Comparison of the spectral biologically effective solar ultraviolet in adjacent tree shade and sun

The solar spectral UVR irradiances in tree shade and sunlight have been mea sured in a sub-tropical southern hemisphere summer. The spectral data allow ed the UVB and UVA irradiances and the biologically effective irradiances t o be calculated for different harmful biological processes to human skin an d eyes. The average of the ratio of the UVA to UVB irradiances was lower by 26% in the shade compared with the same ratio in the sun. The spectral sha de ratio calculated as the ratio of the spectral biologically effective irr adiances in the shade to those in the adjacent sun decreased with increasin g wavelength for all of thr trees. The decrease in the shade ratio was appr oximately 42%at 400 nm compared with the shade ratio at 300 nm. Despite the UVR protection provided by tree shade, the erythemal UVR exposure received in 1 h in the tree shade exceeded the occupational limit for UVR exposure.