It has been estimated that several million workers are required to wear respirators. Elevated inhaled carbon dioxide concentration is an inherent aspect of respirator wear. The physiological effects of breathing 3 - 4% inhaled CO2 are hyperventilation and headache. Previous data suggest that CO2 concentrations in non-NIOSH-approved surgical helmets exceed the NIOSH REL of 0.5% and that CO2 levels exceed the NIOSH STEL of 3% in a prototype powered air-purifying respirator operating with the blower turned off. There is currently no NIOSH certification test for CO2 concentrations in air-purifying respirators. For this study the Automated Breathing and Metabolic Simulator (ABMS), which simulates human metabolism, minute ventilation, and breathing waveforms, was used to characterize average inhaled CO2 in a variety of NIOSH-approved air-purifying respirators. An ABMS CO2 test protocol was developed to test 11 powered airpurifying respirators (PAPR's), 20 airline respirators (SAR's), six gas masks, 27 P-100 air-purifying respirators (APR's), and 26 filtering-facepiece N95 respirators (N95's). The ABMS CO2 protocol consisted of the following levels of O2 consumption, CO2 production, and minute ventilation performed consecutively for a minimum of five minutes each: 0.5, 0.4, and10 L/min STPD; 1.0, 0.8, and 25 L/min STPD; 1.5, 1.3, and 38 L/min STPD; 2.0, 1.9, and 62 L/min STPD; 2.5, 2.5, and 70 L/min STPD; and 3.0, 3.1, and 80 L/min STPD, respectively. The mean across all PAPR models for average inhaled CO2 and O2 ranged from 0.2% and 20.7%, respectively, for the lowest metabolic rate to 0.9% and 20.0%, respectively, for the greatest metabolic rate. The mean across all SAR's for average inhaled CO2 and O2 ranged from 0.5% and 20.3%, respectively, for the lowest metabolic rate to 0.4% and 20.5%, respectively, for the greatest metabolic rate. The mean across all gas masks and APR's for average inhaled CO2 and O2 ranged from 2.6% and 17.5%, respectively, for the lowest metabolic rate to 0.7% and 20.4%, respectively, for the greatest metabolic rate. The mean across all N95's for average inhaled CO2 and O2 ranged from 3.5% and 16.8%, respectively, for the lowest metabolic rate to 2.7% and 18.6%, respectively, for the greatest metabolic rate. These data demonstrate the wide range of average inhaled CO2 concentrations across respirator types and the utility of the ABMS in conducting CO2 testing.