Time-of-day effects of exposure to solar radiation on thermoregulation during outdoor exercise in the heat

Abstract

High solar radiation has been recognised as a contributing factor to exertional heat-related illness in individuals exercising outdoors in the heat. Although solar radiation intensity has been known to have similar time-of-day variation as body temperature, the relationship between fluctuations in solar radiation associated with diurnal change in the angle of sunlight and thermoregulatory responses in individuals exercising outdoors in a hot environment remains largely unknown. The present study therefore investigated the time-of-day effects of variations in solar radiation associated with changing solar elevation angle on thermoregulatory responses during moderate-intensity outdoor exercise in the heat of summer. Eight healthy, high school baseball players, heat-acclimatised male volunteers completed a 3-h outdoor baseball trainings under the clear sky in the heat. The trainings were commenced at 0900 h in AM trial and at 1600 h in PM trial each on a separate day. Solar radiation and solar elevation angle during exercise continued to increase in AM (672-1107 W/m and 44-69°) and decrease in PM (717-0 W/m and 34-0°) and were higher on AM than on PM (both P < 0.001). Although ambient temperature (AM 32-36°C, PM 36-30°C) and wet-bulb globe temperature (AM 31-33°C, PM 34-27°C) also continued to increase in AM and decrease in PM, there were no differences between trials in these (both P > 0.05). Tympanic temperature measured by an infrared tympanic thermometer and mean skin temperature were higher in AM than PM at 120 and 180 min (P < 0.05). Skin temperature was higher in AM than PM at the upper arm and thigh at 120 min (P < 0.05) and at the calf at 120 and 180 min (both P < 0.05). Body heat gain from the sun was greater during exercise in AM than PM (P < 0.0001), at 0-60 min in PM than AM (P < 0.0001) and at 120-180 min in AM than PM (P < 0.0001). Dry heat loss during exercise was greater at 0-60 min (P < 0.0001), and lower at 60-120 min (P < 0.05) and 120-180 min (P < 0.0001) in AM than PM. Evaporative heat loss during exercise was greater in PM than AM at 120-180 min (P < 0.0001). Total (dry + evaporation) heat loss at the skin was greater during exercise in PM than AM (P < 0.0001), at 0-60 min in AM than PM (P < 0.0001) and at 60-120 and 120-180 min in PM than AM (P < 0.05 and 0.0001). Heart rate at 120-150 min was also higher in AM than PM (P < 0.05). Neither perceived thermal sensation nor rating of perceived exertion was different between trials (both P > 0.05). The current study demonstrates a greater thermoregulatory strain in the morning than in the afternoon resulting from a higher body temperature and heart rate in relation to an increase in environmental heat stress with rising solar radiation and solar elevation angle during moderate-intensity outdoor exercise in the heat. This response is associated with a lesser net heat loss at the skin and a greater body heat gain from the sun in the morning compared with the afternoon.

Exposure to high solar radiation reduces self-regulated exercise intensity in the heat outdoors

“…The rate of SHG is influenced by several factors, including solar radiation, solar elevation angle, the projected area, soil albedo and the colour and insulation of the clothing [26]. Although the present study did not measure soil albedo like previous studies [2,5], given that the surface of this asphalt pavement was dark-colored which is low solar reflectance, it might have a moderate impact on the difference in SHG between trials. Also, since the clothing condition was the same in each trial, it may not influence the result of SHG between trials.…”

“…To date only one study systematically investigated the effect of different solar radiations on thermoregulatory responses during outdoor exercise in hot environments [2], though a few studies have examined this effect in cool-to-temperate environments [3,4,5]. The previous study demonstrated a greater thermoregulatory strain when exposed to a higher solar radiation (from 672 to 1107 W•m −2 ) associated with rising solar elevation angle (from 44 to 69) in the morning (34±1°C Ta, 60% RH) compared with a lower solar radiation (from 717 to 0 W•m −2 ) associated with falling solar elevation angle (from 34 to 0) in the afternoon (33±2°C Ta, 54% RH) during a 3-hour baseball training under a clear sky [2]. This was accompanied by a higher tympanic temperature, Tsk and heart rate (HR), a greater body heat gain from the sun (SHG) and a lower heat loss at the skin during exercise in the morning compared with the afternoon.…”

“…There were no obstructions to shield the sun within a 100 m radius from the east, south and west sides of exercising participants, but a building located approximately 8 m from the north side of them as a windshield to minimize the impact of airflow [12]. All trials were conducted between 0900 h to 1200 h from late-July to mid-September to minimize the effects of solar elevation angle [2] and the time-of-day [14] Co., Ltd., Tokyo, Japan). Thermal sensation was measured using a 9-point scale [18].…”

“…Moreover, this study chose the average solar elevation angle of about 55° because the greatest SHG was obtained when it is approximately 55° in quietly standing humans [26] and exercising individuals in the morning [2], but the relationship between this angle and SHG was unclear in sitting humans. Although our subjects faced toward the sun during exercise, this would be the greatest perceived thermal stress from the sun and result in a higher thermal…”

Sustainable solutions to mitigate occupational heat strain – an umbrella review of physiological effects and global health perspectives

“…In addition to the interventions identified through the systematic reviews, original research articles on seven additional interventions, namely: providing shading [24,100], improving clothing design [21,49], utilizing clothing with ventilator incorporated into the clothing [101,102], electrolyte consumption [103,104], taking intermittent rest breaks [105], and slowing pace/reducing work intensity [106].…”