I would note that 2/3+ of Australians will be diagnosed with skin cancer in their life time, compared to ~1/5 of Americans. I think the lesson is that "if you're White, live somewhere with a lot of sun, and spend a lot of time outdoors without mitigation, you'll probably get skin cancer". Note that many Australians aren't White, or spend a normal amount of time in the sun, so constantly getting sunburned is probably even worse than the 2/3 statistic implies.
This is almost certainly due to Australian behavior being an outlier, since Whites in equally sunny places like Arizona have relatively normal skin cancer rates.
Survival rates are like, 95%+ for skin cancer in general, so I wouldn't be too concerned about it. Then again, I'm Asian and tan (too) well, so I'm biased.
Australia (and especially Northern Australia) seems to have more ultraviolet radiation. I don't think it's behavioral. Arizona does not have equal ultraviolet radiation.
That's a really good point. The Nordics and South African Whites also have pretty high skin cancer rates, so maybe it's "fair skin (especially gingers?) is bad", "high UV bad inversely proportional to melanin", and the result is "gingers in Australia will probaby get cancer".
But also, obviously behavior has to play a role, since Solar UV hitting light skin is the direct cause of the vast majority of skin cancer, even in other races.
For behavior to be the cause of why Australian have more skin cancer, they would need to behave differently.
I'm not aware of anyone having documented that Australian behave significantly differently.
https://www.youtube.com/watch?v=sYnUPxG7ONk -- Here is a doctor with a Youtube channel discussing a recent British study showing benefits from UV exposure: "if everyone in the 419K cohort avoided UV light and were followed for 15 years, it would save 39 deaths from melanoma at the expense or 2,982 deaths from everything else".
https://pmc.ncbi.nlm.nih.gov/articles/PMC8411113/ -- UV exposure (namely, 20 to 30 minutes of sunlight exposure without sunglasses or sunscreen every other day) increases levels of the sex hormones in men and women. For men, this increases motivation, drive and interest in sex and romance. For women, it increases interest in sex and romance.
I started routinely getting 20 to 30 min of mid-day sunlight every other day on my bare skin with no sunscreen at all and in my eyes (with nothing in front of my eyes) about 17 months ago. It has positive effects on my mood and motivation and I intend to keep doing it. I've also in the past supplemented testosterone, both via oral administration and via injections. I much prefer getting sunlight: exogenous testosterone was hard on me somehow (on my liver if I had to guess) and I experienced negative behavioral effects such as suddenly becoming much more interpersonally aggressive than the situation calls for. It was a bother as much as a benefit whereas I see the sunlight exposure as pure benefit except for the time expenditure.
I remain as careful as ever of not getting sunburned. Part of that is routinely measuring the duration of my UV exposure, and learning some stuff to help estimate UV intensity. I calculated for example that at my latitude (Bay Area) on a cloudless day, the intensity of UV A at noon mid-winter is about half that of noon mid-summer.
Cohort studies claiming benefits from UV exposure are not credible. It is impossible to separate UV exposure from positive traits correlated with UV exposure, such as exercising or having a social life, and no, you can't just control for it.
It is true that your risk of dying from melanoma is low, and not worth freaking out too much about.
You should do a blinded test of your sun exposure with identical placebo sunscreen to see if the UV exposure itself is necessary for positive effects on your mood, for science.
Thank you for that. I tentatively accept your assertion that the British study (summarized by the Youtube video I referenced) is not credible.
For science, I wrapped my body in a polyester blanket and wore a sun hat with UPF of 50+ and sunglasses so that none of my skin was getting UV at any significant intensity and spent half an hour in the mid-day sun. All sunglasses on the market from name brands reliably block UV. Polyester is very effective at blocking UV and like almost all fabrics will let at least half of the photons at most infrared wavelengths through. I did this prol at least 20 times. This protocol might have had some positive effect, but if so, the effect went unnoticed by the methods I used (namely, mere introspection and curiosity) or the effect was not large enough for me to be sure that the effect is actually there. In contrast, 20 minutes of mid-day sunlight on a cloudless day on my bare skin and into my "bare" eyes has a pronounced positive effect which are obvious to me.
(Also I once had a session in a machine that costs many 1000s of dollars that I climbed into naked that bathed me in all directions with red light and near-infrared light produced by thousands of LEDs. I did not notice the effects that I value the most from sunlight exposure.)
That strongly suggests that some combination of visible light and UV light is the cause of the effects I value the most. Here let us observe that the common LED light bulb produces no UV light.
