Objective To quantify the optimal exposure duration for the human body to synthesize recommended dose of vitamin D through sunlight ultraviolet (UV) radiation without inducing erythema in tree shade, and to analyze key influencing factors. It aims to provide scientific basis for health-oriented urban open space design and residents' outdoor activity scheduling.

Method Eleven typical urban tree species in Beijing were selected to measure UV radiation intensity in tree shade across 4 seasons and 5 time periods (08:00, 10:00, 12:00, 14:00 and 16:00). Optimal exposure durations were calculated based on the minimum erythema dose (MED) for Chinese skin types and the minimum vitamin D synthesis dose (400 IU). Correlation analysis was performed between UV radiation and forest structural characteristics to explore the key influencing factors. A multiple linear regression model was used to establish an exposure duration estimation model.

Result (1) The optimal time for exposure in forest could be 2-13 times longer than that in the open. Summer had the most striking time extension effect, with the duration extendable by more than 8 times; (2) Canopy cover and leaf area index (LAI) were the main structural indicators affecting erythemal UV radiation (UV_er) in the forest, thereby determining the duration of UV exposure during growing seasons. At a canopy cover of 0.4, exposure duration doubled compared to open areas; each 0.2 increase in canopy cover extended the duration by 50%; (3) A model (R² = 0.47) was developed to estimate the optimal exposure time in tree shade with the use of UV index (UVI) and canopy cover using experimental data.

Conclusion Urban tree canopies create a mild UV radiation environment which tends to meet the health needs of Chinese people for moderate exposure to UV radiation. By wisely creating shaded environments with trees, it is possible to achieve a synergistic regulation of UV radiation health benefits and risks, and prolong residents’ outdoor activity time significantly.