How does sun exposure relate to skin cancer risk?
The simplistic, popular story is that sunlight exposure has a linear relationship with cancer, similar to how we view smoking. None is safest and each additional minute in the sun will increase our chance of getting cancer. Many people (maybe most) therefore live in a world with danger lurking beyond every shadow, umbrella, overhang, and roof. You let your kid go outside without a layer of sunblock so thick he looks like he’s been smashed in the face with a whip cream pie, and you’re a terrible mother. And don’t even think about the beach unless you’re wearing a burqa. It’s even got a scary name: ultraviolet radiation. Radiation! Isn’t that the stuff inside nucular bombs?
The relationship between sun exposure and skin cancer is far more complex than you’ve been led to believe. While clearly too much sun is a causative factor in skin cancer of all types, it’s not clear what “too much” means. Is there a U-shaped curve where none is bad, some is good, and too much is bad again? What about other factors, like skin color, hair color, propensity to tan versus propensity to burn, and time of day?
We already know that living in areas that get more UV radiation seems to be protective against other types of cancers, like prostate cancer, lung cancer, colorectal cancer, leukemia, bladder cancer, pancreatic cancer, breast cancer, and lymphoid cancers (I probably could have kept going, but my fingers were getting tired).
Okay, but what about cutaneous malignant melanoma (CMM)? It’s not the most common kind of skin cancer, but it’s by far the deadliest. Is it worth the tradeoff if we increase our risk of CMM?
When we combine the estimated number of annual diagnoses for the cancers I just mentioned in the US, we get a staggering figure: 1,071,000. What about melanoma? 76,100. Incredible, huh? Getting adequate amounts of ultraviolet radiation (scary!) may very well reduce the incidence of cancers that are afflicting over a million Americans each year. Even if the dermatologists and sun alarmists are right and melanoma risk increases linearly with sun exposure and ultraviolet availability, the protection we get against so many other cancers (and the tan, and the sublime feeling of the sun on your skin) might make the tradeoff worth it. That’s clearly, objectively true, at least on the population level. Melanoma just isn’t as large an absolute threat as the totality of other cancers.
Every reader of this blog is likely aware of the importance of vitamin D in the body. The vital pro-hormone, which also comes in supplement form, helps regulate skeletal, cardiovascular, immune, hormonal, and dental health. A deficiency can cause rickets, osteoporosis, muscle wasting, and various hormonal deficiencies. Low levels are linked to heart disease, autoimmune diseases, all cause mortality, and many cancers. These are the systemic effects that get the most attention, but what if the dermatologists and sun alarmists were wrong, or at least a little misguided in their binary opposition to any and all sun? On the surface, it seems counterintuitive that an act that has so many proven anti-carcinogenic and health benefits – getting enough sunlight to optimize vitamin D levels – would increase our chances of getting another type of deadly cancer. It’s possible, but it’s odd.
And there are some other curious facts that muddy the neat, tidy relationship between sun and skin cancer:
Vitamin D (produced in response to UVB exposure) kills melanoma cells. Researchers posit that vitamin D levels around 70 nmol/L may be optimal for protection against melanoma.
There’s also the recent study that suggests smart sun exposure has beneficial, anti-cancer effects at the skin level. Contained within our skin cells are RNAs that do not synthesize proteins, called non-coding RNA. When vitamin D is produced in the skin cells, it triggers these non-coding RNA to go into protective mode. They reduce carcinogenicity and induce tumor suppression. It’s unclear whether oral vitamin D “makes it into” the skin to have the same protective effect. If anything, supplementary vitamin D will deliver the systemic benefits to bone, heart, immune system, and endocrine system thereby reducing the amount we have to produce at the skin level to get the desired anti-carcinogenic effect. But you still need some sun exposure to get it.
Okay, given what we know about vitamin D, it’s easy to buy that it can be protective to a point. Some sun is helpful. Cool. Surely, though, those who get the most sun get the most skin cancer. You have to imagine that outdoor laborers, lifeguards, and other people who spend all day outside in the sun, have way more skin cancer than indoor workers. Right?
