Indoor CO2 levels are an indicator of ventilation effectiveness, ie, ability to exchange
stale indoor air with ‘fresh’ outdoor air. The purpose of ventilation is to avoid the
accumulation of indoor-generated contaminants and odours and to maintain human
comfort levels. Since the pandemic, there has been increased attention on improving
ventilation and indoor air quality to reduce the risk of transmission of current and future
airborne respiratory illnesses.

This pilot aimed to improve understanding of air quality and effectiveness of ventilation
inside urban buses. Indoor air was monitored by Air Matters Ltd on eight in-service
Metlink buses during October 2022 to provide information on the levels of CO2 from
passenger respiration. Sustained and elevated levels of CO2 were found on 22% of
monitored trips indicating inadequate ventilation for the bus occupancy level, which
may pose an increased risk of transmission of respiratory infections and affect passenger
comfort levels.