In-vehicle research is scarce. Therefore, this research aims to determine and compare the CO2 levels as an indicator of air quality in subway wagons and transit buses in the city of Montreal, with field observations from 2021-2022. Data were collected in units of the Société de Transport de Montréal with a NDIR monitor, in trajectories covering three subway and four bus routes. Data consists of hundreds of readings on CO2 levels in one-minute intervals throughout the trip. Additionally, we record the unit’s occupancy level, natural ventilation, and air-recirculating status. Using multilevel mixed-effects models, we identified the salient factors affecting CO2 levels and compared the results between modes.

Results suggest that levels in buses are approximately 26% higher than in subways. Furthermore, the levels show a direct correlation with the occupancy rate, as semi-crowded and crowded units have levels 5% and 15% higher than not-crowded units. In buses, air-recirculating systems are positively correlated with CO2 levels; units with the air conditioning system on show levels 18% higher, and those with heating have levels 19% higher than buses with both systems turned off. Preliminary data analysis shows CO2 levels 8% higher during the colder months (November through February), confirming a seasonality effect.