Solar Radiation

Weather is a regular discussion point irrespective of the conditions. Sure, certain weather events demand an extended discussion, but good, bad or indifferent, there's always something to discuss. Most weather discussions focus on ambient temperature as this variable is routinely reported and forecasted. For planning purposes, it's beneficial to know the weather we are likely to be exposed to but what does the forecast maximum temperature actually mean?

Figure 1. A standard and thermal image of a heat exposed worksite

Take the outdoor workers as an example in the images above. The forecast temperature for this workday (hot/dry summer climate) was 39ºC. By mid-afternoon, the Bureau of Meteorology station (~11km away) reported a temperature of 38.8ºC yet the local ambient temperature measured by a high quality calibrated weather station was substantially hotter. This local heating effect (a substantial 5ºC) was due to the high ground and surface temperatures, in turn, a product of solar radiation (Figure 1). Had the forecast stated 44ºC, it's highly likely that modifications to work schedules would have occurred. The point here is that solar radiation contributes to the thermal load, and therefore needs to be accounted for when planning work activities. Despite the omission of solar radiation from the majority of laboratory-based heat stress research, there's evidence of impact upon endurance, ability to perform complex tasks, recovery and heat stress symptoms.

Figure 2. Time to exhaustion while exercising at 70% maximum in 30ºC/50% relative humidity with various solar radiation loads 

Resting in the shade during rest breaks is a common approach as it expedites the reduction of core temperature compared to resting in the sun (DeMartini et al., 2011). Solar radiation also limits tolerance of physical work in a step-wise manner (Figure 2) (Otani et al., 2016). Furthermore prolonged exposure to simulated solar radiation of the head and neck impairs performance of cognitively dominated (such as mathematic calculations) and motor task performances (fine adjustments of force) (Pill et al., 2020). By impacting work and recovery periods, solar radiation contributes to heat stress symptoms. We recently reported that for workers based in Northern Australia, chronic heat stress was strongly associated with increased exposure to direct sunlight and hot surfaces (Carter et al., 2020).

While the lack of solar radiation - heat stress research requires addressing, the example and limited evidence presented here support the need to account for solar radiation during the planning of work activities. Such planning should include controls to mitigate the impact of solar radiation on workers health, safety and performance.

References

Carter S, Field E, Oppermann E, Brearley M. The impact of perceived heat stress symptoms on work-related tasks and social factors: A cross-sectional survey of Australia's Monsoonal North. Applied Ergonomics. 82:102918, 2020

DeMartini JK, Ranalli GF, Casa DJ, Lopez RM, Ganio MS, Stearns RL, McDermott BP, Armstrong LE, Maresh CM. Comparison of body cooling methods on physiological and perceptual measures of mildly hyperthermic athletes. The Journal of Strength & Conditioning Research. 25(8):2065-74, 2011

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. 116(4):769-79, 2016

Piil JF, Christiansen L, Morris NB, Mikkelsen CJ, Ioannou LG, Flouris AD, Lundbye-Jensen J, Nybo L. Direct exposure of the head to solar heat radiation impairs motor-cognitive performance. Scientific Reports. 8;10(1), 2020