I've never made the experiment as far as I can recall of exposing myself to LED light bulb light that is as intense in the visible spectrum as the mid-day sun is. (Frankly I would probably strongly dislike the experience.) But I definitely notice effects on my internal experience from the brighter variants of the ordinary LED bulbs that most of us have in our homes, and the quality of those effects differ drastically from the quality of the effects from sunlight, which causes me to conclude that the UV fraction of sunlight is in expectation the cause of most of the positive benefit I get from sunlight exposure.
Specifically, within only a few seconds of the start of exposure to bright LED light, I am more alert, maybe it is easier for me to pay a certain kind of attention or focus and on a minute-by-minute basis, it becomes easier (requires less willpower) to turn intentions into sequences of actions. Sunlight has the same (almost instantaneous) effect, but also has other effects that LED light bulb light does not cause. In particular, the effect I value the most persists for hours after the end of the exposure to the sunlight and might persist well into the next day. A kind of optimism and bias for action, but very steady. also, the quality of my cognition seems to be better (in hard to describe ways) on a day when I get 20 or 30 minutes of late-morning or mid-day sunlight and use only the intensity of LED light needed to see comfortably than on a day when I get little or no sunlight, but spend the day under LED light of the intensity one would find in an average office or store (i.e., much more LED light than I tend to use when I am in my home).
There is a lot more I could say on the subject (e.g., on mechanisms) but the essence is that there is a fuzzy argument relying heavily on subjective evidence for benefits (mainly mental) of high expected value from my moderate UV exposure and a very crisp argument, the soundness of which I do not doubt at all, compiled by dermatology researchers and ophthamology researchers, for pernicious harms (namely, melanoma and horrible eye diseases) of low expected magnitude, and I judge that the former easily trumps the latter because I trust my ability to evaluate fuzzy arguments (i.e., to avoid being systematically biased by, e.g., motivated stopping, motivated continuing or confirmation bias).
The expected benefit of my UV exposure is high because it is almost certain that various (mostly mental) benefits are being manifested and because even though I cannot be precise in describing those benefits or in evaluating the (personal) utility of those benefits, the utility is quite significant in magnitude. The expected harm of my UV exposure is low in comparison because the probability of the most pernicious harms (melanomia, etc) is (as you note) low.
"I experienced negative behavioral effects such as suddenly becoming much more interpersonally aggressive than the situation calls for."
I am somewhat confused about how you conclude that this is a liver problem or something unique to exogenous testosterone rather than a completely standard, well-known effect of raised testosterone levels. Not experiencing from UV light indicates that UV light is raising your testosterone levels (much?) less.
UV light is raising your testosterone levels (much?) less
I doubt my doctor chose a dose that would average out to be much higher than the amount produced endogenously by a healthy young man. (I was young at the time.) My guess is that the exogenous testosterone was causing testosterone levels to vary in a pattern different from the pattern caused by endogenous testosterone production. For example, endogenous production might result in a much steadier concentration.
UV light proably has significant positive effects other than tending to raise testosterone. Alexis Cowan asserts that it is a more potent stimulus to endorphin production than aerobic exercise is.
In the environment of evolutionary adaptedness, everybody got sunshine into their blood. We know this is important for Vitamin D synthesis. My suspicion is that it is important for other aspects of biochemistry, which we just haven't found yet. I suspected that I wasn't getting enough sunlight living in a New England winter, so I started going to a tanning salon a few times a winter, enough to maintain a mild tan. I feel better after a tanning session, which is at least a moderate indication that it is good for me. Of course tanning salons are a superstimulus, so the good feeling may be misleading.
Epistemic status: First two sentences are uncontroversial. Everything after that is my own theorizing. My ancestry is Northern Europe, so persons with a more normal skin color may not find this useful.
Also, it’s worth noting that our ancestors have been outside all day every day for millions of years.
It is also probably worth noting that skin color in indigenous populations is extremely correlated with average UV levels in the region, which to me indicates that if you are living in a location other than where your ancestors lived, you can't really use "our ancestors dealt with this just fine" since it seems the ancestors that didn't look like the people who evolved in your region failed to breed at much higher rates than the ones who did.
Of course that cuts both ways, since if it were a pure win to be protected from the sun, we would all be dark skinned.
Of course that cuts both ways, since if it were a pure win to be protected from the sun, we would all be dark skinned.
Not necessarily.
