Playing on natural or artificial turf sports field? Assessing heat stress of children, young athletes, and adults in Hong Kong

Exercising in an unusually hot environment may aggravate exertional heat illness. Turf material significantly affects the microenvironment and heat-stress sensation of sports-field users. However, the difference in humanbiometeorological effects between different sports-field turf materials demands further investigation. This study compared artificial (AT) with natural turf (NT) fields, investigating three age groups (children, young athletes, and adults), two physical activities (playing soccer and walking), and three heat stress indicators (HI, Heat Index; WBGT, Wet Bulb Globe Thermometer; and COMFA, COMfort FormulA). The results showed heat-stress underestimation by HI and WBGT. In contrast, COMFA, incorporating comprehensive environmental and human physiological parameters, provided a more targeted and reliable heat-stress assessment. COMFA indicated a longer heat-stress duration exercising at AT than NT. Compared to NT, children suffered a 24% longer "Extreme danger" duration at AT in sunny daytime. The AT-NT difference in human-biometeorological effect was limited concerning human convection, evaporation, metabolic heat, and emitted longwave radiation, but was considerable in human absorbed radiation. AT had lower albedo than NT, hence field users absorbed more upward longwave radiation but less upward shortwave radiation, highlighting important control by the radiant environment. NT sports fields are recommended for a healthy outdoor thermal environment, especially for children.