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 measured in a sub-tropical southern hemisphere summer. The spectral data allowed the UVB and UVA irradiances and the biologically effective irradiances to be calculated for different harmful biological processes to human skin and 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 shade ratio calculated as the ratio of the spectral biologically effective irradiances in the shade to those in the adjacent sun decreased with increasing wavelength for all of the trees. The decrease in the shade ratio was approximately 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.