Cabin Condition  What to expect when flying  Pressure, oxygen
content and gas expansion  Contrary to popular belief, cabin
pressure is not maintained at sea level pressure. Air pressure
in the cabin is equivalent to the outside air pressure at
1,800–2,400 metres (6,000–8,000 feet) above sea level. As a
result, the blood takes up less oxygen and gases within the body
will expand. The effects of reduced cabin air pressure are
usually well tolerated by healthy passengers. Passengers with
certain medical conditions, in particular heart and lung
disorders and diseases such as anaemia, may not tolerate this
reduced oxygen level. These passengers will need to apply for
medical clearance before travelling.  As the aircraft climbs,
the decreasing cabin air pressure causes gases to expand. Gas
expansion during ascent causes air to escape from the middle ear
and the sinuses, usually without causing problems. This airflow
can sometimes be perceived as a "popping" sensation in the ears.
As the aircraft descends, air must