Introduction

The goal of this review is to summarize recent information on the effect of ultraviolet radiation (UV) on human health in urban areas, and the degree to which urban landscapes might be managed to optimize UV exposure for improved health. Considering both the negative and positive health effects of UV exposure, would urban planning to increase tree cover form an effective public health intervention? Can optimum tree arrangements and species selections be prescribed? More specific questions arise; for example, is it important that shade be provided for school play areas?

To propose answers to these questions, a number of more basic questions must be considered. What are the health impacts of UV on particular populations? What is the relationship between climate and the type of dress, which influences the amount of exposure individuals receive during time spent outdoors? How representative are the UV monitoring networks located within the city? What are the urban atmospheric influences on UV irradiance? Are there trends in UV that may change the importance and effectiveness of proposed shade structures during their term of use? If increased radiation shading is deemed desirable, what types are most effective (e.g., trees or constructed shelters)?

Even though the benefits of UV radiation in photosynthesizing vitamin D and also the consequences of vitamin D for bone health have been known for many years, an increased volume of epidemiological literature published since about 1980 suggests that vitamin D, including that photosynthesized in the body from sunlight, is beneficial for human health (Garland and Garland, 1980; Kimlin, 2004; Engelsen et al., 2005; Turnbull et al., 2005). Relative to recent reviews on urban forest influences on UV radiation (Heisler and Grant, 2000a, b), the apparent beneficial influences of UV radiation in reducing the risk for many cancers through vitamin D production (Garland et al., 2002; Grant, 2002b, c; Grant, 2003; Grant and Garland, 2006) are given greater emphasis in this paper.

Although most urban residents can acquire large doses of UV radiation by spending time in open areas near or within their city, most reports on the subject suggest that vegetation and structures within a city, along with the urban atmosphere, modify UV sufficiently over time to affect human health (Heisler and Grant, 2000a, b; Gies and Mackay, 2004). Tree cover differs from city to city, and can also vary within urban areas. In U.S. residential areas with low building density, trees dominate the environment (Nowak and Crane, 1998; Nowak et al., 2002). Tree cover in urban areas can vary with the climate, culture, local income, city infrastructure, and the tree-management programs carried out by local, state, and federal governments (Miller, 1998).

Other important recent changes that have been published include findings of a reduced downward trend in stratospheric ozone (Newchurch et al., 2003; Malanca et al., 2005; Weatherhead and Andersen, 2006). Most papers written during the last two decades that describe UV effects began with a statement regarding the importance of the research due to probable increases in UV-B (here taken to be radiation between 320 nm and 280 nm, unless specified as the alternate definition of 315 nm to 280 nm) radiation resulting from stratospheric ozone depletion. Although it seems too early to declare a reversal in ozone depletion, recent reports suggest that increased downward trends are unlikely.

The first issue to be considered in regard to the effects of UV radiation on health is the epidemiology of diseases.

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