Low water content of inspired air increases evaporation in the respiratory tract and leads to airway cooling, which can cause increased airway resistance. Conversely, airflow can be improved by the warming and humidifying of inspired air. We conducted a study of the effect of temperature and relative humidity in inspired air on expiratory flowrates in normal subjects under resting conditions of ventilation. Methods: We assigned 40 subjects to four groups of 10 each. One group breathed cold dry air, a second group breathed warm dry air, a third group breathed room-condition air, and a fourth group breathed warm saturated air. Percentage change from baseline in FEV 1 , PEFR, and FEF 25-75% was compared across the four groups immediately and 10 minutes after the subjects breathed the conditioned air. The subjects also used a self-report scale to indicate their comfort/discomfort when breathing the various inspirates. Results: Only the difference in FEV 1 between room air and warm saturated conditions was statistically significant (P < 0.01), although we did not consider the difference clinically important. All other changes in the four groups were varied in direction, and they were neither statistically significant nor clinically important. Subjectively, participants showed a general dislike for inspiring the warm dry air. Conclusion: The results indicate that varying water content of inspired air alone, without the presence of aerosols, has neither a beneficial nor a detrimental effect on expiratory flowrates in normal subjects at rest.
Rau et al. (Thu,) studied this question.