“One beer on the ground is equivalent to two in the air when flying.” ‘Everybody’ knows that’s true. Don’t they? Professor Michael Bagshaw separates fact from fiction.
Back in 1953, in a book about human factors in air transportation, Ross A. McFarland wrote that the alcohol in two or three cocktails would have the physiological action of four or five drinks at altitudes of 10,000 to 12,000 feet. This was based on work done in the 1930s to look at the relationship between alcohol intoxication and lack of oxygen. Obviously alcohol affects performance flying an aeroplane or driving a car, as does lack of oxygen, so it is hardly surprising that the two effects are assumed to be linked. Indeed, up above 20,000 feet they are.
However, this article is considering the effects on the occupants of commercial airliners where the cabin altitude is much lower.
Alcohol Metabolism
Alcohol is readily absorbed from the stomach and the gut, and there is considerable individual variation. It depends on gender (females generally absorb alcohol more quickly than males), body weight, the rate of consumption and obviously the total amount consumed. It is easy to forget that absorption of alcohol still present in the gut continues after drinking has stopped.
There is a range of effects as alcohol gets absorbed, including impairment of vision, balance, cognitive processing, memory and sleep. But of most concern in an aircraft are the behavioural effects of disinhibition, confusion and impulsive, disruptive, violent behaviour. This can be made worse by hypoglycaemia, the drop in blood sugar which occurs 6 to 36 hours after drinking alcohol.
Alcohol is eliminated from the body by a process of oxidation via the liver enzymes and, unlike the rate of absorption, there is little individual variation. The liver processes about 90% and the remaining 10% is eliminated in the breath and sweat. The rate of removal is 0.015% blood alcohol concentration (BAC) per hour. This rate is fixed and cannot be influenced by drinking black coffee, going for a run, taking a cold shower or eating a bacon sandwich; you might feel better, but the blood alcohol level remains unchanged together with the associated cognitive effects. And remember that alcohol enhances the dehydrating effect of caffeine, so perhaps that black coffee is not such a good idea after all!
Pressurisation
The aircraft cabin is pressurised to give a maximum altitude of 8,000 feet during normal operation, and rarely exceeds 5,000 to 7,000 feet during flight whatever the aircraft’s altitude.
Oxygen is carried in the blood-stream attached to haemoglobin and is expressed as a percentage saturation of oxyhaemoglobin, depending on the pressure of oxygen in the surrounding air (related to altitude). The good news is that the oxyhaemoglobin remains well-saturated at normal cruising altitude pressure, with a maximum desaturation of 10% in healthy individuals. This means that the body continues to work normally, including metabolism of alcohol and drugs. So no bad news.