Blood alcohol, what is your limit?

As you should all know, the legal limit for drinking alcohol and driving was lowered in Scotland on December 5^th.  From now on, a blood alcohol concentration of 50mg/100mls blood is considered over the legal limit (down from 80mg/100mls). Personally it made me realise that I did not fully understand the initial drink drive limit. Before it was commonly accepted to have a single pint or glass of wine and still be perfectly acceptable to drive however this 30mg reduction seems to have made all the difference. This article I will look at how your body processes alcohol and at what stage it becomes legal and safe to operate a vehicle.

An example of when it is not ok to drive soon.

Alcohol in your blood

The more I have looked into this topic the more complicated I have found it. Primarily due to the fact that there are many determining factors from the strength and volume of drink, to the size and weight of the drinker. Not to mention this persons particular tolerance to alcohol. Also a lot of research was conducted in America where they measure both blood alcohol content in different units but also have different serving volumes of alcohol. To put it simply, blood alcohol content is the blood ethanol concentration. In the case of Britain, it is expressed in mg/ml.

Without properly calibrated equipment, it is very difficult to provide an accurate reading of blood alcohol content. There are several “calculators” available online that give you an estimation so I tested to see what effect a small amount of alcohol can have on your body. The test provided the following result: As a 75kg male, if I consumed 2 pints and was tested one hour afterwards, my blood alcohol content would be 57mg/100ml, over the legal limit. Not only that, it would take in excess of 4 hours for that alcohol to completely leave my system. Obviously these online calculators must be taken with a pinch of salt but it shows how little alcohol it takes to be over the limit and also how long it remains in your system for.

Alcohol absorption, distribution and elimination. It takes longer than you might think.

Another major cause for concern is people driving after a drinking the night before. Many of us assume that after a few hours’ sleep, all the alcohol in our system will be gone. This however is not the case. Alcohol is absorbed through the GI tract: rapidly in the small and large intestine (due to its large surface area) but also slowly within the stomach itself. Post-absorption, alcohol is passed via the hepatic portal vein to the liver before entering the bloodstream. Alcohol has a high affinity for water and is therefore transported in the blood throughout the body. Once absorption is complete, an equilibrium occurs such that the blood at all points within the body contains approximately the same concentration of alcohol.

From the bloodstream, alcohol must be removed by a combination of metabolism, excretion and evaporation. The predominance of alcohol removal is via liver metabolism. Alcohol dehydrogenase converts ethanol into acetaldehyde (If you remember a previous blog on hangovers you will be familiar with this and if you haven’t read it, read it now! http://www.stannagescience.blogspot.co.uk/2014/08/the-hangover.html). A healthy individual will metabolise alcohol at a fairly consistent rate. Several factors however will influence the rate in which alcohol is metabolised. Rate of elimination tends to be higher when blood alcohol concentration is very high (potentially as the body considers the levels of toxins as a serious threat and invests more metabolic energy to remove it). Also, chronic alcoholics may actually be able to metabolise alcohol significantly quicker due to increased tolerance. The body’s ability to metabolise alcohol quickly diminishes with age, meaning older people take longer to process the same amount of alcohol as a younger person.

In terms of how long alcohol stays in your bloodstream, it is very difficult to predict. As stated before, there are many contributing factors to the speed of alcohol metabolism and as a result, will vary from person to person how quickly it is eliminated. If you consider a normal, healthy liver. It will metabolise alcohol at a constant rate – approximately 1 gram per hour for every 10kg of bodyweight. This is around 1 unit per hour post-consumption.

It is important to note that there is nothing that you can do to speed up the metabolism and therefore the elimination of alcohol in your system. Having a cup of coffee or a cold shower may make you feel more sober and alert but it has in no way reduced your blood alcohol concentration. When it comes to driving, it really is not worth the risk.

Even drinking plenty of water does not reduce the blood alcohol concentration, even if it does make you feel better.

I found a plethora of fairly average blood alcohol concentration calculators but I found this is one considers more factors and provides more information so I would recommend having a play on this: http://www.alcoholhelpcenter.net/program/BAC_Standalone.aspx

(units are in g/100ml so multiply by 100 to get the UK standard measurement of mg/100ml)