2014 年 88 巻 7 号 p. 349-357
In recent years, the importance of solar exposure for vitamin D synthesis in the human body has been pointed out. The solar exposure time necessary for the synthesis of vitamin D depends largely on geographical location, season, time of day, weather, exposed skin area, etc. Using numerical simulations we estimated that if 10 μg vitamin D were to be synthesized entirely by solar exposure to skin type III (SPT), which is considered to be the most typical skin type for Japanese people, it would necessitate 6.4 min horizontal exposure of a 600 cm^2 skin area, corresponding to the face and the back of both hands, under cloudless sky at 12:00 o'clock in July in Tsukuba. Under the same conditions, it would take 20.8 min to reach 1 MED (Minimum Erythemal Dose) which is thought to be the harmful UV exposure level for human skin. In other words, approximately 31% of the time before the skin gets red is enough for the synthesis of 10 μg vitamin D a day. On the other hand, in Sapporo which is located in the northern part of the Japanese Archipelago, the corresponding durations are 8.4 min and 27.0 min, respectively under the same conditions as in Tsukuba, whereas the necessary time in December would be 139 min and 296 min, respectively. Although the sufficient amount of vitamin D cannot be obtained by short-time exposure to solar radiation, it is thought that long-time exposure might not damage the skin. It can be concluded that for a skin area of 600 cm^2 in horizontal position, exposure time until damage would occur is generally 3 times larger than what is necessary for the synthesis of 10 μg vitamin D under strong UV radiation. It should be noted that generally, with a larger exposed skin area the solar exposure time for vitamin D synthesis could be considerably shortened.