Modern aircraft are not pressurised to sea level equivalent. Cabin altitude equivalent is usually between 5,000 and 8,000 feet which means that there is a reduction in barometric pressure and a reduction in the partial pressure of alveolar oxygen (PaO2). Sometimes during flight, although not usually for long periods, oxygen saturation levels can fall to around 90%. A healthy individual can usually tolerate this with no problems but it may not be the same for someone with cardiac or respiratory conditions or with anaemia.
Aircraft cabins also have low humidity levels which can cause dryness of mucous membranes and also the skin. Reduced cabin pressure can also cause gas volume expansion. This can be a problem if there has been recent surgery that has introduced gas into the abdominal cavity or the eye. Gas can also expand if it has been trapped in the ear.