Public transit vehicles represent small, dynamic, indoor spaces, with conditions that can be favourable for the development of poor air quality. The use of CO₂ monitors is recommended as a potential strategy for improving ventilation; however, information around how non-experts can use them for personal monitoring is limited. The purpose of this field study was to assess the real-world usability of a low-cost monitor, and to provide general recommendations around personal risk assessment using a monitor in combination with the evaluation of environmental factors. Measurements of CO₂ and surveying of commute conditions were carried out with two CO₂ monitors, the Vitalight and the Aranet, during 250 public transit trips between October 17th, 2022 and July 18th, 2023. Results show that CO₂ measurements taken by the Vitalight and the Aranet were highly correlated (Time 1 (T1): r = .948; Time 2 (T2): r = .966). Additionally, the Vitalight paralleled the Aranet with respect to its responses to: temperature; relative humidity; occupancy level; seating position; how often the bus doors opened; and whether the windows were open or closed. However, measurements of CO₂ taken by the Vitalight were significantly lower than those taken by the Aranet (T1: t(249) = −22.52, p < .001; T2: t(249) = −32.44, p < .001). Based on these results, we make three recommendations to provide guidance to non-experts around personal monitoring for ventilation on transit buses, including use and interpretation of the Vitalight monitor, and environmental conditions to assess, to inform actions to take to improve ventilation.