Determining an Effective UV Radiation Exposure Time for Vitamin D Synthesis in the Skin Without Risk to Health: Simplified Estimations from UV Observations.

UV radiation contains erythemally weighted UV, as well as UV that synthesizes vitamin D3 . Here, we attempted to determine the relationship between these factors by numerical simulation of atmospheric parameters, such as total ozone, using a simplified "SMART2" model for radiative transfer. Both forms of UV were almost linearly correlated with each other for a comparably large UV radiation exposure, larger than UV Index ~1.6. If erythemally weighted UV, which carries a risk of sunburn, is known, the amount of UV exposure needed for vitamin D synthesis in the epidermis can be estimated using this relationship. The production of 10 µg (400 IU) of vitamin D per day takes approximately 1/3 of the time needed to reach the minimal erythemal dose (MED) for an effective skin area of 600 cm2 for skin phototype III. For an area of 1200 cm2 , 1/6 of that exposure time suffices. From a UV Index that is commonly used, the risks and benefits can be evaluated using this linear relationship, which will enable people to effectively manage their UV exposure and consider the risks and benefits to optimize health outcomes.

The solar exposure time required for vitamin D3 synthesis in the human body estimated by numerical simulation and observation in Japan.

Although the importance of solar radiation for vitamin D3 synthesis in the human body is well known, the solar exposure time required to prevent vitamin D deficiency has not been determined in Japan. This study attempted to identify the time of solar exposure required for vitamin D3 synthesis in the body by season, time of day, and geographic location (Sapporo, Tsukuba, and Naha) using both numerical simulations and observations. According to the numerical simulation for Tsukuba at noon in July under a cloudless sky, 3.5 min of solar exposure are required to produce 5.5 µg vitamin D3 per 600 cm2 skin corresponding to the area of a face and the back of a pair of hands without ingestion from foods. In contrast, it took 76.4 min to produce the same quantity of vitamin D3 at Sapporo in December, at noon under a cloudless sky. The necessary exposure time varied considerably with the time of the day. For Tsukuba at noon in December, 22.4 min were required, but 106.0 min were required at 09:00 and 271.3 min were required at 15:00 for the same meteorological conditions. Naha receives high levels of ultraviolet radiation allowing vitamin D3 synthesis almost throughout the year.