The effect of nosecups on the performance of open circuit self contained breathing apparatus (SCBA) of the type used by firefighters was investigated. Six nonsmoking male volunteers, 18 to 35 years old, performed 30 minutes (min) of treadmill exercise while wearing one of three National Fire Protection Association (NFPA) compliant 60min open circuit positive pressure SCBAs with or without a nosecup. The exercise was performed at a work rate of 3.0 liters (l) per min for 5min, 2.0l/min for 15min, and 0.5l/min for 10min. Average oxygen consumption (VO2), minute ventilation (VE), expired peak flow (PFE), time to alarm (measured as the first sounding of the first stage regulator alarm), minimum (VCO2min) and average inspired carbon-dioxide (124389) concentrations (ViCO2), concentration (VrCO2) and volume of reinspired carbon-dioxide (VRCO2), inspired minute volume (Vim), and breathing frequency were monitored. VO2, VE, and PFE decreased with decreasing exercise work rates. Times to alarm were significantly increased when the nosecups were worn, the percentage increases varied from 7.5 to 18.3%. The mean respirator cylinder pressures at the time of alarm, however, did not vary significantly across the SCBA models when the nosecup was worn compared to when it was not worn, 1133.3 versus 1127.8 pounds per square inch, respectively. VCO2min, ViCO2, VrCO2, and VRCO2 were significantly decreased when the nosecup was worn. Vim was significantly decreased when the nosecup was worn while exercising at the 3.0l/min work rate, but not at the other work rates. The authors conclude that time to alarm is significantly increased when a nosecup was worn with the tested SCBAs versus when it was not worn.