Photochemistry and Photobiology, 2002, 7544): 369-376 Estimation of Pedestrian Level UV Exposure Under Treesn

INTRODUCTION Solar ultraviolet radiation (UV) at the earth's surface has Trees influence the amount of solar UV radiation that many implications for human health. Erythema is caused prireaches pedestrians. A three-dimensional model was demarily by the ultraviolet-B (UV-B) portion of the solar spec"eloped to predict the ultraviolet-B (UV-B) irradiance trum (280-315 nm) and to a lesser extent by ultraviolet-A fields in open-tree canopies where Qhe spacing between (UV-A) (315-400 nm). There appear to be complex relatrees is equal to or greater than the width of individual tionships between skin cancers and sun exposure (1). Nontree crowns. The model predicted the relative irradiance melanoma skin cancers are thought to be correlated with (fraction of above-canopy irradiance) under both sunlit cumulative lifetime UV exposure (2). Though medical opinand shaded conditions under clear skies with a mean bias ions and epidemiological studies vary as to the relationship error of less than 0.01 and a root mean square error of between sun exposure and cutaneous malignant melanoma 0.07. Both model and measurements showed that the 10(3), the statement that "it is now well established that sun cations people typically perceive as shady, low-irradiance exposure is one of the probable causes" is genemlly acloeations in the environment can actually have significant cepted by the research community (4). ~ h e s e health effects UV-B exposure (4040% of that under direct sunlight). may be exacerbated by depletion of the stratospheric ozone The relationship of tree cover in residential neighborlayer and the attendant increases in W radiation (5,6). Alhoods to erythema1 UV-B exposure for children and though changes in habits of recreation and dress, and the adults was modeled for the 4 h around noon in June and increased value placed on a "tan" are evidently largely to blame for most of the increases in skin cancer rates (7), these July. Results showed that human exposures (on the horrates are expected to increase by approximately 2% for every izontal) in cities located at 15 and 30" latitudes are nearly persistent 1% loss in the average ozone concentration (5). identical. For latitudes between 15 and 60°, ultraviolet The human health impact of solar UV at the eanh,s surproteenon (UPF) were less than lor less than face depends on the environmental conditions as well as on 50% tree cover. A UPF of 10 was possible at all latitudes human habits. M~~~ people do not play out their lives in for tree cover of 90%. open fields but reside and work in environments in which there is significant obstruction to the sky and the direct sun by buildings and vegetation. Significant improvemena in the posted on the web site on ~ a n u a i 28,2002. prediction of open-environment exposure of surfaces at varL T ~ ~ ~ , " m y ~ ~ ~ ~ ~ ~ ~ ~ . ~ ~ ~ i d f y ~ ~ ~ a ~ , " ~ $ ious slopes have been made in the past 10 years (8-10). 47907-1 150, USA. F=: 765-496-2926; omail: rgran!Wue. . Previous studies have shown that the-relative irradiance (I,) adu at pedestrian heights is significantly influenced by the can~bbr-iations: G, foliage pjcction a m Iabave-opy direct opies of vegetation and buildings (1 1-13). The influence of bum odiation; 'dh h v e ~ O P y diffuSs dat ion; minhU'l'l smctures on irradiance in 0 p e n e cmopies means measured below-canopy irradiance; 1, modeled beldw-canopy rel.tivs imdiance; I, &red bclow-sanopy =lative irradiance; la that the canopy structure influences the W exposure of peoabove-canopy imdiance; M, tree cover fnnion; MBE. mean bias ple. The mean tree cover of medium-size cities. inTexas, crrdr, MED, minimum erythema1 dose, N. sky radiance distribuOhio, California, Kansas, Alabama, New Yo&, and Penntion; Pu pWility direct beam below sylvania ranges from less than 10% to 37% (14,lS). Because ca-y; Po', probability of unobstructed d i h radiation below PAR photosynthetically active radiation; RMSE, mot the climatic region, population density and socioeconomic mean squared error, S, distance through the canopy; SPF, . characteristics of the population influence the t m canopy protection factor; SW, short-wave radiation; UPF, ultraviolet prostructure in urban areas, these factors also influence UV extection factor; UV, ultraviolet radiation; UV-A, ultraviolet-A raposure. ~~~t~~~ (14) suggested that cities with more humid diation; UV-B,'ul~violet-~' radiation; 3D, three-dimensional; 0, xnith angle; p, canopy vegetation density; 4, hmuth angle; a, subtropical climates t~l'ica'l~ have grater *e canopy cover direction of radiation. than those with northern continental climates, and cities with Q 2002 American Society for Photobiology 0031-8655102 S5.00+0.00 drierclimates typically have less tree cover than those with