Metro tunnels must be ventilated to remove the dangerous build-up of pollutants, reduce the risks of smoke during a fire and to remove the heat generated by the trains and other electrical equipment.
Virtually all of the electrical power consumed degenerates into heat which is removed by a combination of natural and powered ventilation. During free running conditions the piston effect of the trains may be sufficient for ventilation and natural ventilation may maintain this condition even when the trains are not operating. However, when the system becomes congested and trains are running at short intervals or, in the case of an incident, forced ventilation becomes necessary.
The amount of heat generated by trains in rapid transit systems of high density usage is the largest single factor in determining the mass flow of air required to maintain a stable air temperature. However the risk of fire must also be taken into account.
Longitudinal ventilation along the tunnel is common using 100% reversible fans. For an enclosed system large fans connected to atmosphere would be used in a push-pull configuration. If the system is not enclosed, Jetfoils can be used to induce the required airflow along the tunnel to and from atmosphere.