours of protection and whether they are water resistant or not.

Reply on news.ycombinator.com