Why Outdoor Workers Don't Get More Melanoma

For thirty years the public health advice about the sun has come down to one message, cover up. We're told that sunlight causes skin cancer, that every ray adds to a lifetime tally, and that the safest dose is none. It's worth asking, then, why the people who get more sun than anyone else, the farmers, fishermen, roofers, and road crews who spend their working lives outdoors, don't carry the highest rates of melanoma, the most dangerous form of skin cancer. If sunlight were simply toxic the way cigarette smoke is, the dose would track with the disease, and the outdoor worker would be the cautionary tale we all point to, yet that isn't what the research shows.

Set the sun aside for a moment and imagine some other environmental exposure, one where the people who got it in short, intense bursts developed more cancer, while the people who got it steadily, day after day for years, developed no more than anyone else and in some studies a little less. What would you conclude about that exposure, that you should hide from it completely, or that you should get it regularly and simply never overdo it? With melanoma that's close to the picture the evidence paints, and the exposure in question is the sun. The way you get your sun turns out to matter more than how much you get, and the thing that tracks with melanoma isn't steady sunshine, it's the sunburn.

The occupational paradox

Continuous, everyday sun exposure does not appear to raise melanoma risk, and in some studies it lines up with a lower risk. In a 2013 analysis in the International Journal of Cancer, Vuong and colleagues combined two large population-based studies, the Australian Melanoma Family Study and the American Genes, Environment and Melanoma study, more than 1,600 melanoma cases between them, and zeroed in on the sun people accumulated on ordinary weekdays, the closest proxy we have for occupational exposure [1]. They found no increase in melanoma overall or at any body site. In the larger study, more weekday sun actually went the other way for head and neck melanoma, with the most-exposed quartile carrying about half the risk of the least-exposed (odds ratio 0.56, 95% confidence interval 0.36 to 0.86). In their own words, occupational sun exposure did not increase the risk of melanoma, even for melanomas on the head and neck, and weekend sun didn't explain that finding away.

The people at higher risk weren't the ones with steady, day-in-day-out exposure but the ones with a mismatch, lots of sun in one setting and little in another, the pattern of someone who stays covered all week and then burns on a beach holiday. That intermittent, intense exposure is the signal, not cumulative dose.

In a 2005 meta-analysis in the European Journal of Cancer, Gandini and colleagues pooled 57 separate studies and found that intermittent sun exposure and a history of sunburn stood out as the genuine risk factors for melanoma, while high occupational sun exposure was, if anything, inversely associated with it [2]. Two very different methods, a focused case-control analysis and a synthesis of dozens of studies, point in the same direction.

Avoid sunburn

If there's a single villain in this story, it's the sunburn, not the sun. When Dennis and colleagues pooled 51 study populations in a 2008 meta-analysis in Annals of Epidemiology, melanoma risk climbed steadily with the number of sunburns a person had suffered, and it did so across childhood, adolescence, and adult life rather than only in the early years many people assume are the only ones that count [3].

A sunburn, what doctors call erythema, is acute injury, an overwhelming dose of ultraviolet radiation hitting skin that wasn't prepared for it, snapping bonds in cellular DNA, killing cells outright, and setting off a wave of inflammation. That's a fundamentally different event from the gradual, repeated, sub-burning exposure an outdoor worker gets, which prompts the skin to adapt by building melanin, the brown pigment that absorbs UV, and thickening its outer layer. The adapted skin meets the sun on terms it can handle. A burn injures the skin faster than it can defend itself, and that injury, repeated over a lifetime, is what the melanoma data keep pointing back to.

What the skin does when you don't burn

I've come to think of the skin less as a passive barrier the sun damages and more as an endocrine organ the sun switches on, and I suspect the hormonal response to regular, moderate sun is part of why it behaves so differently from a burn.

The clearest example is vitamin D, which is not really a vitamin at all but a hormone your skin manufactures from cholesterol when ultraviolet B light strikes it. It influences hundreds of genes, immune function among them, and you make it in meaningful amounts only with skin exposed to the sun. Sunlight does more than that. Liu, Feelisch, and colleagues showed in a 2014 study in the Journal of Investigative Dermatology that ultraviolet A light striking the skin releases nitric oxide from stores held in the skin itself, widening blood vessels and lowering blood pressure, and it did so independently of the usual enzyme that makes nitric oxide, meaning the skin was acting as a reservoir the sun could tap [6]. A burn delivers none of this gracefully. A daily, graduated dose lets these systems run the way they seem built to.

This is a hypothesis about mechanism, and I hold it loosely, but it fits the pattern, that the body has machinery to turn ordinary sunlight into something useful, and the harm shows up mainly when we blow past that machinery with an intermittent overdose.

