Prolonged exercise in a hot environment increases the minute ventilation (VE) and respiratory rate (RR) with an increase in core temperature. This hyperthermia-induced hyperventilation decreases the partial pressure of arterial blood carbon dioxide (PaCO2). Conversely, nasal breathing during exercise has been reported to produce low VE and RR values and high PaCO2 values; however, no studies have investigated this in hot environments. Therefore, the purpose of this study was to clarify the effect of nasal breathing on estimated partial pressure of carbon dioxide in arterial blood (PaCO2,estimate) during prolonged exercise in a hot environment. Twelve university endurance athletes participated in the study and performed a 40-minutes steady-state cycling exercise at an intensity of 55% of peak oxygen uptake in a hot environment (room temperature 35℃, relative humidity 40%). Using randomized crossover design, two experiments were performed—nasal breathing condition (NB) and mouth breathing condition (MB). Moreover, physiological indices were measured during the exercise. Rectal temperature increased in both conditions, but there was no significant difference between these conditions. PaCO2,estimate values were significantly higher in NB between 10 minutes and 40 minutes of exercise (p < 0.05) compared to MB, and decreased with time in MB. Compared to MB, the VE was significantly lower in NB between 30 minutes and 40 minutes (p < 0.05), while the RR was significantly lower in NB between 25 minutes and 40 minutes of exercise (p < 0.05). Therefore, nasal breathing during a prolonged moderate-intensity exercise in a hot environment prevented the decrease in PaCO2,estimate due to hyperthermia-induced hyperventilation.

The increase in core temperature during exercise in a hot environment causes hyperventilation (hyperthermia-induced hyperventilation), which decreases the partial pressure of arterial blood carbon dioxide. However, the effects on the body of wearing a surgical mask during exercise that would cause hyperthermia-induced hyperventilation have not been clarified. Therefore, we investigated such effects. Ten healthy men participated in the study. A 44-minute steady-state load cycling exercise (55% of peak oxygen uptake) was performed under hot environmental conditions (30℃, 70% relative humidity). Two experimental conditions were set: exercising with surgical face mask (MASK) and without a surgical face mask (CON). The experiment was conducted as a randomized crossover design. Body temperatures, respiratory gas exchange, and subjective indices were measured during exercise. In terms of physiological indices, respiratory rate only revealed a main effect and was significantly lower in the MASK than in CON at 44 min of exercise (CON 41.0±19.2 breaths/min, MASK 35.6±11.2 breaths/min p=0.029). No significant differences were observed between conditions in other physiological parameters. As for the subjective indices, dyspnea showed a condition × time interaction (p=0.016), which was significantly higher in the MASK starting at 5 minutes through the end (CON 4.5±3.5 and MASK 8.5±2.1 at the end of exercise p=0.016). No significant differences were observed in other subjective measures. In conclusion, wearing a surgical face mask during moderate-intensity prolonged exercise under hot environmental conditions significantly worsened dyspnea, but no significant differences were observed in other physiological indices or subjective indices.