Air flow rate through filters and face masks is one important determinant of the final protection factor of a respiratory protective device in use. Respiratory minute volumes and instantaneous breath flow rates were measured in eight subjects during treadmill work using three types of filtering respirators and one control breathing mask. Work comprised five consecutive bouts of walking at 5 km/h with an increase in elevation of the treadmill by 5% every 5 min and a final walk at 6 km/h and 22.5%. Three subjects managed to complete 5 min at the final work rate. Minute ventilation increased in a curvilinear manner with oxygen uptake and reached 88 +/- 20 and 93 +/- 20 l/min at 5 km/h (20%) with the control mask and a half mask with filter (SP), respectively. Mean peak inspiratory flow rate (PIFR) was 273 +/- 39 for Control and 300 +/- 36 for SP at the same work rate. Two power assisted, positive pressure filter respirators (SA and SE) produced higher mask minute volumes at any given work rate compared to respiratory minute volumes in control and SP. PIFR in SA and SE were equal to or lesser than SP. During standardized speech communication, minute volumes decreased. In contrast, PIFR increased by about 100% at low work rates and about 30% at 5 km/h (20%) compared to no speech condition, reaching a highest value of 414 +/- 46 l/min for SE. Filter testing is made at a constant flow rate of 95 l/min, a condition that eventually needs to be reconsidered in order to ensure a relevant and valid function test.