Across 704,808 annual cardiovascular deaths and 565,287 annual cancer deaths in US Health Service Areas, higher ambient ultraviolet exposure was associated with significantly lower mortality from both cardiovascular disease (β = −68.3 deaths/100,000 per IQR increase in UV; P<0.001) and all cancers (β = −43.2; P<0.001). Modelled attributable burdens translated to roughly 36,043 fewer cardiovascular deaths (5.1%) and 49,789 fewer cancer deaths (8.8%) annually. Ten of 18 site-specific cancers showed inverse UV associations in the US, with lung cancer showing the strongest inverse signal in both US and UK Biobank datasets (412,665 participants). Twelve of 18 cancers were directionally concordant across datasets.
Sunlight exposure has long been studied for its role in vitamin D synthesis, nitric oxide release from skin, and immunomodulation — all plausible pathways linking UV to cardiovascular and cancer outcomes. This large ecological analysis adds meaningful epidemiological weight to that literature, with the UK Biobank cohort providing individual-level partial corroboration. However, important caveats apply: this is an ecological study, meaning area-level UV cannot be assigned to individuals, and residual confounding is a serious concern — the authors themselves flag that racial heterogeneity did not follow an expected skin-colour gradient, signalling unaccounted confounders. Lung cancer's strong inverse association is particularly puzzling mechanistically and warrants scrutiny. As a preprint not yet peer-reviewed, these findings should be treated as hypothesis-generating rather than conclusive. Adults should not interpret this as grounds to increase unprotected sun exposure without weighing skin cancer risk.