Submitted 4 weeks ago by Maroon@lemmy.world to [deleted]
These creams have come chemical that blocks the UV with some capacity, say a factor of 50. Why can’t I apply two layers oof 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?
SPF50 means that only 1⁄50 of total UV light gets to your skin. If the SPF50 factor for a narrow light wavelength was tested under 20 μm layer of sunscreen, and you’ll manage to apply 40 μm layer, the resulting UV light would be 100%÷50÷50=0.04%, corresponding to SPF2500 for this wavelength.
Of course, SPF is not measured for one particular wavelength. UV light is a spectrum of sunlight of wavelengths starting at 100 nm to 400 nm. UV radiation of shorter wavelengths contains more energy, but is easier to stop; it usually responsible for short-term damage like sunburns. For longer waves it’s vice versa, they are less powerful and won’t cause a burn, but are harder to stop and penetrate skin deeper and cause cancer and sun damage.
Low-SPF sunscreens are usually achieving their index thanks to stopping easy-to-catch short waves that are most of UV exposure in pure joules, and are having a hard time stopping long-wave UV due to their chemical composition. For further reading I recommend a 2019 article “Critical Wavelength and Broad-Spectrum UV Protection”, Google it, I’m cautious to paste a link in case anti-spam bots are tightly configured here.
SPF is overall not a very good and all-encompassing sunscreen quality indicator. I recommend looking into UVA-centered indexes: CW (over 370 is good, in EU these are marked with UVA circled mark), UVAPF or PPD (over 15 is decent, that’s same system as SPF but for UVA, longer waves) or, common on asian brands, PA++… index (UVAPF=2^x, where x is the amount of “+” symbols after PA).
“Critical Wavelength and Broad-Spectrum UV Protection”
Tell them to add a : so I can trust it.
Like “ “Critical Wavelength and Broad-Spectrum UV Protection: the effect of antidisestablishmentarianism on broad spectrum species “.
Now that sound sciency
If you’ve ever used a UV camera it’s pretty clear that much like lotion not absorbing after a certain point, there’s a limit to how dark your skin gets with a given sunscreen. Also that there’s a massive difference between brands and formulas. I had some photos I took with one but can’t find them. If I do later I’ll update
This is one of my favorite videos for showing this in effect: youtu.be/o9BqrSAHbTc
How did the Vaseline spf50 work? Thats what I use on my body since it seems the most water-resistant, but I still get pretty dark. Did you test face sunscreens?
I wouldn’t say our tests were perfect, just smearing it on cardboard, but we did test like 10 different ones. Few things were obvious, like that the coral reef destroying ingredient ones were more effective than the more eco ones, and that banana boat kinda sucked. Neutrogena was good and even their 15 spf was just about as dark as their 30. I think coppertone was worse than banana boat but I’d wanna retest before I recommend any of them though. My memory isn’t that good, I just remember relief that Neutrogena was good because it’s what I’ve been using.
a.aliexpress.com/_mq8c3YB this is the one I have, although I bought it when it was 50 bucks and a quick glance shows most over 100 now.
Think of the visible spectrum of light. If you were to mix a 30% shade of white with a 30% shade of white, you get…. A 30% shade of white.
Now just switch that coloring to the UV end of the spectrum and it’s the same thing. You’re not layering it realistically, you’re basically just mixing it together and the result is still the same “color” you started with.
Don’t buy American sun screen. It’s always cakey and flakey. Go import some Korean sunscreen. It’s basically feels like you have nothing on and gives you better protection.
There are plenty of great brands of American sunscreen. Just don’t buy the super cheap shit. Which is generally good advice in any country. Some brands that come to mind are Kiehl’s, First Aid Beauty, and EltaMD. To be clear, I love me some Korean skincare, and I think it’s superior to American. I just don’t agree with your premise that all American sunscreen is “cakey and flakey.”
I just looked it up and America will soon get the good sunscreens. Bemotrizinol has just been FDA approved.
Until then, I am sticking with American has some bad sunscreens. Until I guess this month.
Neutrogena works great and feels great. That’s why it’s more expensive.
Us has 16 of uv filters aproved, whereas EU has 30+. Last UV filter aproved by FDA was in 1999. So us had old sun screens
I didn’t know that. Thanks for the info.
By god I think you’ve cracked the thing.
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 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 29/40 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.
That’s a massive reduction and on top of that skin cancer is the world’s easiest treatable cancer with the lowest death rate.
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.
The store increases the price because people will pay more for higher number. You can get spf 80 or 100 cheap elsewhere.
Light clothing, hats are SPF infinity without toxic chemicals. Zinc or titanium cremes work best.
Light clothing
You still need to pay attention to the weave density and thickness. That "infinity" falls somewhat if you're wearing the clothing equivalent cheesecloth.
Source: Once got a severe sunburn on a relatively overcast day through a t-shirt that I soon learned hadn't been thick enough.
Light clothing and hats are absolutely not spf infinity.
Be aware light clothing can still transmit enough light to burn. My bullshit test is “can I look at the sun through this fabric or does it hurt?”. It’s a little conservative, but it’s good enough for me if I plan to be out for 8 hours. I actually just got burned shirtless last week on one side. With a light, black, sports shirt the next day, I was in mild burning pain when the sun hit my burned side. It wasn’t just heat, as lifted the fabric off my skin still hurt the same.
Not a hat guy, and its too damn hot to wear more clothes. Sometimes I see locals wearing puffy jackets that have fans blowing on the inside, but that seems too cumbersome.
slazer2au@lemmy.world 4 weeks ago
SPF rating is subjective at best and not a regulated term in many countries.
Australia is a place where it is a regulated term and a test by Choice showed that 16 of 20 products failed their rating and several have since been recalled.
www.choice.com.au/…/sunscreen-test
Back to your original question which is answered in the article
Maroon@lemmy.world 4 weeks ago
No way! So like I’d be be paying exponentially more for an additional 0.6% if I brought the SPF75!
Damn.
roguetrick@lemmy.world 4 weeks ago
Yeah but any power gamer loves this math because that 0.6% is still a nearly 30 percent damage reduction compared to the lower grade sun armor so you’ll be stomping those pubs that don’t persue it.
protist@retrofed.com 4 weeks ago
Does higher SPF cost more where you live? I just bought both 50 and 100 and they were the same price
slazer2au@lemmy.world 4 weeks ago
Yep. Welcome to capitalism, where the bigger made up number actually mean very little in real terms.
sheridan@lemmy.world 4 weeks ago
I looked up sunscreen recommendations on Consumer Reports and virtually every sunscreen didn’t perform as well as advertised in CR’s lab tests. CR’s top rated sunscreens all happen to be marketed for kids/babies oddly enough.
blarghly@lemmy.world 4 weeks ago
You get sunburned: “oh, silly me, I must have forgot to reapply”
Your baby gets sunburned and spends the next 72 hours crying: “I will murder the CEO of Johnson & Johnson”
lolrightythen@lemmy.world 4 weeks ago
Good writeup, yo
lemonhead2@lemmy.world 4 weeks ago
so if u do two spf 50 layers u should get spf 2500.
but in practice the second layer won’t be as thick…
Korhaka@sopuli.xyz 4 weeks ago
Depends how you apply it. I smear it on like butter.