The Solar Exposure Time Required for Vitamin D3 Synthesis in the Human Body Estimated by Numerical Simulation and Observation in Japan

Fifteen cases of vitamin D-dependent rickets were reported in the national survey of calcium metabolism disorders of children by a study group of the Ministry of Health, Labour and Welfare, Government of Japan during 1971-1980 (1). The study investigated more than 300 medical facilities and 10 of the 15 cases were identified in the Hokkaido and Niigata districts in the northern part of Japan. Seven of the affected children were breast-fed and the study suggested that the condition was possibly caused by insufficient exposure to ultraviolet (UV) radiation in winter. Fujii et al. (2) investigated 34 cases of vitamin D deficiency in various parts of Japan for the period 1995-2005. It was found that 17 cases were speculated and 3 cases were suspected to be due to low solar exposure time (2). According to Yorifuji et al. (3) the incidence of craniotabes in Kyoto, Japan, was present at a high rate in neonates, and that of the birth month had obvious seasonal variations, highest in April-May and lowest in November, which correlates with daylight hours of the month. Ono et al. (4) reported that the mean serum 25-hydroxyvitamin D level of the month was lowest at the end of winter and highest at the end of summer. Corrective countermeasures for vitamin D deficiency include ingesting vitamin D from food, taking medication containing activated vitamin D, and increasing exposure to solar radiation. Among them, everyone can increase their exposure to solar radiation easily, although there is a limitation by location, season, and the time of day. After the discovery of ozone layer depletion, it became well known that excessive exposure to solar radiation can increase the risk of skin cancer and photo-aging (5) and therefore many people avoid exposure to solar radiation for health and cosmetic reasons. Erythemal UV is defined as the component of UV that reddens the human skin, and is known as UV Index to cause skin cancer, eye cataracts, etc. through excessive exposure. Infant sun baths were originally recommended in the Maternity and Child Health Handbook published by the Ministry of Health, Labour and Welfare, Government of Japan, but newer versions suggest avoiding the potential harm of UV exposure and replace the term "sun bath" with "air bath." Accurate information on UV exposure is important to prevent the adverse health effects of excessive UV exposure, but it is also important to receive the UV necessary to generate vitamin D 3 in the body (hereafter, UV for vitamin D3 synthesis). Even slight UV exposure produces adequate vitamin D3 in the body although both erythemal UV and UV for vitamin D3 synthesis are virtually identical on the UV spectrum. Therefore, UV for vitamin D3 synthesis can be equivalent to harmful erythemal UV, which raises questions of how much UV is effective for producing vitamin D3 and how much UV is detrimental to health (6).