Comparing realistic values of UV-B among outdoor recreational spaces in autumn in Hong Kong.

Living organisms, including human beings, spending time outdoors may be harmed by ultraviolet radiation. This research aimed to quantify the effects of vegetation, weather, and their interaction on UV-B irradiances (280-315 nm) in outdoor recreational spaces. UV-B irradiances in a playground shaded by trees and a roof garden in Hong Kong were monitored from 9 October 2020 to 3 November 2020. A turf was used as a control site. UV-B values during the after-school hours (12:00-13:00 h and 15:30-16:30 h) of a kindergarten in sunny and cloudy weather were analysed using mixed-effects models. The results showed high UV-B levels on the control turf (3.23 W/m²) and the roof garden (3.34 W/m²) on sunny midday but significantly lower in the playground (2.41 W/m²). Cloudy weather significantly decreased UV-B irradiances by 40%. The sparse and thin tree crowns in the playground implied equivalent UV-B protection factor values at 1.2-1.4, corresponding to a reduction in UV-B by 17-29%. Behaviourally, playtime could be postponed from midday to afternoon and shifted from sunny to cloudy days to avoid excessive UV-B exposure. This research highlighted the necessity of other sun protection measures despite protection from the tree canopy.