I read in 快猫短视频 that exposure to the sun in winter at latitudes higher than about 35掳 results in negligible vitamin D production (9 August 2014, p 34). For many years here in Tasmania (40掳 south) I have been exercising naked in the sun just after midday to dose up on vitamin D. But have I been wasting my time in winter?
鈥� Contrary to what many experts report, anyone exercising naked at midday during the Tasmanian winter may well be receiving enough ultraviolet radiation to produce adequate vitamin D. Only ultraviolet-B (UVB) radiation is used for the production of vitamin D, and at low winter sun angles above a latitude of 35掳, it is possible for UVB to penetrate the atmosphere.
鈥淎nyone exercising naked at midday in Tasmania may receive enough UV to make adequate vitamin D鈥�
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I live in Oregon (45掳 north) and have been measuring ultraviolet radiation for several years. During the winter months, a clear day can have as much UVB radiation as 100 microwatts per square centimetre at sea level.
This is between 25 and 30 per cent of the UVB we receive in midsummer and about 20 per cent of the levels seen in the tropics. For a person with fair skin, adequate vitamin D is produced by 15 minutes of summer exposure, or an hour during midwinter. Those with more melanin in their skin need more sun exposure.
However, the questioner should make sure he continues his exposure at midday because UVB levels drop off within two hours on either side of the highest sun angle.
Of course, cloud cover, rain and snow all severely reduce or eliminate UVB 鈥� but then, they also make it hard to exercise naked.
Stephen Johnson, Eugene, Oregon, US
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This article appeared in print under the headline 鈥淪unshine down under鈥