Urban Forest Influences on Exposure to UV Radiation and Potential Consequences for Human Health

Abstract

This chapter explores the literature on ultraviolet (UV) irradiance in urban ecosystems with respect to the likely effects on human health. The focus was the question of whether the health effects of UV radiation should be included in the planning of landscape elements such as trees and shading structures, especially for high use pedestrian areas and school play grounds. Ultraviolet radiation can have a strong effect on humans, primarily as a cause or contributing factor for skin cancer and eye cataracts. It is also a probable factor in the development of immune deficiencies. However, UV can also positively affect human health, primarily because it is essential for photosynthesis of vitamin D in human skin. Vitamin D has long been recognized as a requirement for bone health. Recent epidemiological findings attribute vitamin D for the reduction of many types of non-skin cancers. Moreover, there is evidence that it may reduce mortality for those diagnosed with melanoma skin cancer. Alternatively, some public health authorities, particularly those from countries with a large population of northern European descent, strongly recommend that exposure to UV radiation should be minimized to prevent skin cancer. Other agencies, including the World Health Organization, that take a broader world view, differentiate their recommendations according to skin color. They recommend that humans with darker skin, who are likely deficient in vitamin D and also have little access to vitamin D fortified foods, should have moderate UV exposure. Judging from current knowledge of typical spectra of solar radiation in tree shade and the difference between the action spectra (effectiveness versus radiation wavelength) for vitamin D synthesis and that for sunburn in human skin, tree shade has advantages for moderate exposure to solar radiation. Where trees are separated, as is typical in heavily populated urban areas, the shortest wavelengths of UV radiation are scattered from the sky into shady locations with some sky view, and to a small extent, vitamin-D-promoting wavelengths are increased over the radiation spectrum that causes sunburn. The sunburn action spectrum includes longer wavelengths that are less readily scattered into direct beam shade. 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, warming could lead to greater exposure as people adapt to increased temperatures by wearing less clothing. However, empirical evidence for a temperature influence on human exposure independent of radiation climate does not seem to exist.