UV radiation and urban shade trees: How much protection can they provide us?

Authors: Janani Sivarajah*, Faculty of Forestry, University of Toronto
Topics: Earth Science, Environmental Science, Urban Geography
Keywords: urban forestry, trees, shade, UV, health, landscape, canopy, ecosystem services
Session Type: Paper
Day: 4/3/2019
Start / End Time: 4:30 PM / 6:10 PM
Room: Congressional A, Omni, West
Presentation File: No File Uploaded

Trees provide natural shade and protect humans from solar ultraviolet radiation, and
have the potential to reduce UV exposure during childhood especially with the recent emphasis
on outdoor activity. Traditional shading structures reduce UV exposure by reflecting solar
radiation, however, unlike trees, they do not usually absorb or decrease UV radiation reaching
the ground. Little quantitative data exist on how tree shading affects penetration of solar
erythemal UV radiation (i.e. erythema or skin burning). To date, most studies have focused on
the measurement of UV-A radiation within Australian tree canopies, with few measuring UV-B
radiation. As part of a broader effort to understand the shade value of trees for human health, we
measured the penetration of erythema-causing UV radiation (220 to 320 nm) under common tree
species found in the temperate zone of Toronto. Solar UV radiation exposure data were collected
using electronic dosimeters under 14 tree species (n= 56) in urban schools and public parks. The
dosimeters were placed under each tree using PVC tubes at the same height as a small child. The
study was carried out daily during high exposure times. Results show differences among UV
penetration beneath the crowns of different tree species, and our model predicts crown height
and transparency are the driving factors affecting UV penetration. We highlight the importance of
urban trees for human health and provide information on choosing the best tree species for
UV protection. These results can help support landscape designers to increase canopy cover in
building strong outdoor environment

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