Spending more time outdoors in the sun is associated with a higher risk of some diseases and a lower risk of others, according to one of the most comprehensive analyses of sunlight and human health ever conducted. Using data from more than 281,000 participants in the UK Biobank, researchers found that the duration of outdoor sunlight exposure was significantly associated with 36 different diseases after rigorous statistical correction, with effects running in both directions. Each additional hour of average daily outdoor exposure was linked to an increased risk of malignant melanoma of the skin, obesity, cerebrovascular diseases, and osteoarthritis, but also to a decreased risk of osteoporosis, disorders of mineral metabolism, and pressure ulcers.

The study, published in Frontiers in Public Health, is the first outcome-wide cohort investigation to examine sunlight exposure across 134 different diseases spanning 14 major chapters of the ICD-10 classification system. Rather than focusing on a single organ, as most previous research has done, the team led by Sicheng Li of Xiamen Cardiovascular Hospital of Xiamen University aggregated 525 three-character ICD-10 codes into 134 disease categories, each with at least 500 incident cases to ensure adequate statistical power. Over a median follow-up of more than 13.4 years, the participants experienced 1,115,346 incident disease cases, and 223,953 individuals developed at least one of the conditions under study.

The exposure measure was deliberately simple. Participants reported in a baseline questionnaire how many hours they typically spent outdoors on a summer day and on a winter day, and the researchers calculated average daily exposure as the arithmetic mean of the two. The team emphasized that this measure summarizes habitual outdoor duration across contrasting seasons rather than estimating cumulative annual solar radiation. Participants reporting exposure exceeding typical UK day length, extreme values more than two standard deviations above the mean, or missing data were excluded, leaving 281,282 adults with a mean age of 55.81 years, of whom 55.79 percent were female.

Cox proportional hazards models estimated the associations while adjusting for a wide set of covariates, including age, sex, education, employment, skin color, deprivation, location, body mass index, physical activity, sleep, diet, smoking, alcohol, sun protection habits, vitamin D supplementation, and family history of hypertension and diabetes. Restricted cubic splines tested whether the relationships were linear. To guard against spurious findings across hundreds of tests, the researchers applied Benjamini-Hochberg false discovery rate correction, and hazard ratios for the 36 significant associations ranged narrowly from 0.914 to 1.086 per hour of exposure.

The specific numbers illustrate the modest but consistent effect sizes. Each additional hour of average exposure carried a hazard ratio of 1.052 for malignant melanoma of the skin, 1.039 for obesity, 1.028 for cerebrovascular diseases, and 1.042 for osteoarthritis. In the opposite direction, the hazard ratio was 0.975 for osteoporosis, 0.968 for disorders of mineral metabolism, and 0.947 for pressure ulcers. Sensitivity analyses, which included excluding diseases diagnosed within the first two years of follow-up, multiple imputation of missing data, and further adjustment for air pollution, greenspace, noise, self-rated health, medication use, and survey year, produced consistent results in direction, magnitude, and significance.

Perhaps the most striking finding was the prevalence of nonlinearity. Thirty diseases, including depression, anxiety, sleep disorders, arrhythmias, iron deficiency anemia, and kidney failure, showed U-shaped associations, with risk lowest at moderate exposure and rising at both extremes. When the researchers classified all 402 exposure-disease combinations by shape, summer sunlight exposure fell into the increasing category for 37 diseases, the decreasing category for 13, and the U-shaped category for 13, while winter exposure showed more U-shaped patterns, with 27 diseases in that class. The authors suggest that lower winter sun intensity may require longer exposure to reach the same total dose, which could explain why turning points are visible in winter data but may be obscured by the one-hour resolution of self-reported summer exposure.

The associations were not uniform across the population. Interaction analyses revealed effect modification by age, sex, location, and body mass index. The positive association between summer exposure and the aggregated increased-risk disease group was stronger among people under 60, men, and individuals with a BMI below 30, while the inverse association with the decreased-risk group was stronger among women and residents of Wales. For winter exposure, the positive association was stronger among older participants, men, and people in Wales, whereas the protective association was stronger among younger participants, women, and Welsh residents. The authors note that Wales receives higher-intensity sunlight in some comparisons, supporting their interpretation in terms of total exposure amount.

Exploratory mediation analyses pointed to possible biological pathways, though the researchers caution these are statistical decompositions rather than proof of causation, because exposure and mediators were measured at or around the same baseline time. For the aggregated increased-risk group, body mass index accounted for 18.8 percent of the association with summer exposure, biological age for 3.9 percent, neutrophil count for 2.2 percent, and vitamin D concentration for 4.3 percent. For the decreased-risk group, physical activity accounted for 27.6 percent, vitamin D for 7.5 percent, and cholesterol for 2.6 percent. Participants with longer exposure had higher levels of aging and inflammatory markers, higher vitamin D, and higher physical activity, painting a picture of multiple competing mechanisms.

The findings both confirm and complicate earlier research. The elevated melanoma risk aligns with established evidence that ultraviolet radiation damages DNA and triggers inflammatory signaling in the skin, while the reduced osteoporosis risk fits vitamin D’s well-known role in mineral metabolism. But the study diverged from some prior work: cataract risk was not significant, possibly because the exposure measure captured duration rather than specific wavelengths or cataract subtypes, and sunlight exposure was associated with higher rather than lower hypertension risk, in contrast to cross-sectional studies of hemodialysis patients. The authors also note that recent reviews of randomized trials and Mendelian randomization studies do not support vitamin D supplementation for reducing diabetes, cancer, or cardiovascular disease, suggesting mechanisms beyond vitamin D, including photoaging-related systemic inflammation, ultraviolet effects on appetite and fat distribution observed in animal studies, and the emerging skin-gut axis linking sunlight to microbiome changes associated with obesity.

The authors are careful about what their results imply. They describe roughly one hour of summer exposure and one to two hours of winter exposure per day as preliminary population-level reference ranges suggested by the observed curves, not definitive recommendations, and they stress that the observational design, modest effect sizes, residual confounding, self-reported exposure, and a predominantly fair-skinned UK population limit generalizability. Complete-case analysis also excluded a substantial number of participants, and baseline characteristics differed between included and excluded individuals, though multiple imputation yielded similar results. The team concludes that outdoor sunlight exposure shows positive, inverse, and nonlinear associations with a broad range of diseases, that moderate exposure combined with appropriate sun protection deserves attention at the population level, and that validation in other populations is needed before specific exposure recommendations can be established.

Subject of Research: Associations between outdoor sunlight exposure duration and the risk of 134 diseases in the UK Biobank

Article Title: Associations between the duration of outdoor sunlight exposure and the risk of developing 134 different diseases

Article References: Li, S., Zhou, H., Xian, W., & Wang, Y. (2026). Associations between the duration of outdoor sunlight exposure and the risk of developing 134 different diseases. Frontiers in Public Health, 14, Article 1896657. https://doi.org/10.3389/fpubh.2026.1896657

Image Credits: AI Generated

DOI: 10.3389/fpubh.2026.1896657

Keywords: sunlight exposure, UK Biobank, cohort study, melanoma, vitamin D, osteoporosis, obesity, U-shaped association, epidemiology, environmental health, public health, inflammation