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

The short wavelength cut-off (λ_c), the wavelength of the maximum spectral UV (λ_Max) of spectral pre-vitamin D₃ effective solar UV irradiance (UV_D3), and the spectral erythemal UV (UV_Ery) were compared at 5-min intervals over a 6-month period at solar zenith angles (SZA) ranging from 4.7° to 80°. Averaged over the entire period, λ_c for UV_D3 is higher by 1.05 nm than that for UV_Ery. The λ_Max is higher for UV_D3 compared to UV_Ery for SZA < ~50°. For higher SZA (>55°), the ratio of λ_Max for UV_D3 to that for UV_Ery is less than 1. As the erythemal action spectrum extends into the UVA, the ratio of UV_D3 to UV_Ery irradiances decreases with increasing SZA, along with a decrease in the ratio of λ_Max for UV_D3 compared to UV_Ery. The changes in λ_c and λ_Max influence both personal UV_D3 and UV_Ery 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.