Thomas McCabe describes a case of delirium apparently precipitated by flying. ^{Reference McCabe1} This is not unusual. Most liaison psychiatrists, especially old age liaison psychiatrists, will have seen several such cases and many of them after shorter flights. I reviewed the evidence for this in 2002 and 2012 and presented some ideas about how to proceed. ^{Reference Rands2,Reference Rands3}

Planes ascend to and descend from an altitude of between 30 and 40 000 feet above sea level in 20 minutes; in-flight cabin environments are artificially controlled, except for ambient radiation which is higher at these altitudes. The controlled cabin pressures are equivalent to those between 6000 and 8000 feet above sea level. At these altitudes, there is a reduction of between 20 and 26% of available oxygen, resulting in haemoxyhaemoglobin saturations between 83 and 85% of normal. In fit people this can cause tachycardia, tachypnoea, headaches, dizziness, impaired coordination, fatigue and confusion. Reduced air pressure can also cause peripheral oedema and expansion of any air-filled spaces such as the bowel, sinuses and middle ear (Boyle's law).

Air humidity is reduced at altitude. The ‘comfort zone’ for humidity is between 50 and 65%. A centrally heated room has about 25% humidity and air cabin humidity is between 1 and 20%. This can cause dehydration, hypovolaemia and reduced peripheral circulation.

Most research on passenger health has been done on fit young airmen, but the effect on the physiology of older or unwell people, or those with evidence of cerebrovascular disease, is unclear.

The Warsaw Convention (1929) states that airlines are not responsible for their customers' health; the fact that passengers are responsible for their own health may limit the industry's motivation to explore this further. The British Medical Association have published a review of the impact of flying on passengers' physical health. ⁴

For some years I have advised my patients with dementia – especially those with significant vascular aetiology – to avoid flying if possible. If flying is essential, I advise that they sit near the front of the plane (where there is higher oxygen saturation), drink plenty of non-alcoholic fluids and move as much as possible during the flight.

I agree with many of Dr McCabe's recommendations. The most important next step is research into the prevalence of post-flight confusion, its prognosis and risk factors. This would not be difficult to do, although it may be controversial. For instance, people over the age of 65 embarking on flights of more than 4 hours could be asked to complete cognitive tests before departure, within a week of return and a few months after return. Any signs of confusion would indicate further investigations. It may be that those who experience sundowning or psychotic symptoms are more at risk, but this needs researching. Only by identifying the extent and associated risk factors of post-flight confusion can good advice be given to patients, travellers and the travel industry. More knowledge would not only give clues as to aetiology but also inform risk reduction.