This information has been supplied by www.finnair.com
CABIN AIR
Cabin pressure
A person living at sea level will suffer from lack of oxygen (hypoxia)
when the physiological altitude rises above 3000 m (10,000 ft). The cabin
air is pressurised by air from the jet engines. The maximum allowable
cabin pressure altitude corresponds to 2400 m (8,000 ft). During most
flights the cabin altitude remains at 1500-1800 m (5,000-6,000 ft).
Typical flight altitudes for jet aircraft vary between 9-12 km and for
turboprop aircraft between 5-7 km.
Cabin air circulation
In modern aircraft 50% of the incoming cabin air is taken from outside
and mixed with recirculated cabin air which has been passed through highly
effective active filters which remove all particles bigger than 0.3 microns.
Fresh air enters the cabin from above and is removed through openings
at floor level. Clinical investigations show that the recirculated air
does not contain bacteria, viruses or other particles. Modern ventilation
design prevents air in the cabin moving along the aircraft. In all aircraft
the fresh air intake for each passenger is more than 10-20 times higher
than actually needed.
Recirculation decreases fuel consumption and makes the flight more economical.