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Measurements of CO 2 and counting of occupants were carried out in 37 public bus trips during commuting rush hours in Barcelona (NE Spain) with the aim of evaluating parameters governing ventilation inside the vehicles and proposing actions to improve it. The results show that CO 2 concentrations (1039 and 934 ± 386 ppm, as average and median, during rush hours but with average reduced occupancy due to the fair to be infected by SARS-CoV-2 during the measurement period, and measured in the middle of the busses) are in the lower range of values recorded in the literature for public buses, however an improvement in ventilation is required in a significant proportion of the journeys. Thus, we found better ventilation in the older Euro 3+ (retrofitted with filter traps and selective catalytic reduction) and Euro 5 buses (average 918 ± 257 ppm) than in the hermetically closed new Euro 6 ones (1111 ± 432 ppm). The opening of the windows in the older buses yielded higher ventilation rates (778 ± 432 ppm). The opening of all doors at all stops increases the ventilation by causing a fall in concentrations of 200–350 ppm below inter-stop concentrations, with this effect typically lasting 40–50 s in the hermetically closed new Euro 6 hybrid buses. Based on these results a number of recommendations are offered in order to improve ventilation, including measurement of CO 2 and occupancy, and installation of ventilation fans on the top of the hermetically closed new buses, introducing outdoor air when a given concentration threshold is exceeded. In these cases, a CO 2 sensor installed in the outdoor air intake is also recommended to take into account external CO 2 contributions. • Measurements of CO 2 were carried out in public buses during commuting rush hours. • An evaluation of factors governing ventilation inside the vehicles was carried out, and actions to improve it were suggested. • The opening of the windows in the older buses yielded higher ventilation rates. • The opening of all doors at all stops increases the ventilation by causing a fall in concentrations lasting 40–50 s. • Measurement of CO 2 and occupancy and installation of roof ventilation fans in the hermetically closed new buses. [ABSTRACT FROM AUTHOR]