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Comment by sovnade | Hacker News Reader

I've seen this number given before but I have literally never seen someone get a sunburn through a t-shirt. I've seen plenty of people with awful, terrible, red, peeling burns that required a trip to urgent care on their arms, and their torso is completely fine because they had a tshirt on. Is SPF 7 really all that's required? I usually use SPF30 sunscreen but it seems like that's overkill by a significant amount.

edit: UPF vs SPF is a distinction I hadn't made before I guess. Is the difference in effectiveness that large? Does traditional sunscreen offer UVB protection also, but it's not advertised?

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I have a very sensitive skin. Unprotected in full summer sun (latitude 51 north), I'll burn within 15 minutes. In typical hiking attire, only my hands are in the sun, and I keep them in my pockets a lot of the time. My cheeks and lips receive only indirect sunlight, the rest is covered by a hat, sunglasses and a beard. In this attire, I've had sunburn through t-shirts a few times. They are never too bad, though, as I'll overheat way before I burn.

In non-fully protected clothing, something else burns before my chest has the chance to. A major problem with sunscreen is that it degrades over time, and is severely affected by sweat and abrasion. I've read that you should be reapplying every 30 minutes. I typically don't reapply nearly that often, so some parts of my skin are not fully protected. Once those start to burn, I leave direct sunlight, so the skin under my t-shirt (which has consistent coverage) never really gets the chance to burn.

I guess it depends on quality/thickness of T-Shirt. The cheap value-weight t-shirts you get at the supermarket is almost transparent if you hold it up to the sun. Heavy-weight t-shirts that are just a bit more expensive you can't see through at all and never got a sunburn through those.

Not a hiker myself: What is typical hiking gear such that it (mostly?) prevents sunburn in someone with sensitive skin?

I'm not a hiker but I do cycle and a few times I was exposed for hours to direct noon sunlight. The contrast is clear, exposed skin is red and irritated where the skin under the clothing is white and unaffected. Any kind of synthetic clothing has a greater amount of sunburn protection when compared to cotton but most of the specialized gear comes with additional UV50+ protection.

Wide brimmed hat, pants and long sleeves.

'Tech' fibers such as capilene for shirts do a great job without making you absolutely miserable like cotton would in the heat. For multi-hour mid-day hikes in the middle of summer (>100F daily temperatures) I wear my 50 SPF 92% polyester/8% spandex long sleeve hoodie. It's white so it reflects heat, and the synthetic mix allows it to breathe and be flexible. As it gets soaked through with sweat, it does tend to retain more heat, so it takes careful planning to find shade and dry off.

Maybe as a possible help/hint. There are theories that you get sunburned more if you wear glasses. Something with a part of the region that does some kind of hormonal regulation.

Sun protection factor 7 is quite a lot; it means only one seventh of the UV rays reach the skin, which is less than 15 percent.

Thanks for this factoid. I didn't know that the SPF is literally a ratio. Thanks!

'factoid' means that something is fact-like, not that it is a tiny fact. Assuming of course one cares about linguistic precision. If you're the kind of person who uses 'literally' to mean 'figuratively' then you can ignore this, I make no value judgement.

To be both precise and accurate, factoid also means a brief / tiny bit of trivia.

Personally, I seldom (if ever) hear it used how you’re describing.

Yes, in the same way that 'literally' also means 'figuratively', which is to say that colloquial usage of the word is incorrect as regards it's intended meaning.

Whether or not that is a useful distinction is subjective.

“Literally” has been used figuratively for hundreds of years and is more akin to hyperbole than additional definitions like we see with factoid.

I think it’s fair to say that hardlining a correct vs. incorrect distinction with how literally is used today is mostly pointless.

I disagree. By surrendering the meaning of words we lose the ability to communicate precisely. That may well always be a losing battle, linguistic evolution being what it is, but that doesn't mean it isn't worth resisting.

You might be bummed to learn about hyperbole, then. Or anthropomorphisms. Or similes.

I guess a silver lining (not literally, of course) is that you're at least aware of the difficulties - good luck!

I'm ginger, as white as can be without being albino. I got a second degree burn on my shoulders through a t shirt. It was field day at school, spent 8 hours in the sun with nothing on my shoulders except a cotton T. Literally had dozens of bubbling blisters. SPF 7 for me means about an hour tops, not 8 hours.

If the t-shirt is wet, it might reduce the SPF.

Is that because it's closer to the skin, or changes the refractive properties? (or both?)

Closeness doesn't matter, I suppose. Make a t-shirt partially wet, hold it in front of some light source and you will see that the wet parts are much brighter. This will probably be the case too for UV light.

> It was field day at school, spent 8 hours in the sun with nothing on my shoulders except a cotton T

My skin is brown-reddish, so I rarely get sunburns and never use sun blocker, but I fear I would get sunburned too after eight hours!

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