Solar radiation is a physical phenomenon which interacts with any human being throughout its life on earth. This interaction is associated with numerous physiological functions, as well as a broad spectrum of deleterious effects on human health and wellbeing. The aim of this study is to elucidate the effects of solar radiation on human function and cognition in environmental conditions characterized by the same thermal stress.

Volunteers were exposed to four different environmental conditions [two hot environments: 30°C WBGT (1st: air temperature = 32.6°C; relative humidity = 30%; wind speed = 0.5 m/s; solar radiation = 800 W/m2 and 2nd. air temperature = 40.6°C; relative humidity = 30%; wind speed = 0.5 m/s; solar radiation = 0 W/m2) and two neutral environments: 20°C WBGT (1st: air temperature = 19.1°C; relative humidity = 30%; wind speed = 0.5 m/s; solar radiation = 800 W/m2 and 2nd. air temperature = 28.5°C; relative humidity = 30%; wind speed = 0.5 m/s; solar radiation = 0 W/m2], over an equal number of days. The aforementioned environmental scenarios were randomly allocated for each volunteer. To minimize participant bias, the true purpose of the study was hidden from the volunteers. Of course, once the data collection was completed, all volunteers were informed about the true purpose of the study and gave their permission to analyze and publish these data. During this blinded randomized controlled trial, all volunteers participated four times in an experimental protocol with a total duration of 03:20:00, following an hour of preparation and baseline tests. The protocol had three main time periods [rest (two hours), work (cycling) at 100 W (one hour), and recovery (twenty minutes)]. Anthropometrical data [age; body stature (Seca 213; seca GmbH & Co. KG; Hamburg, Germany); body mass (BC1000, Tanita corporation, Tokyo, Japan); body fat (DXA scan: Lunar DPX Madison, GE Healthcare, Wisconsin, USA); and lean mass (DXA scan: Lunar DPX Madison, GE Healthcare, Wisconsin, USA)] were collected two days prior to the experiments.

Volunteers were euhydrated prior to the experiments. Hydration status was assessed using a handheld refractometer (ATAGO Ltd, Tokyo, Japan) and determined as either euhydrated (USG < 1.020) or dehydrated (USG ≥ 1.020) according to the current guidelines. Water consumption was restricted during the experimental protocol. The same clothing consisting of a light-blue t-shirt (100% cotton), a black exercise short pant (100% polyester), and a pair of medium-high socks (100% cotton) were used by volunteers throughout the experiments. A pair of sunglasses was mandatory to be worn throughout the experiments. Furthermore, experiments took place during the same hour of the day for each participant and an 8-hour fasting was ensured prior to the experiments. Furthermore, participants were requested to avoid caffeine and alcohol consumption for at least twelve hours before the experiments, as well as to avoid salt and sugar consumption eight hours before the experiments.

During the study, continuous heart rate, core temperature, mean skin temperature, skin blood flow, and sweat rate were measured. Specifically, heart rate was collected using wireless heart rate monitors (Polar Team2. Polar Electro Oy, Kempele, Finland). Core temperature was collected using telemetric capsules (BodyCap, Caen, France). Skin temperature from four sites was measured using wireless thermistors (iButtons type DS1921H, Maxim/Dallas Semiconductor Corp., USA) and was expressed as mean skin temperature according to Ramanathan [mean skin temperature = 0.3(chest + arm) + 0.2(thigh + leg). Skin blood flow was measured with a laser Doppler flowmeter (PeriFlux 4000, Perimed, Stockholm, Sweden) at right forearm (brachioradialis) and leg (gastrocnemius). The probe (PROBE 413 Integrating Probe, Perimed, Stockhold, Sweden) was held in place with a plastic holder (PH 13, Perimed, Stockhold, Sweden). Sweat rate was measured at three regions [forehead, thigh (quadricep), and arm (bicep)] using ventilated capsule method. Thermal comfort (1 = comfortable; 5 = extremely uncomfortable), thermal sensation (-3 = cold; +3 = hot), and perceived exertion (6 = no exertion at all; 20 = maximal exertion), alongside cognitive performance (vigilance, divided attention, memory test, and reaction time) were assessed at baseline, pre work condition (01:40:00), and post work (03:00:00) conditions. Volunteers entered the chamber five minutes prior to the experiments to install data loggers and accompanied sensors on their body.](streamdown:incomplete-link)

Inclusion Criteria:

• Healthy non-smoker individuals who exercise at least three times per week

Exclusion Criteria:

• Non healthy individuals and smokers