INTRODUCTION: Carbon monoxide (CO) is a toxic gas with potential for detriment to spaceflight operations. An analytical model was developed to investigate if a maximum CO contamination of 1 ppm in the oxygen (O₂) supply reached dangerous levels during extravehicular activity (EVA). Occupational monitoring pre- and postsuited exposures provided supplementary data for review.METHODS: The analytical model estimated O₂ and CO concentrations in the extravehicular mobility unit (EMU) based on O₂ and CO flow rates into and out of the system. The model was based on 3 h of prebreathe at 15.2 psia, 8 h of EVA at 4.3 psia, and 1 h at 15.2 psia for suit doffing. The Coburn-Forster-Kane equation was used to calculate crewmember carboxyhemoglobin saturation (COHb%) as a function of time. Monitoring of hemoglobin CO saturation (S_pco) with a CO-oximeter was conducted pre- and post-EVA during operations on the International Space Station and in ground-based analog environments.RESULTS: The model predicted a maximum P_CO in the EMU of 0.061 mmHg and a maximum crewmember COHb% of 2.1%. Operational S_pco measurements in mean ± SD during ground-based analog testing were 0.7% ± 1.8% pretest and 0.5% ± 1.5% posttest. S_pco values on the ISS were 1.5% ± 0.7% pre-EVA and 1.1% ± 0.3% post-EVA.DISCUSSION: The model predicted that astronauts are not exposed to toxic levels of CO during EVA and operational measurements did not show significant differences between S_pco levels between pre- and post-EVA.Makowski MS, Norcross JR, Alexander D, Sanders RW, Conkin J, Young M. Carbon monoxide levels in the extravehicular mobility unit by modeling and operational testing. Aerosp Med Hum Perform. 2019; 90(2):84–91.