Oxygen must be available for the crew and all passengers if the aircraft is to be flown above 15,000 feet. The reaction to a lack of oxygen varies considerably between different individuals.
Smokers are more likely to suffer hypoxia symptoms at lower altitudes than non-smokers.
Many sailplane pilots avail themselves of altitude training in a pressure chamber to help them identify their own individual symptoms of hypoxia. For altitudes up to about 25,000 feet, a simple flow-type mask is usually adequate. For altitudes above 35,000 feet the mere availability of oxygen in the mask is insufficient. The oxygen must be available at a positive pressure to force the oxygen into the lungs, thus a pressure-demand type of mask is necessary. Dehydration is a very subtle physiological problem which is easily brought on by the activities associated with sailplane flying.