Every Dermatologist Says to Stay Out of the Sun. A 20-Year Study of 29,518 Swedish Women Found That Avoiding Sun Exposure Carried the Same Mortality Risk as Smoking.

Slip, Slop, Slap

The message is so universal it barely counts as advice anymore. Wear sunscreen. Seek shade. Cover up. The American Academy of Dermatology recommends seeking shade whenever possible. Cancer Research UK tells people to stay indoors between 11 AM and 3 PM on sunny days. The WHO classifies UV radiation as a Group 1 carcinogen, the same category as plutonium and mustard gas. Billions of public health dollars have gone into convincing you that sunlight is trying to kill you.

Pelle Lindqvist wanted to know what happens to the people who actually listened.

Thirty Thousand Women, Twenty Years

Lindqvist is an obstetrician-gynecologist at Karolinska University Hospital in Stockholm. He and colleagues at Lund University followed 29,518 Swedish women enrolled between 1990 and 1992 as part of the Melanoma in Southern Sweden (MISS) cohort. Each woman, aged 25 to 64 at baseline, answered four questions about her sun habits: How often do you sunbathe in summer? Do you sunbathe on winter holidays? Do you use sunbeds? Do you sunbathe when traveling abroad?

The researchers scored each woman from 0 (active sun avoidance on all four measures) to 4 (highest exposure across all domains) and then waited. For twenty years, they tracked deaths through Sweden's national population and cause-of-death registers, adjusting for age, smoking, income, education, marital status, BMI, physical activity, and preexisting disease.

By 2011, 2,545 of the 29,518 women had died. The dose-response curve was monotonic. Each additional sun-exposure behavior reduced all-cause mortality. Women in the highest exposure group had an adjusted hazard ratio of 0.66 compared to avoiders, meaning avoiders were roughly 50% more likely to die during the study period after controlling for everything measurable. The 2016 competing risk analysis revealed where the bodies were: cardiovascular mortality was more than double among sun avoiders compared to sun-seekers (subdistribution HR 2.2, 95% CI: 1.5–3.2). Non-cancer, non-CVD mortality showed a nearly identical pattern (HR 2.1, 95% CI: 1.5–2.9).

Then came the comparison that made headlines. Nonsmokers who avoided sun exposure had a life expectancy similar to smokers in the highest sun exposure group. Avoiding sun, the researchers concluded, "is a risk factor for death of a similar magnitude as smoking." The difference in life expectancy between avoiders and the most sun-exposed women: 0.6 to 2.1 years.

What's the Sun Actually Doing?

If you accept that sun exposure reduces cardiovascular mortality, you need a mechanism beyond vitamin D. Swedish winters deliver minimal UVB for half the year, and vitamin D supplementation trials have mostly disappointed for cardiovascular outcomes.

A 2014 study by Donald Liu and colleagues at the University of Edinburgh offered one. In 24 healthy volunteers, they exposed skin to UVA — the longer-wavelength UV that penetrates clouds, glass, and winter skies — and measured what happened in the bloodstream. Blood pressure dropped. Circulating nitrite rose. Confocal microscopy showed why: the upper layers of human skin contain large stores of nitric oxide bound in photolabile forms. UVA cracks these molecular vaults open, releasing nitric oxide into the circulation, where it dilates blood vessels and lowers blood pressure independently of vitamin D and independently of the body's standard nitric oxide synthase enzymes.

Cardiovascular disease kills roughly a hundred times more people than skin cancer worldwide. "We are concerned," Liu's team wrote, "that well-meaning advice to reduce the comparatively low numbers of deaths from skin cancer may inadvertently increase the risk of death from far higher prevalent CVD."

The Strongest Counterargument

The most damaging critique is one word: confounding. The MISS cohort is observational. You cannot randomly assign 30,000 women to avoid the sun for 20 years, so every observational study of sun exposure faces the same challenge. People who sunbathe differ from people who don't in ways that a regression model can only partly capture. Sun-seekers may be wealthier, more social, less depressed, more likely to exercise outdoors. Lindqvist adjusted for income, education, BMI, exercise, smoking, marital status, and comorbidity, but unmeasured confounders could still drive the association.

Dermatologist Antony Young of King's College London, responding to the study for the Science Media Centre, made precisely this point: "It is possible that the women in the study who had high sun exposure differed from the women who had low sun exposure in ways that may explain their reduced cancer risk." The objection applies to every cohort study of behavioral exposure. The question is whether the effect is large enough, consistent enough across adjustments, and backed by a plausible mechanism to resist it.

What We Didn't Prove

This study followed Swedish women only, a genetically and geographically specific population with low ambient UV for much of the year. The findings may not generalize to populations with year-round intense sun, darker skin types, or different melanoma incidence patterns. Sun exposure was self-reported at a single time point and never updated over the 20-year follow-up; women who changed their habits after enrollment would be misclassified. The study measured four binary sun behaviors and combined them into a simple score, capturing nothing about sunburn frequency, sunscreen use, or cumulative UV dose.

The study does not prove that deliberately increasing sun exposure will reduce your risk of dying. It shows that among Swedish women who naturally sought out the sun, mortality was lower. Conceptual replication exists: a UK Biobank analysis of approximately 377,000 white-skinned participants found higher UV exposure associated with lower all-cause mortality (Stevenson et al., 2024), and the US Adventist Health Study 2 found similar patterns for outdoor time and cardiovascular death. But no randomized trial has tested the causal question, and given the 20-year timescale, none is likely.

What You Can Do

Do not interpret this study as permission to burn. The relationship between UV exposure and skin cancer is well established, and melanoma kills roughly 60,000 people worldwide each year. What the evidence suggests is that total sun avoidance may carry its own cardiovascular risks. Moderate, regular exposure of 15 to 30 minutes of unprotected skin several times a week, short of burning, is what the dose-response data support. If you live at a northern latitude with limited winter sun, this finding may be especially relevant. Ask your doctor about your individual skin cancer risk before changing your behavior, but know that the blanket advice to always cover up may not reflect the full balance of evidence.