A commonly asked question is why filtration systems are not installed in Australian tunnels as part of their tunnel ventilation system.
Studies have found filtration systems would not provide any measurable improvement to the air quality in the surrounding community.
In NSW we have an Advisory Committee on Tunnel Air Quality, which is chaired by the NSW Chief Scientist & Engineer and consists of a range of experts in fields related to road tunnels, air quality and human health. This includes the NSW Chief Health Officer, and an independent, international air quality expert.
The Committee released a report in relation to road tunnel air quality, which found emissions from well-designed road tunnels cause a negligible change to surrounding air quality, and as such, there is little to no health benefit for surrounding communities in installing filtration and air treatment systems in such tunnels.
This is because Sydney's motorway tunnel ventilation outlets are designed and operated to propel tunnel air high into the atmosphere. Once in the atmosphere, the ejected tunnel air is diluted hundreds of times as it mixes with the surrounding air, before mixing down to ground level resulting in little, if any, change to the quality of the air people breathe.
This dispersion is so effective that if you put a filtration plant in the bottom of the ventilation outlet and had monitoring stations in the local area it would be difficult to measure a difference in air quality between when the filtration plant is working and when it is not. Remembering that filtration does not remove all pollutants from tunnel air so regardless we still need to run ventilation outlets to protect local air quality.
Our major NSW tunnels are also continuously monitored at the ventilation outlet to control the ventilation system. Since the Lane Cove Tunnel, air quality monitoring data is required to be made publicly available on the tunnel’s website.
Internationally there are few tunnels with filtration systems and those that do rarely use them.
Of the small number of filtration systems installed around the world, most have been switched off, or are currently being operated infrequently – in some cases only a few hours a year. Often they are used to improve in-tunnel visibility and rarely solely for environmental purposes.
Did you know the Stockholm bypass tunnel currently being constructed uses a longitudinal ventilation system to manage tunnel air quality with no filtration. Once open this will be one of the longest tunnels in the world - about 18 kms long.
Filtration systems also use large amounts of energy to operate, and has significant costs to both build and operate.
The best approach to reducing community exposure to motor vehicle emissions is to continue to adopt cleaner fuels and vehicles to reduce emissions at the source, rather than attempt to filter them out of the air once they have been released. This approach delivers the greatest environmental benefits as they will improve air quality for the community as a whole as motor vehicles and trucks travel throughout the state.