Short sessions of UVB or UVAB radiation with the same vitamin D-weighted exposure increased 25(OH)D levels. The UVA dose does not influence 25(OH)D levels under short exposure times. However, there was a significantly lower increase of 25(OH)D levels during longer UVA irradiation (≥9 min).

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Conflicts of interest:None declared.Our study (1) investigated reported UVA photodegradation effects (2, 3). We exposed three groups of subjects to mainly UVB ('UVB', n = 23), UVB + UVA ('UVAB', n = 23) or, as in sunbeds, mainly UVA ('UVA', n = 10). Exposure times were calculated from measurements of the exposure cabin's spectral irradiance convoluted with the CIE previtamin D3 action spectrum to give the same vitamin-D-weighted radiation dose (≤ 1 standard erythema dose, SED). A fourth group (controls, n = 19) was unexposed.Our responses to Norval and de Gruijl's numbered comments are below. 1. Mary Norval and Frank de Gruijl argue that uncertainty may be the reason for our lack of evidence of a correlation between change in 25(OH)D levels and skin type. We observed a correlation between skin type and baseline 25(OH)D but noted that skin type does not seem to matter for the change in 25(OH)D levels during very short UV exposure, as also observed by others before us. Concerning over-representation of skin types V and VI in our comparatively small 'UVA' group, we noted and showed in Table 4 that 'a subset consisting of the skin types II and III is congruent with the overall result'. 2. Norval and de Gruijl complain that none of our UV sources (designated 'UVB', 'UVAB' and 'UVA') emitted solely UVB or UVA wavebands. We investigated the impact on 25(OH)D of the same spectrally CIE-weighted vitamin-D-effective UV exposures but with different UVA content, that is, with unweighted UVA doses (320-400 nm) 10 and 100 times as large as in our 'UVB' exposures. The small difference in exposure time between our 'UVB' and 'UVAB' exposures is due to the low factors of spectral weighting in the UVA region and a long wavelength end point at 330 nm for the CIE vitamin D3 action spectrum. Exposure time of the 'UVAB' cabin was settable in seconds and was calculated from spectral irradiance measurements during steady-state conditions about 3 min after start, then repeated after another 2-3 min with similar results. Several more measurements had also been made on other occasions, again with similar results. The relative spectral distribution of low-pressure fluorescent tube lamps do not significantly change during 'warm-up'. However, total irradiance level vs. time does have a small but wide temperature-dependent peak. Therefore, the cabins were 'warmed up' for 3 min before the first subject was exposed. Thereafter, each exposure followed upon a previous one, delayed only by change of subjects and restart of the cabin. Concerning the single-grating spectroradiometer being used (SolaHazard), it was not a spectrally scanning type of instrument. The UV sensor of the SolaHazard instrument is an array of CCD diodes simultaneously accumulating radiation in the whole spectrum (250-400 nm) during a sample time chosen to be as long as possible (i.e. 20 s) without risk of saturating the part of the array that records the highest intensity of the radiation (usually the UVA peak). Therefore, the part of the array measuring the lower range of the ...

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