Perhaps the characteristic that most caucasian skin-types, when placed in quality raw sunlight, are able to generate 5,000iu in 15 to 20 minutes. QRsunlight would be that obtained when the person’s shadow is well shorter than the height of the person. As I live near Austin, Texas, for MOST of the year I can go out anytime between 10am and 4pm, and my shadow will be less than 6’5″ long, so I can assume that UV-B wavelength is making it through the atmosphere sufficient for my exposed legs/trunk/back/arms to generate that amount of D. I do wear a hat to provide cosmetic protection for my face and chronically-overexposed redneck neck. 🙂 It’s November, so that 10-to-4 window is more like 11am to 1pm. And there will be days to come this winter where I can stay out for hours and not get the same UV-B 15 minutes would give in July. So taking 5,000iu daily (or more, frankly) has for years given me a blood level in that “mother nature” range of 50ng/ml. I know this through blood tests conducted several times a year over a number of years, personally. It’s just a generalized rule of thumb, in other words. Some folks’ genetics and lifestyles will require more, or less. But there does not appear (in the literature) to be ANY demonstrable HARM from taking what the body WILL make given “reasonable” sun exposure.