The effects of sunlight exposure on mortality: a systematic review of epidemiological studies

Introduction:
Current sun safety advice focuses on minimizing exposure to sunlight, due to the relationship between ultraviolet radiation and skin cancer. However, sunlight also has beneficial effects, and there are calls for guidance to reflect these alongside the harmful effects. To examine the net effect of harmful and beneficial aspects, we aimed to determine the association between sunlight exposure and all-cause mortality. Additionally, we examined cause-specific mortality and whether the associations varied according to skin type/colour or ethnicity.

Methods:
We conducted a systematic review, searching MEDLINE, Embase, Web of Science and the Cochrane Central Register of Controlled Trials (Nov 2023) for reports of epidemiological studies in the general population investigating the effect of long-term sun exposure on all-cause, cardiovascular-related, or cancer-related mortality. We conducted a narrative synthesis of the findings and assessed risk of bias using the ROBINS-E tool. PROSPERO: CRD42023474157.

Results:
The search identified 73 eligible articles, with 55 included in the narrative synthesis. Methods of measuring sunlight exposure comprised radiation, proxy measures of radiation (e.g., latitude) and behaviour associated with sunlight exposure. The evidence was mixed. While most studies of skin cancer mortality found a higher risk associated with more exposure to sunlight, many studies of other cancers reported lower associated risk. Evidence for all-cause mortality was mixed, as were findings for cardiovascular mortality. Results were subject to high risk of bias, largely due to the likelihood of uncontrolled confounding and the use of indirect measures of sunlight exposure. There were insufficient data regarding any differential effects of sunlight on mortality for those of different skin types/colours or ethnicity.

Conclusion:
Findings from observational epidemiological studies of the association between sunlight exposure and mortality vary across different disease outcome and location being investigated. As such, the findings do not provide a strong rationale for changes to sun protection guidance.