The Covid-19 pandemic led to the widespread closure of events. Between April and July 2021, the AIRBODS consortium carried out an Environmental Study as part of the UK government Events Research Programme to assess environmental risk factors for Covid transmission at events. A detailed post-occupancy evaluation of Indoor Air Quality was employed to assess the effectiveness of ventilation systems in operation. CO2 monitors were installed at high spatial resolution throughout the occupied spaces of ten venues around the UK. Data from 55 events was obtained and average and maximum CO2 values were used to classify the spaces in relation to a proposed Air Quality Index. Indoor spaces where ventilation could be improved were rapidly identified and mitigations were tested to reduce the risk of airborne transmission of respiratory diseases.