Fresh air is drawn by an underground ventilation station on the north shore and is pumped to all sections of the tunnel through vented ducts. The air supply uses fourteen axial flow fans, each up to in diameter. The exhaust uses sixteen (eight in each northern pylon tower) fans that draw exhaust air through two underground ducts from the tunnel ventilation station and transfer the air to the top of the pylon. They can expel it at up to – equivalent to changing all the air in the tunnel every two minutes with the capability of running in reverse in an emergency and all fans are rated for smoke extraction. Each of the fans has a duty of . The testing of the fans was one of the most comprehensive ever, covering flowrate and pressure, power measurements, sound levels, bearing vibration, x-raying of all impeller components, high-temperature tests at for 2 hours, impeller strain, and 24-hour run tests for reversals.