Travelling in an aircraft is an unusual experience as passengers
are enclosed within a cabin designed to protect them from the hostile external environment for a period which may last for a few minutes or even several hours.
The cabin’s internal environment is made to meet a balance between the physiological needs of the occupants and the engineering constraints of an aircraft operating at an altitude. A difference thus exists between the fresh air at the sea level and that in the cabin.
This can be very significant to the well-being of the healthy individual. However, the potential adverse effect is more likely to impair that good health the individual came in with. This is for the healthy passenger. For some very ill and some individuals is could portend great danger with serious effects.
The cabin air pressure is often reduced to that of air pressure at 8000 feet. This is the maximum permitted cabin altitude. Why is it so? Sobsonic aircraft fly at altitude of up to and around 40,000 feet in order to avoid turbulence and reduces drag on the aircraft.
The external ambient evniornment at this height is hostile with temperature below minus 50 C and the pressure about 140 mmHg.
Note that the pressure at sea level is 760 mmHg. Though the normal proportion of oxygen remains at 21% yet the pressure is too low to maintain life.
Hence the need to ensure a maximum permissible pressure within the cabin that will also allow physiological functions without expressing danger on the differential pressure across the wall of the cabin as well as on the aircraft design.
Of prime importance to respiration and so to passengers with cardiovascular and pulmonary disease is this reduction in cabin pressure when compared to that on ground level. This may not be acceptable to an elderly with pulmonary disease. Other physiological factors are:
Humidity: This is usually low and may cause drying of the mucous membrane with the sensation of thirst. It does not mucous membrane with the sensation of thirst. It does not lead to systemic dehydration.
Affectation of high level cognitive functions. This may be very marginal in passenger. It is taken care of the crew cockpit.
Effect of ascent on body cavities: As the aircraft ascends the cabin pressure falls. Thus the body cavities with moist have the gas within cavities. A health distensible viscus will cause no problem, e.g the intestines, also the air within the rigid well vented structured such as the parnasal sinusers and middle ear.
The rapid rate of ascent is well tolerated. However, problems may arise with descent because of occlusion of the Eustachian tube. Small change in pressure may not precipitate problem in healthy organs.  Passengers with cold, sinusitis, otitis media with rigid ear drum many experiences sever ear aches with headache, so also  are those who just had surgeries of the eye, ear and dental fillings,etc.
Decompression sickness: This is commonly seen in passengers who had previously before the travel, been exposed to hypobaric pressures due to reasons of work or leisure, such as the divers, caisson workers and tunnellers.
Occupational physicians advices should be sought by such workers before travel. However, compression may occur if there is cabin structural defect in the aircraft, failure of pressure controller, or the discharge valve with reduction of inflow of air or increase in the discharge of air. The effect of any of these can easily be corrected by the pilot by descent or ignored if insignificant.
Risks of decompression
Hypoxia
Loss of consciousness
The time of useful consciousness is short hence the advice to adorn the overhead oxygen mask on yourself first before helping others whenever it is released.
Implications of the cabin pressure on the sick
Apart from the sick, it is advisable that the healthy take sufficient time to plan the travel.
Do not travel if you have just undergone a surgery-ear, sinus, eye, dental filling, abdominal surgery.
Do not travel if you have bad erache /infection, sore throat.