Any factor that inhibits the ability of an individual with asthma to recognize their symptoms appropriately may contribute to treatment delay, "near miss" events, and death during acute severe asthma. The purpose of this study was to investigate the effects of two common features of acute severe asthma--hypoxia and hypercapnia--on respiratory sensation. Sixteen individuals with stable asthma were exposed to three gas conditions (34 minutes each): isocapnic hypoxia (arterial blood O2 saturation of approximately 80%), hypercapnia (increase in end-tidal CO2 of approximately 5-10 Torr), or isocapnic normoxia on 3 separate days. The perceived magnitude of externally applied resistive loads, measured during each gas condition, was reduced throughout hypoxia compared with normoxia, and there was a trend for a progressive decline during hypercapnia. Within the 15-minutes postgas inhalation period, methacholine-induced symptoms of difficult breathing, chest tightness, and breathlessness, measured using modified Borg scales, were 25-30% lower after hypoxia compared with normoxia but were not reduced after hypercapnia. We conclude that 30 minutes of sustained hypoxia and possibly hypercapnia impair sensations of respiratory load and that the effects of hypoxia persist for at least 10 minutes after returning to normoxia.