Background: Sunlight contains ultraviolet B (UVB) radiation (290–315 nm) that affects human health in both detrimental (skin cancers) and beneficial (vitamin D3) ways. Serum 25-hydroxyvitamin D concentrations from young Americans (≤ 19 years) show that many have deficient (< 50 nmol/L, 20 ng/mL) or insufficient (< 75 nmol/L, 30 ng/mL) vitamin D levels, indicating that they are not getting enough sun exposure. Those findings are in conflict with some calculated, published values that suggest people make “ample” vitamin D3 (~ 1,000 IU/day) from their “casual,” or everyday, outdoor exposures even if they diligently use sunscreens with sun protection factor (SPF) 15.Objective: We estimated how much vitamin D3 young Americans (n = ~ 2,000) produce from their everyday outdoor ultraviolet doses in the North (45°N) and South (35°N) each season of the year with and without vacationing.Methods: For these vitamin D3 calculations, we used geometric conversion factors that change planar to whole-body doses, which previous calculations did not incorporate.Results: Our estimates suggest that American children may not be getting adequate outdoor UVB exposures to satisfy their vitamin D3 needs all year, except some Caucasians during the summer if they do not diligently wear sunscreens except during beach vacations.Conclusion: These estimates suggest that most American children may not be going outside enough to meet their minimal (~ 600 IU/day) or optimal (≥ 1,200 IU/day) vitamin D requirements.

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Most solar UV measurements are relative to the horizontal plane. However, problems arise when one uses those UV measurements to perform risk or benefit assessments because they do not yield the actual doses people get while they are outdoors. To better estimate the UV doses people actually get while outdoors, scientists need geometric conversion factors (GCF) that change horizontal plane irradiances to average irradiances on the human body. Here we describe a simple geometric method that changes unweighted, erythemally weighted and previtamin D(3)-weighted UV irradiances on the horizontal plane to full cylinder and semicylinder irradiances. Scientists can use the full cylinder model to represent the complete human body, while they can use the semicylinder model to represent the face, shoulders, tops of hands and feet. We present daily, monthly and seasonally calculated averages of the GCF for these cylinder models every 5 degrees from 20 to 70 degrees N so that scientists can now get realistic UV doses for people who are outdoors doing a variety of different activities. The GCF show that people actually get less than half their annual erythemally weighted, and consequently half their previtamin D(3)-weighted, UV doses relative to the horizontal plane. Thus, scientists can now perform realistic UV risk and benefit assessments.

Many solar UV measurements, either terrestrial or personal, weight the raw data by the erythemal action spectrum. However, a problem arises when one tries to estimate the benefit of vitamin D(3) production based on erythemally weighted outdoor doses, like those measured by calibrated R-B meters or polysulphone badges, because the differences between action spectra give dissimilar values. While both action spectra peak in the UVB region, the erythemal action spectrum continues throughout the UVA region while the previtamin D(3) action spectrum stops near that boundary. When one uses the previtamin D(3) action spectrum to weight the solar spectra (D(eff)), one gets a different contribution in W m(-2) than what the erythemally weighted data predicts (E(eff)). Thus, to do proper benefit assessments, one must incorporate action spectrum conversion factors (ASCF) into the calculations to change erythemally weighted to previtamin D(3)-weighted doses. To date, all benefit assessments for vitamin D(3) production in human skin from outdoor exposures are overestimates because they did not account for the different contributions of each action spectrum with changing solar zenith angle and ozone and they did not account for body geometry. Here we describe how to normalize the ratios of the effective irradiances (D(eff)/E(eff)) to get ASCF that change erythemally weighted to previtamin D(3)-weighted doses. We also give the ASCF for each season of the year in the northern hemisphere every 5 degrees from 30 degrees N to 60 degrees N, based on ozone values. These ASCF, along with geometry conversion factors and other information, can give better vitamin D(3) estimates from erythemally weighted outdoor doses.

A recent article by Lazovich et al. (1) reported a multivariate odds ratio (OR) for melanoma for ever use of indoor tanning of 1.74 (95% CI, 1.42-2.14). Another recent article reported slightly lower relative risk (RR) results for indoor tanning in Scandinavia (2). The OR reported by Lazovich et al. (1) is much higher than the IARC meta-analysis of 19 studies [overall RR, 1.15 (1.00-1.31); ref. 3]. Furthermore, when the five studies from the United Kingdom, where many have a genetically related high melanoma risk, are omitted, the RR drops to 1.04 (0.91-1.14; ref. 4). Why the OR found by Lazovich et al. (1) was very high compared with the other studies is puzzling. There are a number of well-known risk factors for melanoma, including skin pigmentation, hair color, moles, and painful sunburns (1, 2, 5). Currently, industry standards, recommendations, and training warn very fair-skinned persons (skin type I) against indoor tanning. Having very fair skin [OR, 5.50 (2.70-11.18); ref. 1] seemed to contribute 30% of the increased melanoma risk associated with indoor tanning, assuming equal distribution among categories. Likewise, red hair contributed 38% of the risk, and many moles, 27%. These categories seem to be largely independent (5); thus, when combined, they may explain nearly all of the increased risk found. In addition, the study failed to explicitly categorize sun sensitivity. Lazovich et al. (1) found for more than five sunburns lasting more than 1 day an OR for melanoma of 2.56 (1.67-3.93). Any significant overlap between the 734 melanoma cases who reported indoor tanning use and the 739 melanoma cases with such a history of sunburns may cause conclusions in the report linking indoor tanning with melanoma to be invalidated, as a history of severe sunburns is one of the strongest risk factors for melanoma (1, 2). Whereas the multivariate analyses should, in principle, account for the risk factors, a way that seems guaranteed to do so is to remove cases and controls associated with each high-risk factor and repeat the analysis, as well as study each factor separately with respect to indoor tanning. We would like to see these analyses as well as the distribution of high-risk factors with respect to use of indoor tanning. The lesson seems to be that those with preexisting high-risk factors for melanoma should be careful in using indoor tanning facilities and sun exposure.