INTRODUCTION: Astronauts undergo CO₂ exposure training to recognize their symptoms that can arise acutely both on the ground and in spaceflight. This article describes acute CO₂ exposure training at NASA and examines the symptoms reported by astronauts during training.METHODS: In a controlled training environment, astronauts are exposed to up to 8% CO₂ (60 mmHg) by a rebreathing apparatus. Symptoms are reported using a standard form.RESULTS: Symptom documentation forms between April 1994 and February 2012 were obtained for 130 astronauts. The number of symptoms reported per session out of the possible 24 was related to age and sex, with those older slightly more likely to report symptoms. Women reported more symptoms on average than men (men: 3.7, women: 4.7). Respiratory symptoms (90%), flushing sensation/sweating (56%), and dizziness/feeling faint/lightheadedness (43%) were the top symptoms. Only headache reached statistical significance in differences between men (13%) and women (37%) after adjustment for multiple testing. Among those with multiple training sessions, respiratory symptoms were the most consistently reported.DISCUSSION: CO₂ exposure training is an important tool to educate astronauts about their potential acute CO₂ symptoms. Wide interindividual and temporal variations were observed in symptoms reported during astronaut CO₂ exposure training. Headache could not be relied on as a marker of acute exposure during testing since fewer than half the subjects reported it. Our results support periodic refresher training since symptoms may change over time. Further study is needed to determine the optimal interval of training to maximize symptom recognition and inform operational decisions.Law J, Young M, Alexander D, Mason SS, Wear ML, Méndez CM, Stanley D, Meyers Ryder V, Van Baalen M. Carbon dioxide physiological training at NASA. Aerosp Med Hum Perform. 2017; 88(10):897–902.