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The Pall bacterial filter was tested as a potential heat and moisture exchanger on a model patient, placed on a circle absorber system, and clinically. The laboratory study was conducted during mechanical ventilation at a V of 6 L/min with fresh gas inflows of 1,3 and 6 L/min. The model patient introduced carbon dioxide into the circuitry at a rate of 200 ml/min. The resistance of the filter was tested before and after each experiment. With all fresh gas inflows, absolute humidity increased from around 19 mg H2O/L at the start of experimentation to about 27 mg H2O/L within 30 mm. Maximum humidities reached were 28 ± 0.7 mg H2O/L, 27.6 ± 0.5 mg H2O/L, and 27.7 ± 0.5 mg H2O/L within 3 hr, with fresh gas inflows of 1, 3, and 6 L/min, respectively. Variations in inspired humidity were also assessed at minute volumes of 4 and 5 L/min with fresh gas inflows of 6 and 3 L/min. Increases in percent dead space were negligible when the filter was inserted between the model patients (assumed to weigh between 70–40 kg) and the circuit. There was no statistically significant increase in pressure with gas flows of 50 L/min when the instrument was dry (0.02 ± 0.001 cm H2O/L-min−1) or when it was wet (0.02 ± 0.002 cm HzO/L-min−1). The clinical study was conducted on ten adult anesthetized patients breathing through the bacterial filter and ten controls. The loss of body temperature was 0.2°C when the filter was used and 1.5°C when the filter was not used. Arterial blood gas tensions were within normal limits when the bacterial filter was used as a humidifier.