We know that humans need sunlight for optimal Vitamin D production, so this does happen to be true. But your logic here doesn't hold up. Synthesizing melanin is metabolically costly. Cave creatures, which have no use for pigmentation but also incur no obvious direct disadvantages from it, often evolve albinism because of the benefits of repurposing melanin precursors for other needs.
Humans aren't cavefish, but the principle applies. If you don't need melanin, making it is a waste of energy, and benefits from sunlight aren't necessary to explain the evolutionary pressure against pigmentation.
I admit to having missed that footnote, but I think I am intending to make a subtly different point here, which is that not only does a mismatch with ancestral environment undercut the "our ancestors didn't have sunscreen" argument, but rather the strong correlation between UV intensity and skin color indicates that this is not a weak selection pressure and getting exactly the right amount of sun is probably kind of a big deal.
Especially if it is true that it's just vitamin D that was imposing pressure towards lighter skin and we can get the same effect that through dietary supplementation.
Sure, but that could still be consistent with “sunburns bad, suntans fine” theory, I figure. Maybe even if our ancestors were outside all the time, they would still sometimes lose their tan during a cloudy week and get sunburned?
I’m definitely open to the possibility that e.g. people of Scandinavian descent living in Nairobi simply cannot accommodate to the UV exposure by tanning, i.e. even if they are as tanned as they can possibly be, they’ll still get burned, there’s just too much UV. If so, then sunscreen (+ clothes, shade, etc.) is their only option to avoid sunburns, and again everyone agrees that sunburns are bad, both immediately and long-term.
I may be wrong here, but IIRC the trade-off is:
So in an area with less sunlight, evolution leads to people with brighter skin color and vice versa. In the modern world where people are mobile across continents, they need better information about technology that can help against both problems (like supplements and protection against skin cancer).
My best guess would be that the sun breaking down folate (Vitamin B9) would be more of a reproductive fitness disadvantage especially during pregnancy compared to the very few skin cancer cases late in life. I don't have strong evidence either way though. Probably both effects are roughly within one order of magnitude? Also light skin could be faster selected for than dark skin since vitamin D is so important. As Carl Feynman notes, there could be other biochemistry stuff.
…So why do people sell and use sunscreen with way way higher nominal SPFs, like 30 or more?
Personally:
If there's no downside then there's no downside.
Sometimes you say cancer and sometimes you say deadly cancer. How careful were you to distinguish these? My understanding is that sun exposure is positively correlated with melanoma and negatively correlated with melanoma deaths.
But also, spending lots of time in full sunlight seems to be protective against nearsightedness
I'm curious about that. I suspect something like "I'd guess it's because being lots in sun means you're staring way less on tiny things very close to you not because UV is good for eyes" but I'm curious why you think this is case in the first place
My impression (I’m open to correction) is that studies have tried to tease out the influence of inside-vs-outside from time-spent-looking-at-books-and-screens, and they reliably find that the former makes a big difference in myopia, while the latter makes no difference at all (once you control for the former).
There’s some proposed mechanism involving bright light increasing dopamine in the retina, which in turn impacts myopia via [not sure what the pathway is] (example). I don’t know how strong the evidence is for that mechanism.
Anyway, if you or anyone else tries to make sense of this literature, I’d be interested in what you learn. :-)
What you've suggested here is pretty much what I already try to practice. When I'm about to be outside a bunch, I try to avoid getting sunburned on my way to a base tan. And I use sunscreen during the transition and mostly don't afterward.
But I do try to check the UV index and let that guide me too.
The main reason that I use sunscreen is vanity. Sunscreen is the ultimate cosmetic anti-aging agent; nothing can even come close. I put sunscreen on my face almost every day, including in winter. It is very simple to look 10+ years younger than your actual age if you avoid alcohol, cigarettes, and wear sunscreen everyday. Huge +EV move.
The agricultural worker data is interesting, if chronic sun exposure barely increases cancer rates, is that because the tan is absorbing most of the UV, or because there's a genuine non-zero damage threshold (similar to the linear no-threshold debate in ionizing radiation)? The two explanations would be distinguishable by measuring how much UV a tan actually blocks, if it's only SPF 2-4 as some estimates suggest, the tan alone probably can't account for it.
I don’t understand your comment. Getting sunburned is obviously a threshold thing, right? If I get 5× more sun than normal on some day, I don’t get a 5× bigger sunburn, instead I go from no sunburn whatsoever to yes sunburn.
A tan being SPF 2–4 sounds reasonable, see §2.3.
