See also inflight emergencies.
Fit to Fly
Two sets of interests are involved in advising on fitness to fly. The first is as usual that of the patient - if their life is likely to be materially worsened by flying then advice to not fly is likely to be appropriate, but illness per se is not necessarily the only consideration. The second is that of the airline, and of the other passengers on that flight and subsequent flights of that aircraft. Declaring that someone with a contagious disease should not be permitted on an aircraft is straightforward, but a diversion because a passenger became suddenly ill in flight would be a very expensive business. Airlines reasonably hope to minimise these. In case of doubt it is possible to discuss risks with the airline - the larger ones have a medical department, smaller ones will have means to obtain their own medical advice.
A chartered aircraft or air ambulance is a different matter from a seat on a scheduled flight.
Cabin pressure in modern jet aircraft at cruise altitude is usually equivalent to the air pressure at an altitude of about 6000 ft (say m or around the altitude of Denver). At this pressure/altitude, the inspired partial pressure of oxygen is the equivalent of 15% O2 at sea level. Normal air at sea level is 20.9% O2 by composition. Piston-engined or small passenger aircraft are likely to have a cabin altitude above this, of around 8000 ft, and light aircraft may not be pressurised at all. Travel with an oxygen mask in an unpressurised aircraft at high altitude is beyond the scope of this article.
Symptoms related to these lower partial pressures are possible in airline passengers who have normal arterial oxygen saturations at sea level. These include fatigue, malaise, backache, headache, light-headedness, shortness of breath, alertness and impaired coordination and appear to be a function of both the cabin pressure and time of exposure. However it is important to note that identical symptoms will occur in passengers confined to a simulated airline cabin at their normal functioning air pressure for similar lengths of time and in one study only simulated cabin pressures at 7000 feet or more for more than 5 hours created fairly clear symptom deviation from normal. This study also suggested that acute mountain sickness has a negligible incidence in up to 10 hour flights with cabin pressures 6000 feet or under.
Standard fitness-to-fly assessments give patients 15% O2 to breathe, followed by assessment of arterial oxygen saturation and possibly blood gases. A saturation over 90% (on 15% O2) suggests a short haul or domestic flight is unlikely to cause them great trouble. The British Thoracic Society offers detailed advice. 
The mean SaO2 for healthy passengers at cabin pressure is 97%. However 54% of passengers have O2 sat of 94% or less. 
- Flying and lung disease
The main issue is potential hypoxia. This is much more important in unpressurised aircraft.
- Flying and heart disease
Cardiac arrhythmias are a significant issue, even to pilots who if they have them are likely to lose their commercial license. There are distinct changes in the human ECG with altitudes above those commercial aircraft are pressurised to and pacemaker threshold can increase, so again much more important in unpressurised aircraft.
Tuberculosis and flying
Flying and Pregnancy
Typically passenger travel is unrestricted up to 28 weeks, patients are asked to carry a letter including their expected date of delivery after then, women with uncomplicated singleton pregnancies are carried without argument up to the end of the 36th week and those with multiple pregnancies up to the end of the 32nd week.
Insurance may be an issue.
Flying and DVT
Depending on the relative risk from patient history, recent events and duration of travel (consider coach transfers which may be no less hazardous than flights) advice to move the ankles, wearing compression "flight socks" and injections of low molecular weight heparin may be beneficial. BCSH offers a guideline noting that there is no evidence for Aspirin.  
Flying and Diabetes
Aircraft holds are less likely to reach freezing temperatures than used to be the case. The effort to ensure insulin arrives together with the patient is a more compelling reason for it going in hand baggage. Customs and airlines may want proof that the person carrying syringes and ampoules is diabetic - a letter from a clinic or general practice to this effect should suffice. Logically, one must be ready to confirm this at arbitrary local times, but in practice it is probably just another piece of paper of little or no actual security value. (See also Get a note from your doctor).
Provided travel is only to the North or South no special problems of dosage and timing arise, however when travelling East or West a gradual adjustment from departure location to destination is needed. Probably the best way to do this is to travel by ship, walking two miles around the upper deck each day.
