All three of the skin cancers, including the most deadly—melanoma, are related to sun exposure, and although action spectra are not clearly defined, the UV-B band is apparently the most responsible for all three cancers. Though use of sunscreens is widely recommended, there remains an uncertainty about the effectiveness of sunscreens in preventing cancer. The incidence rate of skin cancers has been rising rapidly, even though the use of sunscreens has become widespread. Recent studies muddy the picture about the effect of sun exposure on survival of melanoma victims.
The increasing evidence that adequate levels of vitamin D reduce the incidence of many non-cutaneous cancers suggests a role for tree cover in urban areas for pedestrians and for children's playgrounds as an intervention that reduces sun exposure for the erythemal action spectrum somewhat more than for the vitamin D response spectrum. This difference is caused by the greater scattering of the shortest-wavelength radiation and the apparent limit to the vitamin D action spectrum to wavelengths below 320 nm. Determining if this difference is of practical significance requires further study, especially because the interpretations of the vitamin D action spectrum that are currently in use are based on only one study published in 1982. Additional research on the effective spectra in human skin is underway and seems well-justified.
Methods of modeling UV radiation exposure to pedestrians in different urban neighborhoods are being developed. These methods should be verified by additional UV radiation spectral measurements (Webb, 1991) above and below canopy over extended time periods, and by the use of personal dosimeters with volunteers. The dosimeter studies should include school children and structurally different neighborhoods over a range of building structures and tree densities to evaluate human exposure during everyday activities.
Major modifications to the structure of urban areas, including residential neighborhoods, may be accomplished over time with policies for tree management—including tree species selection for planting. Selection for the eventual size and shape of crowns of planted trees, which is generally known, is more important than considerations of tree-crown leaf density, which is less well known.
Studies on above-canopy UV irradiance in urban versus rural areas are also needed. Such research is especially important because epidemiological studies have shown different disease incidences that may be related to differences in exposure to UV radiation caused by different tropospheric atmospheric constituents.
Below typical street trees at mid-latitudes, where the visible solar irradiance is 10% or 15% of irradiance above trees, UV-B relative irradiance is commonly 30% or 40%. For typical solar radiation spectra during mid-day at mid-latitudes in summer, estimates are that Caucasians with medium-skin color may acquire a daily recommended dose of vitamin D from the sun in one-fourth of the time sunburn would occur. Alternatively, when arms, hands, and face are exposed on sunny days, a standard vitamin D dose could be acquired in just 4 or 5 minutes when in the open, and 12 to 15 minutes when in typical tree shade. Dark-skinned individuals would require about 30 minutes in shade of scattered trees where the view of the sky was substantial. Thus tree shade tends to make possible the recommendations that some researchers make for moderate exposure to UV radiation. An advantage of obtaining UV exposure in tree shade is that the reduction of the longer wavelengths of solar radiation will often produce thermal comfort for people when it would be too hot in direct sun.
Global climate change, which is expected to increase temperatures, especially at higher latitudes, may have a variety of effects on UV exposure for human populations; for example, it might be expected that warming would lead to greater exposure as people adapt to the warmer temperatures by wearing less clothing. However, empirical evidence for the influence of temperature on human exposure, independent of radiation climate, does not seem to exist.
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