The most common idea (zero-threshold hypothesis) is that if you get, let's say, 10 joules of energy into 1 square centimeter of skin (after it passes through the tan), you get, let's say, 1% more probability of having cancer (I'm inventing the numbers), and it doesn't change whether you received it over 1 second or 10 years or whether it caused a burn or not.
Sunburn happens when this damage happens at once, so many cells die, triggering an inflammatory response, but the zero threshold hypothesis says that whether the exposure was over 1 second (thus giving you a sunburn) or over 10 years is irrelevant to cancer rates.
However, the agricultural worker data suggests that they received, let's say, 1000x sunburns' worth of UV along many years, even taking into account their tans, but have much lower cancer rates than a person who received 1000 actual sunburns.
So this suggests that maybe the DNA can repair itself over time in a way that fixes many cancer-causing mutations, such that only a big dose in a small amount of time causes cancer. If this were true for all organs, then, for example, much less money would be spent on nuclear shielding, or there would be much less worry about increasing cancer risk when doing a CT scan. But the increased cancer rate from small doses is very hard to experiment with because you need very big populations to get a statistically significant measurement, like let's say a group of 10M people where half undergo a CT scan randomly.
Edit: In fact, the DNA repairs all the time, otherwise you would suffer from Xeroderma pigmentosum which causes your skin to burn in minutes on sunlight, and increases cancer risk by a factor of 10000. The question might be whether sunburns overload this repair mechanism in a way that triggers irreparable damage/cancer while constant exposure doesn't.
Edit 2: (found in a Reddit comment): "If a thymidine dimer isn’t repaired before the next time the DNA is replicated, it can cause major issues. The daughter cells can become either non-viable ... or they can become unregulated and cancerous ..."
Skin damage happens before you start burning. Sunscreen helps little because its a poor medium for protection (and also because sunscreen can be inconvenient). Also SPF I think measures quantity, but not depth which is as important.
For UV protection supplements are better:
Table of contents:
Part 1: In which I use my optical physics background to share some hopefully-uncontroversial observations
1.1 UV depends a lot on “solar zenith angle” [a.k.a. “angle of the sun away from directly overhead”], not on how hot it is outside
That means: you should mainly be thinking about UV exposure in proportion to how close it is to (1) the summer solstice and (2) solar noon.[1]
Here, I made this handy widget.[2] Select a city in the drop-down at the bottom, and mouse over (or tap) the colored area for specific datapoints:
I find that people intuitively judge sunburn risk based on temperatures being high, instead of shadows being short. So they worry about UV too much in the hot late summer, and/or not enough in the cool early spring; and they worry about UV too much in hot late afternoons, and/or not enough in cool late mornings.
(Of course, temperature matters indirectly, because if it’s hot, you’re probably more likely to be outside, and also less likely to be covered by clothes.)
Here’s an example plot showing how UV exposure falls off with solar zenith angle (SZA) at some location:
Left: sunburn-causing UV. Right: wrinkle-causing UV. Source.
Looks like the wrinkle-causing UV is roughly proportional to the cosine of SZA, while the sunburn-causing UV falls off with angle a bit faster.
If this all sounds to you like a small effect (“only” 2× difference in sunburn-rate between 20° vs 45° SZA), then see §2.3 below.
1.2 Other things matter too, so just check your local UV index
Solar zenith angle is important, but ozone, clouds, and elevation are important too, and there are additional minor things on top of that. So just install a weather app that shows the current UV index (or better yet, a forecast of UV index over the day).
By the way, UV index is a linear scale, so e.g. you get the same cumulative dose from 1 hour at UV index 10, versus 2 hours at UV index 5. (Consensus seems to be that “cumulative dose” over the course of a day is what actually matters. Seems plausible.)
1.3 Around half of UV is diffuse (mostly coming from the blue sky) not direct
(Specifically, Figure 6 here seems to suggest that maybe 40% of UVA [which causes wrinkles] and 50% of UVB [which causes burns] is diffuse, although the ratio depends on solar zenith angle, atmospheric conditions, etc.)
So if you’re sitting in a narrow spot of shade, with almost full view of the blue sky, the amount of visible light hitting you might be 5× lower, but you might get a sunburn only 2× slower.
(Don’t get me wrong—2× slower is still a big deal! See §2.3 below.)