Sun exposure and surviving melanoma

The story gets stranger once someone already has melanoma. In a 2005 study in the Journal of the National Cancer Institute, Berwick and colleagues followed melanoma patients and found that the markers of chronic sun exposure predicted better survival, not worse [4]. Solar elastosis, the sun-weathered collagen a pathologist can see under the microscope as a sign of years of sun, carried roughly a 60% lower risk of dying from the melanoma (hazard ratio 0.4, 95% confidence interval 0.2 to 0.8), and people who paid attention to their own skin fared better too. The authors concluded that sun exposure was associated with improved survival from melanoma.

Vitamin D shows up again here. Newton-Bishop and colleagues, studying 872 melanoma patients in Leeds in a 2009 report in the Journal of Clinical Oncology, found that higher vitamin D levels at diagnosis went with thinner tumors and better survival [5]. None of this proves the sun cured anyone, and part of the survival edge may simply be that sun-exposed people catch their melanomas earlier, but it sits very awkwardly beside the idea that sunlight is uniformly the enemy.

The cost of hiding from the sun

Avoiding the sun is not a free choice with only an upside. Lindqvist and colleagues followed nearly 30,000 Swedish women for 20 years in the Melanoma in Southern Sweden cohort and reported in 2016 in the Journal of Internal Medicine that avoiding sun exposure carried a mortality risk on the order of smoking [7]. The women who avoided the sun lived an estimated 0.6 to 2.1 years less than those with the highest exposure, a gap that came mostly from more cardiovascular and other non-cancer deaths. The study doesn't show that sun prevents those deaths, and confounding is a real worry, but it does land the point that treating sunlight as nothing but a hazard ignores the other side of the ledger.

Keeping it honest

A few cautions keep this from tipping into bad advice. Despite the evidence I've laid out that the melanoma link is questionable at best, basal and squamous cell carcinomas, the far more common and rarely fatal skin cancers, do appear to climb with cumulative lifetime sun exposure. Current research struggles to separate that signal from the confounders of burns, unhealthy short bursts, and others, so I hold it loosely. Either way, these less dangerous cancers may track with sun exposure, or we have yet to find the confounding variable that raises their risk. Almost all of this evidence is observational, so it shows associations rather than proof, and confounding is a genuine problem, since the fair-skinned person who burns easily is also the one most likely to slather on sunscreen and stay indoors. Melanoma is a real and sometimes lethal cancer, and nothing here is permission to bake. Fair skin can burn in minutes, and the whole argument rests on not burning. Tanning beds are exactly the wrong lesson to draw, since they deliver intense, intermittent, artificial ultraviolet, the very pattern the data implicate, and they cause skin cancer, so leave them alone. Genetics, a personal or family history of melanoma, and a lot of atypical moles all change the calculus, and they're reasons to be more careful, not less.

What to do

The aim is regular, moderate sun without ever reaching a burn. Build your exposure gradually as the season turns rather than going from a winter of covered skin to a full day at the lake, and let your skin develop its own defenses the way it's meant to. Get out in the daylight most days, aim for short exposures around midday when your skin makes vitamin D most efficiently, and stop well short of any pink. When you've had enough, reach for shade, a hat, and clothing rather than using sunscreen as a license to stay out for hours, since covering up before you burn protects you without blocking the everyday exposure that seems to do good. Don't trade the harm of burning for the harm of hiding, because both carry a cost. Whatever your sun habits, watch your skin, since a mole that's changing, growing, or behaving differently from its neighbors deserves a look from a physician no matter how careful you've been.

If you want the bigger picture on how much of cancer traces back to how we live rather than the genes we're born with, we've written about what chimpanzees and the Amish reveal about cancer, and about how your skin is downstream of your gut.

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References

1. Vuong K, McGeechan K, Armstrong BK, et al. Occupational sun exposure and risk of melanoma according to anatomical site. Int J Cancer. 2014. PMID: 24288300
2. Gandini S, Sera F, Cattaruzza MS, et al. Meta-analysis of risk factors for cutaneous melanoma: II. Sun exposure. Eur J Cancer. 2005. PMID: 15617990
3. Dennis LK, Vanbeek MJ, Beane Freeman LE, et al. Sunburns and risk of cutaneous melanoma: does age matter? A comprehensive meta-analysis. Ann Epidemiol. 2008. PMID: 18652979
4. Berwick M, Armstrong BK, Ben-Porat L, et al. Sun exposure and mortality from melanoma. J Natl Cancer Inst. 2005. PMID: 15687362
5. Newton-Bishop JA, Beswick S, Randerson-Moor J, et al. Serum 25-hydroxyvitamin D3 levels are associated with Breslow thickness at presentation and survival from melanoma. J Clin Oncol. 2009. PMID: 19770375
6. Liu D, Fernandez BO, Hamilton A, et al. UVA irradiation of human skin vasodilates arterial vasculature and lowers blood pressure independently of nitric oxide synthase. J Invest Dermatol. 2014. PMID: 24445737
7. Lindqvist PG, Epstein E, Nielsen K, et al. Avoidance of sun exposure as a risk factor for major causes of death: a competing risk analysis of the Melanoma in Southern Sweden cohort. J Intern Med. 2016. PMID: 26992108