Arising from the Antoine equation and the ideal gas law, the volume of exhaled water has been calculated. Air temperature, humidity and minute ventilation has been taken into account. During physical exercise amount of exhaled H₂O is linear, but not proportional to heart rate. And so at the heart rate of 140 bpm amount of exhaled water is approximately four times higher than during the rest and equals about 60–70 ml/h. The effect of external temperature and humidity on water lost via lungs was assessed as well. When temperature of inspired air and its humidity is 35°C an 75% respectively loss of water is 7 ml/h. Whereas when above parameters are changed to minus 10°C and 25% lung excretion of H₂O increases up to 20 ml/h. The obtained results may become the basis for the assessment of osmolarity changes on the surface of the lower airways. The increase of which is recently considered as one of the factors responsible for exercise induced bronchospasm.