Given the importance of vitamin D sufﬁciency for optimal health, and the fact that solar UVB irradiation is the primary source of vitamin D for most people, it is imperative that guidelines for solar UV exposure be revised in consideration of overall health, rather than only for reducing the risk of skin cancer and melanoma.
The amount of UVB irradiation required for vitamin D sufﬁciency can be calculated from the amount of vitamin D produced from one minimal erythemal dose (MED) – 10,000-25,000 IU of oral vitamin D.17 If 10,000 IU of vitamin D is produced from exposure of the full body to one MED, exposing the full body to 25 percent of the MED would produce 2,500 IU. In order to achieve 1,000 IU, 40 percent of the body should be exposed to 25 percent of the MED; if production is more efﬁcient, less of the body need be exposed.
For pale skin, the exposure time for one MED in the summer noonday sun in the southern United States is about 4-10 minutes; for dark skin, such as for African Americans, the corresponding time is 6080 minutes.17,63 Exposure times should be 25-50 percent of the MED. The length of time varies with geographical location, skin pigmentation, percent body fat, and age.
The best time of day for vitamin D production is near solar noon, when the ratio of UVB to UVA is highest. Typically, vitamin D3 can be produced from 10 a.m. to 3 p.m. during the spring, summer, and fall.17 Because UVB radiation occurs at shorter wavelengths than UVA, it experiences greater attenuation from atmospheric scatter than UVA. Also, UVB is absorbed by ozone. Thus, the exposure time required for a given level of vitamin D photoproduction is lowest near solar noon. In addition, basal cell carcinoma (BCC) and cutaneous malignant melanoma (CMM) are probably more susceptible to UVA irradiation than UVB irradiation,170-172 so that minimizing UVA rather than UVB exposure may be appropriate. For these two reasons, midday solar UV irradiation, short of erythema, will reduce the risk of both BCC and CMM. BCC and CMM are also linked more to intermittent UV exposure, such as during a vacation in a sunny location, than to occupational exposure, which seems to be protective.173-175 This protective effect of regular exposure may be via vitamin D production176 or perhaps through conditioning of the skin for higher UV radiation. BCC is the most common form of skin cancer for those with lightly pigmented skin, whereas CMM is the most deadly. On the other hand, actinic keratosis (AK) and squamous cell carcinoma (SCC) are more likely related to total lifetime UVB irradiation. SCC, although a rarer form of skin cancer, is more deadly than BCC and accounts for most non-melanoma skin cancer deaths in the United States. Thus, sunscreens, which have much greater protection against UVB than UVA radiation, appear to protect against AK and SCC but not BCC177 and CMM.178,179
In addition, indoor tanning using artiﬁcial lamps with a UV spectral output that mimics that of solar UV radiation reaching the Earth's surface near summertime noon at midlatitude (3-5% UVB, 95-97% UVA) can also be used to produce vitamin D.76 Lower fractions of UVB, such as 1.5 percent in France and Sweden, are associated with increased risk of mela-noma.180 However, those who do not tan easily should not use such lamps since they are less well protected against free radical formation. Higher fractions of UVB may be more beneficial, but research on this topic has not been conducted. The vitamin D-production potential of both the sun and artificial UVB sources can be determined by various means.181 A summary of the advantages and disadvantages of various sources of vitamin D is given in Table 3. While solar UVB is the natural way to obtain vitamin D for most people, other sources may be more convenient or have other health advantages. However, the disadvantages have to be weighed as well.