A laboratory test system has been designed and built to measure the efficiency of filter material from several commonly used respirator filter cartridges, approved disposable dust masks, nuisance dust masks, single-use surgical procedure masks and a resuscitation mask against aerosols of the bacteria Bacillus subtilis subspecies globigii, Micrococcus luteus and Pseudomonas alcaligenes. All tests were carried out at a flow rate of 30 l min(-1). The same materials were also tested with monodisperse urea aerosols in the size range 1.5-9 mu m and with the British Standard BS 4400 sodium chloride aerosol, which has a mass geometric mean diameter of 0.6 mu m and a geometric standard deviation of 2.3. One type of surgical mask allowed penetration of up to 83% of a microbiological aerosol and 87% of a non-biological aerosol, and nuisance dust masks allowed penetrations of up to effectively 100% for a microbiological aerosol and 91% for a non-biological aerosol. The materials of the other surgical masks and the resuscitation mask, all of which were electrically charged, performed better against all the test aerosols. Penetration of microbiological aerosols through the respirator filter materials was no greater than 0.88%, and penetration of non-biological aerosols no greater than 1.72%. The results for microbiological aerosols generally corresponded to those for the non-biological aerosols in the same size range. Crown