Comparison of biologically effective spectra for erythema and pre-vitamin D3 synthesis.

The short wavelength cut-off (lambdac), the wavelength of the maximum spectral UV (lambdaMax) of spectral pre-vitamin D3 effective solar UV irradiance (UVD3), and the spectral erythemal UV (UVEry) were compared at 5-min intervals over a 6-month period at solar zenith angles (SZA) ranging from 4.7 degrees to 80 degrees. Averaged over the entire period, lambdac for UVD3 is higher by 1.05 nm than that for UVEry. The lambdaMax is higher for UVD3 compared to UVEry for SZA<approximately 50 degrees. For higher SZA (>55 degrees), the ratio of lambdaMax for UVD3 to that for UVEry is less than 1. As the erythemal action spectrum extends into the UVA, the ratio of UVD3 to UVEry irradiances decreases with increasing SZA, along with a decrease in the ratio of lambdaMax for UVD3 compared to UVEry. The changes in lambdac and lambdaMax influence both personal UVD3 and UVEry exposure and, to take this into account, a dual calibration technique for polysulphone dosimeters has been developed to simultaneously provide measurements of both types of exposure.