Action Spectrum Conversion Factors that Change Erythemally Weighted to Previtamin D 3 -weighted UV Doses † Stanley J. Pope 1 , Michael F. Holick 2 , Steven Mackin 3 and Dianne E. Godar 4 * 1 Sun Systems & Svc, Inc., Oak Park, MI 2 Boston School of Medicine, Boston, MA 3 Solartech, Inc., Harrison Township, MI 4 U.S. Food and Drug Administration, Center for Devices and Radiological Health, Silver Spring, MD Received 2 July 2006, accepted 28 March 2008, DOI: 10.1111 ⁄ j.1751-1097.2008.00373.x ABSTRACT 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 contribu- tion 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° from 30°N to 60°N, based on ozone values. These ASCF, along with geometry conversion factors and other information, can give better vitamin D 3 estimates from erythe- mally weighted outdoor doses. INTRODUCTION Solar terrestrial UV radiation (290–400 nm) affects human health in both detrimental and beneficial ways. Sunburn (1) is among the detrimental health effects (2,3), while vitamin D 3 production (4) is among the beneficial health effects (5–7). The erythemal action spectrum can estimate the risk of getting a sunburn (8); however, it cannot correctly estimate the benefit for making vitamin D 3 . Erythemally weighted terrestrial UV doses are available worldwide from calibrated R-B meters and Brewer spectrophotometers. Erythemally weighted personal UV doses are readily available from calibrated polysulphone badges and minimum erythemal dose (MED) meters. Most outdoor UV measurements are weighted by the erythemal action spectrum so that action spectrum conversion factors (ASCF) are needed to convert those doses to previtamin D 3 - weighted doses (9) in order to do accurate benefit assessments. Because most benefit assessments for vitamin D 3 production from solar UV exposures are based on erythemally weighted data, they are incorrect overestimates (10). These estimates also do not account for human body geometry, which can decrease the amount by 50% or more, because the UV doses used in the calculations are relative to the horizontal plane. Furthermore, most estimates also do not account for the declining ability to make vitamin D 3 with age (11), which can reduce the amount by 50% or more after the age of 60. Thus, the estimates for vitamin D 3 production currently available are much higher than what people actually make from casual outdoor UV exposures. Although both action spectra peak in the UVB region, the previtamin D 3 action spectrum stops near the UVA boundary (9) while the erythemal action spectrum continues throughout the UVA region to 400 nm (8). If one uses the previtamin D 3 action spectrum to weight the solar UV spectra, one finds it contributes a different amount toward vitamin D 3 production than what the erythemally weighted UV doses predict. Thus, to do proper benefit assessments for making vitamin D 3 , the difference between the contributions of these action spectra must be accounted for by using ASCF that change erythemally weighted UV doses to previtamin D 3 -weighted UV doses. Simply weighting solar spectra by the previtamin D 3 action spectrum to get the effective irradiance (eff) will not render useable data because the original studies used MEDs produced from tanning lamp exposure to measure the amount of circulating 25-hydroxyvitamin D 3 produced in humans. Thus, those human studies actually weighted the tanning lamp’s spectral output by the erythemal action spectrum. To get accurate ASCF for solar erythemal UV doses, one must normalize the ratios between the effective irradiances of previtamin D 3 and erythema by the ratio of the effective irradiance delivered by the tanning lamp’s spectral emission that was used to get a given amount of that biologic effect †The opinions and ⁄ or conclusions expressed are solely those of the authors and in no way imply a policy or position of the Food and Drug Administration. *Corresponding author email: dianne.godar@fda.hhs.gov (Dianne E. Godar) Ó 2008 U.S. Government. The American Society of Photobiology 0031-8655/08 Photochemistry and Photobiology, 2008, 84: 1277–1283 1277