UNITS OF ALCOHOL

Alcohol is back on the agenda following the newly introduced Railways and Transport Safety Bill which limits pilots to 20mg in 100ml of blood when "performing an aviation function or an activity ancillary thereto. There has been much talk in the past of "Units" of alcohol and how many are "safe", how much they increase blood alcohol levels and how quickly they are eliminated by the body. Indeed, you are still asked to estimate your weekly average intake of units each time you fill in your JAA medical application form.

It is still said that a unit is equivalent to half a pint of beer, a small glass of wine or a pub measure of spirit (1/6 of a gill if that means anything to anyone), that a unit increases the blood alcohol by about 20mg per 100 ml blood so that four units will reach the 80mg per 100 ml drink/drive limit in the UK, and that the alcohol is then broken down and eliminated by the liver at the rate of about 20mg per hour. Many pints have been sunk in the pub by patrons while calculating when it is safe to go home!

Of course, all this unit stuff is nonsense. Firstly there is immense individual variation on how quickly the alcohol is absorbed and eliminated, depending on such things as body weight, recent food intake, rate of ingestion, "training". Secondly, not everybody metabolises alcohol at 20mg per hour. Some will do so at 15mg per hour and a small percentage are known to do so at 10mg per hour.

More importantly it is most misleading to suggest the number of units a drink might contain, as naturally it depends on the strength of the drink. 

The true definition of one unit in the UK is 10ml (8g) of pure Ethyl alcohol.

But in Australia 1 unit = 10g, in USA 12g, in Russia probably a bottle of vodka!

In the UK, the ml of ethyl alcohol in a drink = volume x (% Alcohol by Volume(ABV) / 100)

So 1 pint (568ml!) of Bass (4.4 % ABV) = 568 x (4.4/100) = 25ml Ethyl Alcohol = 2.5 units

A few years ago it was recommended that each bottle, can etc., had the equivalent units displayed on them. This did not happen, but what we do have on them, and also on most beer pumps in the pub, is the ABV. This can vary enormously; beers for instance are usually between 3 - 9 % ABV (Grolsh, Speckled Hen are 5 %) though the more specialised Belgian beers can exceed 11 %! This means that a pint can be equivalent of anything between 2 - 5 units! Most wine, including Champagne, is around 12 %, spirits around 40 %.

Beware! Counting units or relying on "8-hours from bottle to throttle" are unreliable methods of assessing your fitness to fly.

Dr S A Goodwin
21st July 2003

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