Part 2: In which I boldly defy Public Health Orthodoxy on the whole UV situation
(UPDATE: For a counterpoint to this part, check out “Contra Byrnes on UV & cancer: You should wear sunscreen instead of trying to get a tan” (substack, lesswrong, reddit). So far my take is: I acknowledge I have some mistakes to correct, but I basically stand by my bottom line. See the ongoing discussion here.)
[…Well, “boldly” is the wrong word. This is “strong opinions, weakly held”. Even I myself am too nervous to fully act on these opinions; instead I kinda split the difference between Public Health Orthodoxy and what I actually believe. And I certainly wouldn’t suggest that readers make health decisions based on poorly-researched contrarian blog posts by randos on the internet.]
Public Health Orthodoxy in the USA—or at least my vague impression from the messaging that trickles down to me and my non-scientist friends and neighbors—is something like: “sunlight and UV are dangerous, and you should stay safe by always wearing sunscreen and sunglasses”.
My current thinking is that this has some kernels of truth, but is oversimplified and misleading. In particular, if sunlight-without-sunscreen causes deadly skin cancers, then why is the correlation between sunscreen use and deadly skin cancers so weak (after controlling for confounders like skin fairness and sun exposure)? This 2003 review found no effect, and neither did this 2018 review. (Indeed, the latter found a small, non-significant increase in skin cancer from sunscreen!)
Also, it’s worth noting that our ancestors have been outside all day every day for millions of years.[3] So UV-induced health problems are only plausible if they’re pretty rare, mild, and/or late-in-life, albeit with some caveats.[4]
So anyway, here’s where I’m at right now.
2.1 I lean towards: (1) sunburns are bad, (2) tans are neutral (in themselves), (3) tans are good all things considered (because they prevent sunburns), (4) Sunscreen is for sudden transitions in sun exposure, and then you should try to wean off it (if you can)
(…This is only in terms of deadly skin cancer; no comment on wrinkles etc.)
The evidence linking sunburns to skin cancer seems quite robust. Also, sunburns are painful. So we should definitely be avoiding sunburns.
If you get some unprotected sun exposure, but not enough for a sunburn, then you instead get a tan. So what about tans?[5] Are they linked to skin cancer?
My answer: The best I can tell right now is that if tans are bad at all, they’re very very much less bad than sunburns.
For example, there are people who work outside, without a shirt or sunscreen, basically all day every day for months on end. Maybe they have 50× more sun exposure than an office worker. Do they get 50× more skin cancer? No way. This would be a huge, easily-observable effect, and as far as I can tell, it has not been observed.
Do they get any more skin cancer than office workers? Maybe slightly—e.g. 20% more in this study of agricultural workers. Or, maybe not even slightly—there are enough confounders (e.g. sunburns, exposure to pesticide & fertilizers) that this little 20% effect might be unrelated to tans. So if sun-tans carry any cancer risk at all, I’m rounding it to zero.
So, if tans are neutral in themselves, then that means tans are good all-things-considered, because they prevent sunburns (well, by and large, see addendum below). It’s nature’s sunscreen.[6]
So my claim is: if you’ve been getting a similar amount of sun exposure every day (or every few days or whatever), then you can get into a rhythm where you never wear sunscreen, and never get sunburned, because you always have an appropriate tan. And this is fine. Indeed, it’s better than sunscreen, because you don’t have to worry about getting burned whenever you miss a spot, or when you sweat it off, or when you forget to pack it, etc. (Plus the sunscreen itself is annoying, and might or might not have health risks of its own.)
Sunscreen would be important during sudden transitions. Maybe you’ve been wearing a shirt every day, but today you’re in a bikini for hours. Or you’re inside all day during school, but it just ended and now it’s summer break. Or you have a desk job but you’re flying off to a tropical cruise. With sudden transitions like that, you’re a sitting duck with no protective tan, and you need sunscreen (and/or clothing, shade, etc.) to avoid getting burned.
But the idea would be to wean off it. For example, instead of putting on sunscreen before going out, set a timer and put it on after some time in the sun (which you can ramp up over time). (Also good for Vitamin D!)
ADDENDUM APRIL 16: I should clarify that for some people in some situations (apparently white people in Australia are often in this category), it might be the case that your body is simply incapable of developing enough of a tan to avoid getting a sunburn. If so, then you should obviously wear sunscreen! At the end of the day, if you’re getting sunburns, then whatever you’re doing is the wrong thing to do, and you should do something different. Sunburns are bad.
