What does the sun actually do to your health?

Short answer

Sunscreen works: in a randomized trial of 1,621 Australians, daily SPF 15 cut squamous cell carcinoma by 38% over eight years, and no sunscreen ingredient has a known mechanism for causing cancer. Twenty minutes of whole-body sun makes roughly 10,000 IU of vitamin D3, so you almost certainly don't need a supplement, and randomized trials of vitamin D supplementation show no benefit for cancer, heart disease, diabetes, falls, or mortality. Sunlight does appear to help mood, and seasonal affective disorder responds to early-morning blue light therapy, though not to vitamin D. Do not sun your perineum.

If you were a kid in the 1980s, sun protection meant someone yelling at you to come inside. Now there's an Instagram genre arguing that we overcorrected, that sunscreen is the real danger, and that a large enough serving of watermelon does the same job. So it's worth going through what sunlight actually does to you, and what it doesn't.

Sunscreen does not cause cancer. It prevents it.

The damaging part of sunlight is ultraviolet light, in two flavors. UVB is the shorter, higher-energy wavelength. It doesn't penetrate deeply, but it damages DNA, and that's one of the mechanisms by which sun exposure causes skin cancer. UVA goes a bit deeper and is more responsible for the cosmetic stuff: sunspots, wrinkling, the general business of looking older than you are. A good sunscreen blocks both.

The anti-sunscreen argument you see online goes like this: sunscreen arrived in the 1930s, got FDA approval around 1980, and melanoma rates have climbed ever since. Therefore sunscreen causes melanoma. This is a textbook ecological fallacy, and it's also backwards. Before cheap travel, pale skin was how wealthy people signaled that they didn't work outdoors. Being tan became desirable, people started spending far more time in the sun, and they wore sunscreen to keep from burning while they did it. More sun, more cancer. Detection also improved enormously. In 1850, if you had a melanoma, nobody was checking your moles every six months. You just died, possibly of something else first.

None of the chemicals in sunscreen have been linked to cancer. None damage DNA. None have a plausible mechanism for causing it. If I want to argue the other side as hard as I can, I'd say sunscreen might give you a false sense of security, so you stay out longer and don't reapply. That's a plausible risk factor, but it isn't the sunscreen doing the damage.

Meanwhile there's randomized evidence. A trial published in the Lancet in 1999 assigned 1,621 Australians to either daily SPF 15 or "discretionary use," which in practice meant considerably less. Over eight years of follow-up, the daily-sunscreen group had a 38% lower rate of squamous cell carcinoma. There were 3 invasive melanomas in the sunscreen group versus 11 in the comparison group, which is not enough events for statistical confidence but is not nothing either.

That's with SPF 15, which is on the low end of what anyone recommends today. Observational studies on sunscreen are a mess in both directions, and you can find ones pointing either way, because people at higher risk of skin cancer use more sunscreen. In medicine we call that confounding by indication; economists call it reverse causality, which is admittedly easier to say. The same phenomenon makes statin users look like they have more heart attacks than non-users. They take statins because they're already at risk. This is why you need a randomized trial, and for sunscreen we have one.

Which sunscreen, and how much SPF

Mineral sunscreens use zinc oxide or titanium dioxide, physical particles that reflect UV off your skin. Zinc covers both UVA and UVB. Titanium is more UVB-focused, so if you're using titanium alone you want something else covering UVA, unless you're relaxed about skin aging. These are the ones that make you look like an 18th century consumptive, though the newer nanoparticle formulations are much less stark and work just as well.

Chemical sunscreens use compounds that absorb UV instead, and they don't turn your face white. The worry you'll hear about is endocrine disruption. Feed a rat enormous doses of some of these chemicals for its entire life and its reproductive system suffers. But people have worked out how much sunscreen a human would have to apply, over a lifetime, to reach those exposures, and it isn't feasible.

Use whichever one you'll actually put on. That's the whole calculus. Clothing works too, if you can tolerate it, which most kids can't. Look for the phrase "broad spectrum" on the label, which is an FDA-regulated term meaning it covers both UVA and UVB. And you don't need SPF 100. The American dermatology guidance is SPF 30 or higher, so 30 or 50 is fine, and you can skip the expensive stuff.

Tanning beds are essentially the same exposure as sun, with some odd wrinkles. A 2025 molecular study in Science Advances found tanning bed users had higher melanocyte mutation burdens spread across a broader field of body sites, and they're more likely to develop multiple primary melanomas, meaning separate cancers rather than one that spread. That's marginal stuff. Neither is safe. If you want to look tan, there's a spray for that.

You are almost certainly getting enough vitamin D

Vitamin D is unusual among vitamins in that you make most of it yourself. In your skin, you have 7-dehydrocholesterol, a cholesterol derivative. UVB converts it to vitamin D3, which is the stuff on the shelf at CVS and is biologically inert. Your liver converts it to 25-hydroxyvitamin D, also inert. That travels to the kidneys and becomes 1,25-dihydroxyvitamin D, and that, and only that, is the active hormone. Your body has built an elaborate multi-organ pipeline whose entire purpose is to turn sunlight into a functioning skeleton.

