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During the SARS-CoV-2 pandemic, large-scale events were initially suspended categorically without consideration of ventilation technology. In this study, CO2 concentrations were measured during three trade fairs in the fall of 2021 at Messe Frankfurt with the aim of assessing air mixing and dilution of loads emitted through human respiration. 24 sensors were installed on columns near aisles, in peripheral areas and exhaust/ambient air ducts. At three positions, in addition to a height of 2 m, measurements were taken at 3.2 m height to reveal potential stratification effects. Finally, attendances were estimated based on exhaust/ambient air CO2 levels and ambient air volume flows. Results show that the maximum CO2 concentration during visiting hours never exceeded 835 ppm. Indoor air quality thus qualified as hygienically insignificant at all times. Furthermore, no stratification was detected, indicating that air was mixed well without apparent local accumulations of CO2 and thus potentially infectious aerosols within the breathing zone. Estimated mean attendance corresponds to information provided by Messe Frankfurt. Finally, results indicate that the exhaust CO2 concentration represents a good indicator to assess air quality within the occupied zone in large-scale halls, making elaborate measurements within the occupied zone obsolete. Further evaluation is needed to support this.