A Sure Way of Avoiding Being Caught By the Breathalyser


The Sunday Nation


11 December 2005


The legal alcohol limits announced by the Director Of Medical Services, Dr James Nyikal, have come as a surprise to many Kenyans. A contributor to the Cutting Edge column in the ‘Nation’ of December 8, said: "I find the maximum limit of alcohol allowed ridiculous and unrealistic. Who gets drunk after taking two bottles of beer or two glasses of wine?”

Dr Nyikal placed the maximum Blood Alcohol Content (BAC) limit at 50 milligrams of alcohol for every 100millilitres of blood. This is similar to many other countries in the world where the legal limits vary from 20mg per 100ml (e.g., Norway) to 80mg per 100ml (e.g., the United Kingdom).

The limit does not mean that if you have that much alcohol in your blood you will be staggering and singing songs from the sixties. No. It means that your reflexes and judgement will be impaired to level that makes it unsafe for you to drive. All the same, how does the 50mg per 100ml translate into volume of drink taken?

Regular beer contains about 4 per cent alcohol by volume. Therefore, one bottle (500ml) will have about 20ml of alcohol. With a density of about 0.8g per millilitre, this is equal to about 16 grams. If all of it was absorbed into the blood stream, what would be the Blood Alcohol Content (BAC)?

The average adult person has about five litres of blood, that is, 5,000ml. Adding 16g of alcohol to this gives a concentration of 0.32g per 100ml, or 320mg/100ml. This would be more than six times the legal limit of 50mg/100ml! However, alcohol is not absorbed at once after consumption and, in addition, the body begins to break it down as soon as it enters the blood stream. The ultimate rate of retention depends on a person’s weight, age, sex, race etc.

On average though, consuming one gram of alcohol raises the BAC by 1.5mg to 2mg per 100 ml within the first hour. In other words, one beer (500ml) with 4 per cent alcohol by volume would increase the BAC by 24mg to 32mg per 100ml in one hour. Therefore, taking two beers takes the BAC just below the limit.

Wines usually contain much more alcohol than beers. The concentration ranges from about 10 per cent to 20 per cent by volume. A typical 125ml wineglass contains between 12.5ml and 25ml of alcohol, that is 10g to 20g. This is approximately the same amount as that contained in a 500ml bottle of beer.

However, due to the high concentration, the alcohol in wine is absorbed into the blood stream faster than that in beer. As a result, one glass of wine increases the BAC by about 50mg/100ml, that is, just on the legal limit.

Clearly, the variations are very wide and it is difficult to tell whether what you drink will take your BAC above the limit. Therefore, the sure way to avoid being caught by the breathalyser is simply not to drink at all if you intend to drive.

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