Aircraft moderately often decompress in flight, and this has occasionally produced otic barotrauma. Cabin crew with perforated eardrums should be rested from flight until the perforation has healed, and the tympanum is expected to be solidly healed, as regular changes in pressure seem likely to delay or prevent the hole sealing. Depending on the size of the perforation and the speed of return of normal hearing this may require varying time. If the perforation leaves a thin patch in the drum further trouble may ensue.
Flying and Diving
There is no problem flying and diving, but diving and flying increases the risk of decompression sickness - "the bends". The risk depends on many factors some of which are individual to the diver, and need some thought. Pragmatically, for ordinary sport divers coming home from a week or two diving on air, a 24 hour gap between surfacing and their next ascent is probably all right.
Flying and Influenza
A low but measurable risk of transmission of influenza was able to be established in modern commercial air travel at the start of the A/H1N1 pandemic. However the risk is much greater if the airliner's normal ventilation fails. Modern aircraft usually have HEPA filters, which filter out viruses. Air is recirculated in the coronal plane of the aircraft, so the risk is greatest in the same row as the infectious patient, and falls off fairly quickly with distance forward and aft of the patient.
The British Committee for Standards in Haematology (BCSH) is a sub-committee of the British Society for Haematology (BSH).
- "Pulse" article advising GPs how to respond to queries about when it's [un]safe to fly.
- Air New Zealand "Travelling with a Medical Condition" information, including information on fitness to fly.
- Aerospace Medical Association website
- Guidelines on health issues related to flying from WHO, International Civil Aviation Organization and the International Air Transport Association
- "Assessing fitness to fly. Guidelines for medical professionals from the Aviation Health Unit, UK Civil Aviation Authority."
- Patient Flying with Medical Conditions page
- British Airways health services: your patient and air travel - a guide to physicians.
- ↑ Muhm JM, Rock PB, McMullin DL, Jones SP, Lu IL, Eilers KD, et al. Effect of aircraft-cabin altitude on passenger discomfort. The New England journal of medicine;357(1):18-27. (Direct link – subscription may be required.)
- ↑ Mantziari L, Styliadis C, Kourtidou-Papadeli C, Styliadis I. Arrhythmias, Sudden Cardiac Death and incapacitation of pilots. Hippokratia. Aug; 12(Suppl 1):53-8.
- ↑ Tuberculosis and Air Travel: Guidelines for prevention and control (second edition,) – WHO Global Tuberculosis Programme, Geneva (p16)
- ↑ British Committee for Standards in Haematology Guidelines
- ↑ Prodigy
- ↑ Baker MG, Thornley CN, Mills C, Roberts S, Perera S, Peters J, Kelso A, Barr I, Wilson N. Transmission of pandemic A/H1N1 influenza on passenger aircraft: retrospective cohort study. BMJ (Clinical research ed.). 340:c2424.(Epub)
- ↑ Moser MR, Bender TR, Margolis HS, Noble GR, Kendal AP, Ritter DG. An outbreak of influenza aboard a commercial airliner. American journal of epidemiology. 1979 Jul; 110(1):1-6.
- ↑ Nicholson A, Bagshaw M, Giangrande P, Archard G. Need to know - fitness to fly. Pulse. Last viewed 10 December.
- ↑ Air New Zealand. "Travelling with a Medical Condition" Copyright. Last viewed (2 June)
- ↑ International travel and health: situation as on 1 January. World Health Organisation
- ↑ Patient Flying with Medical Conditions page. Acknowledgements EMIS is grateful to Dr M Preston for writing this article and to Dr Sean Kavanagh for earlier versions. The final copy has passed scrutiny by the independent Mentor GP reviewing team. ©EMIS. DocID: 2156. Document Version: 21. DocRef: bgp2179. Last Updated: 11 Sep. Review Date: 11 Sep
- ↑ British Airways health services: your patient and air travel - a guide to physicians. Undated. Last viewed 7 Apr.