It doesn’t seem to be the case:
- “Recreational” sun exposure – weekend outings, vacations, and other forms of intermittent exposure to the sun – is usually associated with greater melanoma risk than chronic sun exposure.
- “Occupational” sun exposure – the kind outdoor workers or other groups who spend lots of time in the sun on a regular, consistent basis – is associated with lower rates of melanoma. In fact lifetime sun exposure is associated with a lower risk of melanoma.
- In Ireland, non-melanoma skin cancer is increasing among the young, urban, and affluent. The authors suggest that the young, urban, and affluent have more free time on their hands, and they’re spending it outdoors. Okay, but the biggest increases in skin cancer in this demographic are occurring in clothed body parts - the swathes of skin that get the least amount of sun exposure.
- Body parts that get the most sun have the lowest rates of melanoma.
It actually makes sense that going from habitual sun avoidance (working inside all week, getting out on the weekends every once in awhile when you can find the time) to full-on sun exposure (vacation in the tropics, an all day beach excursion) would be bad for you. Indoor skin simply isn’t ready for high UV exposure, and it will burn more easily in response to it. Sunburning definitely increases the risk of melanoma.
Plus, indoor workers still receive UV exposure, just differently. They might see the sun in the morning, through the windows during the day, and after work in the late afternoon. Both morning and late afternoon sunlight have very little vitamin D-producing UVB; it’s almost entirely UVA. Midday sun (you know, the sun they tell you to avoid at all costs) is the only kind with appreciable levels of UVB. And even though office workers get modest levels of sunlight through windows, those windows filter out the UVB and leave only the UVA.
As explained in a 2008 paper (PDF), isolated UVA degrades cutaneous vitamin D while failing to produce any to replace it. So these indoor workers aren’t making any vitamin D and what little they do produce is being broken down from all the UVA they receive.
But wait: although UVA promotes melanoma progression, UVB initiates its formation. Animal models show this. And the connection between sunburns (which are caused by UVB) and melanoma supports it. So isn’t UVB the real problem?
As stated earlier, intermittent and excessive exposures of UVB (“recreational sun exposure”) which often result in sunburns are the problem. It just so happens that the people most likely to get recreational, intermittent, condensed sun are the people who work indoors and go most of the year with low vitamin D – from lack of consistent UVB and from excessive low level window-filtered UVA. People with occupational exposure are getting UVA and UVB in consistent doses, enough to keep the vitamin D levels topped off and nip any budding melanoma cells before they can progress.
How does all this translate to the real world?
Don’t fear a healthy base tan. Contrary to popular opinion, a tan isn’t “already too late.” People who tan well (as opposed to the sun-sensitive types who burn easily) have greater resistance to skin cancer and people who maintain a year-round tan have a lower risk of melanoma.
Avoid burning. Burning is still bad. It’s negative feedback from your body regarding your excessive exposure levels.
Go out at noon. This is when UVB and vitamin D production are highest. The popular advice “that sun exposure should be avoided for three to five hours around noon and postponed to the afternoon” is likely “wrong and may even promote CMM.” Another way to determine UVB availability: go out when your shadow is shorter than you.
Don’t go from zero to sixty. Don’t go from spending the past three months indoors to tanning for hours-long stretches. Work your way up gradually, minute by minute. Start small, especially if light skinned/red haired/blond.
Move around. Don’t “lay out.” You don’t want to expose a single body part to the sun for an extended amount of time. Go do stuff that exposes different parts of your body: garden, lift weights, hike, swim, that sort of thing.
Consider vitamin D supplementation. It delivers the desired systemic effects to bone, heart, immune system, and endocrine system, thereby taking a load off the skin, which can save some D3 for its own protection, and increasing your resistance to the sun. Anecdotes frequently report increased resistance to sunburns upon vitamin D supplementation.
Get your nutrition in check. There are many “internal” measures you can take to increase your skin’s resistance to sun damage. I outlined several of them here.
Get your sleep in check. Our resistance to the sun follows a circadian rhythm, with our defenses strongest in the first half of the day. So you might not want to go tanning after an all-night bender, nor do you want to get too much UVA-rich sun in the late afternoon when your defenses are down.
I hope I cleared up some confusion. If not, feel free to send along any questions.
Thanks for reading!