In the field of road tunnels, air quality is traditionally considered in relation to the level of concentrations of vehicle exhaust inside a tunnel. However, the concentrations of pollutants outside a tunnel can be harmful or annoying to people. Such pollutant concentrations rapidly reduce from a portal or exhaust shaft to the surrounding environment according to complex mechanisms such as the speed and direction of the wind and the neighbouring topography. Consequently, it is recognized that air quality in the vicinity of tunnel portals or other exhaust points is of interest when the traffic intensity increases and when tunnels are constructed in an urban environment.
Above a tunnel, the air quality is expected to be better than if an open air road section was situated at the same location. However at the portals and shafts, polluted air is set free when a longitudinal or transverse airflow is discharged by the piston effect of traffic and/or by ventilation systems.
Depending upon background concentrations and other sources localized close to a tunnel portal or shaft, the concentration levels of pollutants can exceed the maximum levels set by authorities. In that case measures must be taken to improve the air quality in the vicinity of the tunnel. Measures may include civil or mechanical works, planning of the land use around the tunnel, etc. Most often it may be possible reduce the pollution concentrations based on operational measures such as changes in the ventilation regime. Chapter III “Environment” of the 1996 PIARC report 05.02 “Road Tunnels: Emissions, Environment, Ventilation” provided background information on the behaviour of a tunnel air jet from portals.
PIARC has published the Technical Report 2008 R04 "Road Tunnels: A Guide to Optimizing the Air Quality Impact upon the Environment", which focuses on outside air quality related to tunnels and it is a guide to enhancing the urban environment by altering the emissions from vehicles and changing their spatial distribution within the space surrounding a tunnel. The guide considers a wide range of design and operation opportunities to mitigate the impact of tunnels on outside air: from the selection of the most optimum location of a tunnel, to gradients, ventilation type, air discharge management, traffic management, tunnel maintenance and finally (if still useful) contaminant removal techniques (see also Section 4.4.5 "Air cleaning" of the PIARC report 2017 R02 "Road Tunnel Operations: first steps towards a sustainable approach").
The environmental issues linked to ventilation, besides the energy consumption and the related carbon footprint, are linked to the localised, concentrated discharge of polluted air from the portals and stacks. Reducing their impact on the tunnel surroundings is part of good environmental design : see Section 4.3. "Tunnel air dispersion technique", Section 4.6. "Operational aspects" and Appendix D. "Overview of dispersion modeling in designing ventilation systems" of the PIARC report 2008 R04 "Road Tunnels: A Guide to Optimizing the Air Quality Impact upon the Environment".
Additional information specific for complex tunnels can be found in section 8.1 "External Air Quality" of the PIARC Report 2016 R19 “Road Tunnels: Complex Underground Road Networks”.