BACKGROUND: Elevated levels of carbon dioxide in gym air can diminish the benefits of physical activity and pose health risks for children.. AIM: to access carbon dioxide concentration in the air of school gyms during physical education classes. MATERIAL AND METHODS: A total of 612 measurements were taken to estimate the concentration of carbon dioxide in the air. These measurements were conducted in two separate gymnasiums: in Gym 1, designated for primary classes with an area of 77 m2, and Gym 2, used by middle and high school students with an area of 293 m2. Measurements were taken at 12 different points, both around the perimeter and in the central part of each gym. The height when measuremenmts were taken ranged from 0 to 230 cm. To assess the carbon dioxide concentration in the gym air, the background level was calculated based on GOST 30494-2011 standards (761.5 ppm). Student’s t-tests for independent samples were used to compare the data. Additionally, a regression analysis was utilized to estimate the spatial distribution of carbon dioxide within the gymnasiums. RESULTS: In Gym 1, the initial concentrations ranged from 845 to 1267 ppm, slightly exceeding the expected throughput. Throughout the training session, the carbon dioxide content increased by 1.6 to 2.3 times. By the end of the session, the carbon dioxide content reached 1934 to 1948 ppm at an estimated respiration level of 1.0 to 1.9 m. In Gym 2, the carbon dioxide content increased by 1.1 to 1.2 times by the end of the class. At a height of 0.0 to 1.7 m, the concentration of carbon dioxide was measured at 1016 to 1023 ppm. CONCLUSION: After 20 minutes of training at the expected intensity, carbon dioxide levels in the air exceed not only the background level of 761.5 ppm, but also the permissible level of 1000 ppm. This study highlights the importance of daily monitoring of carbon dioxide levels in school gymnasiums during training sessions and sporting events. Such monitoring is crucial for ensuring the health and safety of students and athletes.