Respirators are worn in only about 20-30% of the appropriate circumstances. This research examined the effect of inspired air conditions on performance time. An environmental chamber supplied air at 27°C, 28°C, 37°C, and 55°C to a powered air-purifying respirator (PAPR) worn by the subject while exercising in a neutral environment. A mathematical model of performance time as a function of the heat index (HI), respirator familiarity (RF), personality type (SN), and minute volume (Ve) indicated that performance time increased with a decrease in HI and Ve and with an increase in RF and SN ratio. A model of performance time as function of user acceptability and the heat index indicated that time to reach a level of "fairly uncomfortable" decreased exponentially from 5.34 minutes to 2.85 minutes with an increase in the heat index. Performance at the heat index conditions may be described by physiological, subjective, and individual characteristics.