Air quality pilot study on board Metlink services
People breathe out CO₂ (carbon dioxide) and indoor CO₂ levels can build up when there is not enough fresh air coming in from outside. Improving indoor ventilation is one important approach to reducing concentrations of any airborne infectious particles (such as viruses) and other air pollutants. Monitoring CO₂ levels indoors can help us identify when ventilation is poor.
Greater Wellington (GW) commissioned a pilot study¹ in October 2022 to measure indoor air quality on our bus network. Air Matters Ltd installed SAN-10 CO₂ sensors in eight buses to measure air quality across variety of bus types and routes. In total, they surveyed 64 trips.
Bus network pilot study findings
- CO₂ levels fluctuated during journeys
- Levels of CO₂ were very strongly correlated with passenger numbers
- All buses reached levels that may represent a moderate and high risk of transmission of airborne infectious particles (based on OzSAGE guidelines) for varying lengths of time
- CO2 levels were higher on the upper floor than on the lower floor on double decker buses
Rail network monitoring
In October 2022, TransDev commissioned an air quality study on the Wairarapa line between Masterton and Wellington². Air Matters Ltd installed SAN-11 CO₂ sensors across four trains over eight trips.
The rail findings were similar to the bus study, but also noted that CO₂ concentrations were most elevated when air intake vents were closed when travelling through tunnels.
Otago University (public health) also undertook a study in during April – May 2023 to measure air quality on Public Transport². Using Aranet 4 devices, they measured CO₂ levels on both buses and trains across three cities.
Like our pilot study, they found CO2 levels strongly correlated with passenger numbers. This was also consistent with international research.
Metlink supports public health advice encouraging people to wear face masks in crowded places, such as public transport. This helps prevent the spread of COVID-19 and other seasonal illnesses.
Next steps
Metlink follows advice and guidance from the Ministry of Health, the Ministry of Transport and Waka Kotahi on health guidelines. Our bus fleet meets the current regulations for urban buses set by Waka Kotahi. Currently, there are no regulatory indoor air guidelines for CO₂ on public transport.
Metlink is advocating to Government, through the Ministry of Health, to develop a common set of indoor air quality standards for public transport in New Zealand. Once in place, Waka Kotahi and public transport authorities across the country will be able to work to a consistent framework, with appropriate funding, procurement, testing and compliance.
In the meantime, we are investigating the feasibility of improving ventilation on upper decks of double decker buses as well as testing the effect of adding fresh air through the air conditioning system where this is possible.
However, we will look to international practice for any new bus procurements to ensure we are keeping step with the latest design and thinking.