Does sun exposure cause skin cancer, and how do you get sun safely?

Not all sun is equal. The main skin cancers follow different patterns. Melanoma, the most dangerous, is tied to intermittent exposure, the holiday or weekend burst, and to a history of sunburn. In a meta-analysis of 57 studies, steady high exposure, such as outdoor work, was if anything inversely linked to melanoma⁶. Squamous cell carcinoma tracks accumulated exposure instead: outdoor workers had 77% higher odds in a pooled analysis of 18 studies³.

Burns count at every age. Melanoma risk rose with the number of sunburns in childhood, adolescence and adulthood alike, and the increase per burn was greatest for adult and lifetime burns². Avoiding burns is worthwhile at any age.

Skin type and place. Pigmentation matters enormously: about 151,000 fewer melanomas a year would occur worldwide if every population had the rates seen in dark-skinned populations¹. UV is stronger nearer the equator, and the link between work in the sun and squamous cell carcinoma was stronger at lower latitudes³. In winter at high latitudes, sunlight cannot make vitamin D at all⁷.

Time of day. The UV Index forecasts the day's peak, which falls in the four hours around solar noon. WHO advises shade at midday, plus a shirt, hat and sunscreen, when it is 3 to 7, and avoiding midday outdoors at 8 and above⁴. For vitamin D, brief midday sun is the most efficient time, but that is also when UV is strongest⁷.

A tan is not armour. Two weeks of daily sub-burn doses raised the dose needed to redden skin by only 1.4 to 2.3 times in 16 volunteers⁸. Build exposure gradually to avoid burning, but do not count a tan as protection.

Shade and clothing first, sunscreen second. The WHO advice is shade, a shirt and a hat, with sunscreen added⁴. In the Nambour trial of 1,621 Australian adults, daily sunscreen cut squamous cell tumors by about 40%⁹ and, in long-term follow-up, new melanomas by about half, a borderline result¹⁰. But a higher SPF can lead people to stay out longer¹¹. See Choosing a Safe Sunscreen.

Food. Carotenoid-rich foods and lycopene give a little protection against sunburn, far less than sunscreen, and take weeks to build up¹². Beta-carotene pills did not lower skin cancer rates in the Nambour trial⁹.

Sunbeds raise melanoma risk, especially when first used before age 35⁵. For vitamin D and the other benefits of sunlight, see The Sunshine Vitamin and More Than Vitamin D.

How much skin cancer is due to the sun? Arnold and colleagues estimated that 168,000 new melanomas worldwide in 2012 were attributable to excess UV, 75.7% of all new melanoma cases, rising to 96% in Oceania. Roughly 151,000 fewer cases would occur if every population had the rates of dark-skinned reference populations¹. This is a modelled estimate, not a trial.

Pattern of exposure. Gandini and colleagues' meta-analysis extracted relative risks from 57 studies published before September 2002. Intermittent sun exposure and sunburn history were considerable risk factors for melanoma, whereas high occupational, continuous exposure seemed inversely associated. Better-conducted studies supported that pattern, and studies at higher latitudes found higher risks for a history of sunburn⁶. Dennis and colleagues pooled 51 study populations, 26 of them with dose-response data. Melanoma risk rose with the number of sunburns in childhood, adolescence, adulthood and over a lifetime, not only with childhood burns². For squamous cell carcinoma, Schmitt and colleagues pooled 18 studies (6 cohort, 12 case-control). Sixteen found higher risk with occupational UV exposure, and the pooled odds ratio was 1.77 (95% CI 1.40 to 2.22), stronger at lower latitudes³.

Sunbeds. Boniol and colleagues' meta-analysis of 27 studies found a summary relative risk of 1.20 (1.08 to 1.34) for ever using sunbeds, and 1.87 (1.41 to 2.48) when first use came before age 35 (13 studies). Each additional session per year raised melanoma risk by 1.8%. They estimated that 3,438 melanomas in western Europe in 2008 were attributable to sunbeds⁵.

How protective is a tan? Sheehan and colleagues gave 16 healthy adults (8 skin type II, 8 type III) solar-simulated radiation daily for two weeks at 0.5 or 0.75 of the minimal dose that reddens skin. A week later the dose needed to redden skin had risen by factors of 1.4 and 2.1 in type II and 1.5 and 2.3 in type III, which the authors called only moderate protection⁸. The sample was small, healthy and short-term.

Sunscreen trials. The Nambour trial randomised 1,621 Australian adults aged 25 to 75 to daily or discretionary SPF 15+ sunscreen on head, neck, arms and hands, with or without 30 mg a day of beta-carotene. After 4.5 years there was no difference in people treated for a first basal cell carcinoma (rate ratio 1.03) or squamous cell carcinoma (0.88). Daily sunscreen did cut the number of squamous cell tumors (0.61, 0.46 to 0.81) but not basal cell tumors, and beta-carotene had no effect on either cancer⁹. Ten years after the trial ended, 11 new primary melanomas had occurred in the daily-sunscreen group and 22 in the discretionary group (hazard ratio 0.50, 0.24 to 1.02; P = .051). Invasive melanomas numbered 3 versus 11 (0.27, 0.08 to 0.97)¹⁰. The numbers are small and the main result narrowly missed conventional significance, so this is suggestive, not definitive.

A conflicting signal. Observational studies have linked sunscreen use with more melanoma, basal cell cancer and moles, and Autier and colleagues suspected longer sun exposure might be the reason. They randomised 87 young Europeans to SPF 10 or SPF 30 for their summer holidays. The SPF 30 group had longer total sun exposure (72.6 versus 58.2 hours, P = .011) and longer daily sunbathing (3.1 versus 2.6 hours), with no difference in sunburn¹¹. Sunscreen helps as a backup only if it does not simply extend exposure.

Food and photoprotection. Stahl and Sies reviewed human studies showing protective effects of beta-carotene or lycopene, from food or supplements, against sunburn. They concluded that the sun protection factor from such components is considerably lower than from sunscreen and that several weeks are needed for an effect to appear¹². This agrees with Nambour, where beta-carotene pills did not reduce skin cancers⁹.

Vitamin D, latitude and time. Webb and Engelsen calculated that, where solar UV is sufficient, the best time for brief sun exposure to make vitamin D is the middle of the day. At high latitudes in winter vitamin D cannot be made, or the time needed is impractical, and at low sun angles there is a fine line between adequate UV and sunburn⁷.

Limits. Most of this evidence is observational, because people cannot be randomised to burns. The sunscreen and diet trials are few and small. Individual skin type, medicines and diseases also change risk.

Bottom line (confidence 5). That UV exposure causes skin cancers is very well established: mainly burns and intermittent intense exposure for melanoma, and accumulated exposure for squamous cell cancer. The practical strategy of avoiding burns, using shade and clothing first and sunscreen as backup is sound, though the best evidence for sunscreen against melanoma is a single borderline trial.