The overall impression left by medical literature is that UV radiation in urban areas does have important consequences for human health, but that because little is known about the influence of urban structure on UV exposure, the epidemiological effects of UV radiation in urban areas are not being fully evaluated. Thus, knowledge and methods to predict spatial and temporal distribution of UV would be very beneficial to epidemiology. Such knowledge could also benefit public education and urban planning.
There can be significant differences between reductions of the visible portion of the solar spectrum (that is, the shade pattern we see) and reductions of UV by trees and other structures in urban areas. Large differences may occur in relative irradiance (below/above urban canopy) between the visible and thermal radiation that people see and feel and the ultraviolet spectrum. The differences occur partly because visible and UV radiation differ in the diffuse fraction of total irradiance (Grant and Gao, 2003), in the distribution of sky radiance (Grant, 1985; Grant and Heisler, 1997; Grant et al., 1997a, b) in reflectivity of urban structural surfaces (Heisler and Grant, 2000a), and in optical properties of leaves at different wavelengths (Grant et al., 2003).
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