Travel by air is not only for healthy people. Over one billion people travel by air each year and thousands of those transferred by air ambulances. Fit to fly report is requested for all those unhealthy passengers, even for the air ambulance flights.
The effect of mild hypoxia and decreased air pressure in the cabin, the ability to adopt the brace position in emergency landing, the ability of the patient to cope mentally and physically with travel to and through the airport to reach the flight and on disembarkation can be counted as couple concerns for a regular flight.
Modern aircrafts are not pressurised to sea level equivalent, which means that there is a reduction in barometric pressure and a reduction in the partial pressure of alveolar oxygen (PaO2). At high altitudes blood oxygen levels fall in everyone, and some people may feel a little breathless. 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.
In case of any critical medical problem on board, an emergency landing would be required. 80% of emergency landings are due to medical reasons. Such emergency landings do not only change the travel plans but also costs a lot to the airlines companies. Those medical and operational concerns show the importance of the fit to fly reports. Since it is not a very common practice, most of the medical doctors would not be aware of the fit to fly criteria. Some airlines require medical certificates confirming that a patient is currently stable and fit to fly. Most have medical advisors who provide advice and 'clear' passengers as fit to fly.