We’ve demonstrated that despite its popularity as a heat stress control, resting in shade does not reverse the elevated core temperatures of heat-exposed workers in a timely manner (Brearley et al., 2023). As a result, scheduled breaks may need to be extended and/or utilised more frequently, which may not be commercially viable. More effective cooling strategies during rest periods are an obvious solution here, but what about shade as a heat stress control during work?
Figure 1. Basic thermal imagery analysis of ground surface temperature of shaded and non-shaded areas on a clear day at ~38ºC
It’s important to consider the differences between working while exposed to sunlight (solar radiation) compared to shade. Solar radiation is comprised of ~50% infrared rays that provide heat, ~42% visible rays that provide light and the remaining ~8% are ultraviolet (UV) rays known for their causal link to skin cancer. Blocking solar radiation provides protection from heat, light and UV rays, and is commonly achieved with relatively inexpensive shade sails/cloth. Typical shade sails block 80-95% of solar radiation. A simple method of visualising the impact of a shade sail (or similar) is to measure surface temperature in shaded and proximal non-shaded areas of a work site. Figure 1 demonstrates a ~20-25ºC surface temperature difference between the shade provided by a typical shade sail and non-shaded areas on a clear and dry ~38ºC day.
Table 1. Classification system for human shade protection provided by knitted and woven shade fabrics as in Table 3.1 of the Australian Standard AS 4174:2018 Knitted and woven shade fabrics
Australian standard AS 4174:2018 details the classification of shade sails/cloth solely based upon blocking of UV radiation, as per Table 1. Despite the lack of standardised reporting for infrared or light blocking, it’s generally assumed that as UV blocking increases, so too does infrared and light blocking. In our experience, ~340gsm shade sail fabric produces ~95% UV block but this may vary with shade sail quality and colour.
Figure 2. Time to exhaustion while exercising at 70% maximum in 30ºC/50% relative humidity with various solar radiation loads
Is shade worth the effort? Shade sails or equivalent can introduce a work site hazard during storms and high wind periods and flammability of the fabric may be a hazard for some sites. But in general, we consider shade a low cost - high return control. We’ve reported on the impacts of solar radiation on physical work performance previously (Figure 2), noting the steep decrease in time to exhaustion as the solar radiation load increases (Otani et al., 2016). Clearly, working in shade is associated with delayed fatigue at a set workload. In our opinion, establishing shade for work sites is worthy of consideration to limit both the absorbance of heat by the work environment, and by the workers within that environment.
References
Brearley M, Berry R, Hunt AP, Pope R. A systematic review of post-work core temperature cooling rates conferred by passive rest. Biology. 2023;12(5):695
Otani H, Kaya M, Tamaki A, Watson P, Maughan RJ. Effects of solar radiation on endurance exercise capacity in a hot environment. European journal of applied physiology. 2016;116:769-79