2.2 Wear sunglasses for comfort if you want, but they’re not a health product
UV is a risk factor for cataracts. But also, spending lots of time in full sunlight seems to be protective against nearsightedness. So for people with good vision,[7] wearing sunglasses would presumably trade off cataracts versus nearsightedness.[8]
For kids, that seems like a bad trade. The cataracts in question would be developing in like 2080, which will be a very different world. If the superintelligent AI apocalypse or utopia hasn’t come by then, we should at least have better much cataract treatments. By contrast, nearsightedness is annoying and immediate.
(My own kids do have sunglasses, and if they want to wear them, that’s fine with me! But I’m not proactively encouraging them to wear sunglasses, the way some parents do.)
For adults, the nearsightedness-vs-cataracts tradeoff is less obvious, but I still think it goes weakly in the anti-sunglasses direction, at least for people like me who are spending most of their life indoors.
(But I absolutely do wear sunglasses when I’m driving in glare, for safety.)
2.3 An appropriate “effective SPF” in most situations is usually like 3, maybe up to 10 tops
The math here is pretty simple, but it took a long time before I really thought it through.
As background, SPF (Sun Protection Factor) is a linear factor on sun exposure. I.e., in theory, you get the same UV exposure from e.g. 1 minute with no sunscreen versus 20 minutes with “effective SPF” 20.
(I’m using the term “effective SPF” because you can have a sunscreen labeled “SPF 20” on the bottle, but you apply it too thinly, or it’s not mixed properly, or whatever, and so you wind up with “effective SPF” much less than 20.)
Now, this will be different for different people (e.g. infants burn especially quickly), but I’m a white guy with usually no tan (since I’m usually inside all day), and if memory serves, I’m nevertheless always fine for 30 minutes, maybe more, in almost-overhead sunlight (at sea level), before I start to burn. That suggests that for me, “effective SPF” of 5-10 would probably get me from dawn to dusk on a maximally sunny day without burning, even with no tan at all.
Indeed, even “effective SPF” of 2 or 3 would often be enough! This explains why solar zenith angle (§1.1) makes such a big difference in practice, as does shade (§1.3), tans (§2.1), etc.
(Also, dry white t-shirts are supposedly only SPF 5-10, and I don’t recall ever getting a sunburn through a white t-shirt. Or even really a tan. And I don’t think I wear unusually-densely-woven t-shirts?)
…So why do people sell and use sunscreen with way way higher nominal SPFs, like 30 or more? I think part of it is the difference between nominal SPF and “effective SPF” from poor application, etc., mentioned above? (But then shouldn’t you be focusing less on the nominal SPF and more on how easy it is to properly apply?) Or maybe part of it is that a high SPF for UVB may be a barely-adequate SPF for UVA or vice-versa? Or people just buy crazy-high SPF because higher numbers seem better? I dunno.
Solar noon is the time of day when the sun is highest in the sky. It mainly depends on your time zone and longitude, and I think for most places it falls somewhere between 12:00 and 1:00pm. You can just look it up for where you live (see widget below), but note that it shifts by an hour due to daylight savings, plus it bounces around slightly (maybe ±15 minutes) due to orbital stuff I don’t understand.
Thanks Gemini 3.1 Pro
People sometimes object to this argument on the grounds that the UV situation today is dramatically different today than in the past, because of CFCs depleting the ozone layer. But I’m pretty sure that’s wrong. Thanks to the Montreal Protocol, ozone depletion never really got that bad. Indeed, the extra UV exposure you get from CFC-related ozone depletion is less than the extra UV exposure you would get by moving a mere 200 km closer to the equator. (Source.)
This argument doesn’t rule out health problems from e.g. light-skinned people of Scandinavian heritage living at the equator, or interactions between UV exposure and diet, or various other caveats. So it’s not a strong argument, but I think it’s context that’s worth keeping in mind as one reads the literature.
I’m only talking about real sun-tans, not tanning beds, which I didn’t look into, they’re not my scene.
I guess some people have a skin type where they don’t tan at all? I don’t know what to make of that. Is it just impossible for those people to be outside every day year-round without sunscreen?? Like, what were their ancestors doing 500 years ago?? My hunch is instead that their skin “gets used to” sun exposure in some other way that doesn’t involve visible tans. But I dunno. YMMV.
If you wear glasses or contacts outside, then you can check whether they block UV. If they do, then I guess you get the best of both worlds, and sunglasses would seem to become a pure eye-health negative.
I’m assuming the sunglasses reduce the UV and not just the visible light. Not sure how common that is. If your sunglasses are letting the UV through, then that’s the worst of both worlds.