The objective of this study was to determine the average reduction in inhalation exposures produced by intermittent use of filter cartridge respirators by cadmium workers. Inhalation exposure was estimated by measuring the cadmium concentration inside the respirator while it was worn or hanging around the worker's neck. Air concentrations of cadmium were measured simultaneously inside the respirator and at the worker's lapel with a dual sampling system. Each of nine workers were measured on three consecutive days for a full work shift. The average inhalation exposures ranged from 3 to 67 micrograms/m3 while the TWA lapel concentrations ranged from 19 to 3600 micrograms/m3; respirator use produced a substantial reduction in inhalation exposures when lapel concentrations were above 100 micrograms/m3. On the average, the inhalation exposure was 26% of the lapel concentration, but the effective protection varied widely between individuals and from day to day. If used cautiously, this relationship may be useful for estimating the approximate average inhalation exposure of a group of workers routinely using half mask respirators.