Here's the part that surprises people. Recommended daily intake is somewhere in the range of 200 to 400 IU, and supplements typically come in 1,000 to 2,000 IU. Twenty minutes of whole-body UVB exposure in a light-skinned person produces roughly 10,000 IU of D3. Twenty minutes on the beach is about ten times what a pill gives you.

You cannot overdose this way. Sunlight also degrades vitamin D3 in the skin, so the system brakes itself. Swallowing that much is a different matter. Vitamin D toxicity is real, it can drive calcium levels high enough to cause kidney damage, arrhythmias, and coma, and as a nephrologist I have a professional interest in you not doing that.

The reason vitamin D keeps coming up is a mountain of observational data. Low blood levels of 25-hydroxyvitamin D, which is what labs usually measure because it's the easier test, are associated with higher risk of cancer, cardiovascular disease, diabetes, falls, COVID, and all-cause mortality. Read only that literature and you'd conclude vitamin D is the most important molecule in the body.

Then you look at the randomized trials, which are numerous and large, some with 25,000 or 26,000 participants and true placebo controls. They show nothing. Supplement people with vitamin D and the outcomes don't move.

The reason is selection. Roughly half of Americans take vitamin D, and it's the same half that exercises, eats vegetables, and doesn't smoke. We spent years telling people to take vitamin D to prevent cancer, which means the advice itself made the confounding worse over time. And even if you exclude supplement users entirely, blood level still tracks with bad outcomes, because vitamin D level is a lifestyle biomarker. Go outside, move around, eat a varied diet, and your level is higher. Be sick, sedentary, and nutritionally depleted, and it's lower. It's a good marker of overall health. Improving the marker doesn't improve the health.

Two caveats that matter. Winter and high latitudes make sun-derived vitamin D harder to come by. And melanin, which protects DNA from UV mutations, also reduces UVB-driven vitamin D synthesis, so there's more rationale for attention to dietary intake if you have darker skin. I'm still not enthusiastic about supplements, because the randomized trials don't show a benefit at any skin tone.

Sunlight, mood, and the things sun does that aren't about vitamin D

People are happier outside, and there's observational data from the UK Biobank covering more than 400,000 people showing each additional hour spent outdoors during the day is associated with lower odds of major depressive disorder. Whether that's causal is unclear. But seasonal affective disorder is real, and it responds to blue light therapy in randomized trials, so there's a mechanism by which light exposure genuinely lifts mood.

It is not vitamin D mediated. Vitamin D supplementation does not improve seasonal affective disorder. Light does. Much of the mechanism appears to be circadian and sleep-related, which is why the light is given early in the morning, to reset the clock. If this podcast has a recurring theme, it's that sleep is upstream of nearly everything.

Two other associations came up. There are strong and consistent links between low childhood sun exposure and later multiple sclerosis. MS is diagnosed more often at northern latitudes, and this holds even within the United States, so it isn't just a rich-country, poor-country artifact. I don't have a good mechanism, and most of these are case-control studies, which pull together people who already have the diagnosis and ask them about the distant past. That design carries every problem of observational data plus problems of its own. It generates hypotheses rather than settling them. Emily's guess, and I don't disagree, is that sun exposure won't turn out to be the main driver.

Better evidence, some of it randomized, suggests outdoor time reduces myopia in kids. That's probably less about sunlight than about being outside and having to focus at distance instead of at a screen 14 inches from your face. Our mothers were roughly right about the television.

The perineum question

I debated including this. Then Tucker Carlson put it in a documentary about manliness, so here we are. The claim, delivered online by a shirtless and largely pantsless man with his legs over his head, is that 30 seconds of sun on the anus equals eight hours of clothed sun exposure, and that this boosts testosterone.

It doesn't work. Human testosterone levels do peak in late summer and early autumn observationally, but that has nothing to do with your perineum and probably little to do with sun at all, given how many things move testosterone around.

There is real risk, though. Patients with psoriasis are treated with UV light, partly because UV is immunosuppressive, which is also one of the reasons excess UV lets cancers develop. When whole-body UVA/UVB therapy started, clinicians found a fourfold increase in genital tumors, because the skin there is thin enough for high-energy UVB to reach tissue it can't reach elsewhere. Guidelines now specifically tell you to shield the genital area during therapeutic UV. Also, and this deserves saying plainly, you will get a sunburn in a place where you very much do not want one.

Bottom line

Wear sunscreen. Broad spectrum, SPF 30 or above, whichever formulation you'll reliably apply. It reduces skin cancer in randomized data and there is no mechanism by which it causes any. Skip the vitamin D supplement; twenty minutes outside beats the pill ten times over, and the trials of supplementation are resoundingly null. Go outside, because it's good for your mood and your kids' eyes, and get your morning light if winter flattens you. Leave your perineum out of it.

Here's our discussion from the episode:

I covered this in depth on Wellness, Actually, listen below.

Wellness, Actually Podcast

"What's the deal with the sun?" — Listen to the full episode, including the week's health news and listener Q&A.

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