Alcohol and cars – a volatile mix
Key text
This topic is sponsored by NRMA – ACT Road Safety Trust.
Alcohol produces physiological effects on the human body that can turn safe drivers into potential killers.
According to the Australian Transport Safety Bureau, alcohol was implicated in about one quarter of all road deaths.
What is it about alcohol that turns even good drivers into potential killers? And how can individuals manage this pervasive drug so that they don’t become a danger on the roads?
The absorption of alcohol
The alcohol in every kind of drink – beer, wine and spirits – is called ethanol. There are many other kinds of alcohol, but most are more toxic than ethanol and not fit for human consumption.
Ethanol is a small but very active and volatile molecule. It dissolves readily in water and is quickly absorbed into the bloodstream. The key measure in determining the effect of ethanol on the body is its concentration in the blood. This is usually called the blood alcohol concentration (BAC) and is measured in grams of ethanol per 100 millilitres of blood. For example, men and women with a BAC of 0.05 grams per 100 millilitres – the legal limit for most drivers – have 0.05 grams of alcohol in their body for every 100 millilitres of their blood.
The rate of absorption of ethanol into the bloodstream helps determine the effect the ethanol has – the higher the rate of absorption, the higher the BAC will reach before the body’s metabolising processes convert the ethanol into other substances.
When you have an alcoholic drink, ethanol is absorbed in three different places: to a minor extent through the tissues of the mouth and throat, to a greater extent through the stomach wall, and to the greatest extent through the wall of the small intestine, which the ethanol enters through a sphincter at the bottom of the stomach. The small intestine absorbs alcohol much more quickly than do other tissues and organs. Thus, the rate at which material passes from the stomach to the small intestine – ‘gastric emptying’ – is a key factor in determining how quickly ethanol is taken up into the bloodstream.
Metabolising of ethanol
Most of the body's metabolising of ethanol into other substances takes place in the liver, which is the major detoxifying organ in the body. The liver first converts the ethanol to acetaldehyde, then to acetic acid and finally to carbon dioxide and water. Some ethanol is also expelled from the lungs through the act of breathing. This is handy for police: breathalysers analyse the ethanol content of air exhaled from the lungs and convert it to BAC by way of a special formula.
Effects on the brain
Alcohol – ethanol – is a depressant, which means that it slows the activity of the central nervous system (Box 1: Ethanol and the brain), increasing the time taken to react, diminishing judgment and reducing motor control. The depressant effect of alcohol puzzles some people; after all, some of the expressions associated with drinking – like ‘happy hour’ and ‘high as a kite’ – as well as the antics of many intoxicated people, seem to contradict this idea.
But the apparent stimulatory effects of alcohol can be explained in terms of its depressive effects. The brain contains inhibitory centres that ordinarily moderate a person’s behaviour. Ethanol depresses the activity of these centres, thereby removing some of the barriers to extroverted, loud and aggressive behaviour. This effect can be dangerous on the road. While under the influence of alcohol we may be more inclined to drive above the speed limit, take unnecessary risks or otherwise engage in acts of foolish bravado, especially in the presence of our peers.
There are other depressant effects that influence our capacity to perform well on the road. Several studies have demonstrated that a BAC of 0.1 – twice the legal limit – results in a 300-400 per cent increase in body sway, due to the inhibition of the brain’s sensorimotor coordination function. In another study, a BAC of 0.065, only marginally above the legal limit, resulted in a 10-12 per cent reduction in measures of hand-eye coordination. And other studies suggest that a BAC of 0.05-0.l can decrease perceptive abilities to such an extent that reaction time is slowed by up to 20 per cent. The general conclusion of scientists is that impairment of the brain’s ability to process information, make decisions and carry them out is evident above a BAC of 0.05 and markedly affected above 0.1.
The net effect on driving
Competent driving requires a variety of different skills – an ability to physically operate the car, an ability to perform more than one task at a time, an awareness of potential hazards, a capacity to react quickly to danger, and so on. Virtually all these skills diminish with increasing BAC. This is borne out by research suggesting that the risk of involvement in a motor vehicle crash in which one or more people are killed or require hospital treatment doubles with a BAC of 0.05 and increases more than four-fold with a BAC of 0.12.
How much can I drink and still drive?
In all Australian States and Territories, you commit an offence if you drive while your BAC is 0.05 or above. The legal limit is lower for certain road user groups (eg, those who hold learner or provisional licences). Depending on the State or Territory, this lower limit is either zero or 0.02.
How does this translate into a ‘safe’ number of drinks? For those who must stay under 0.02, that’s easy – you can’t drive a car or ride a motorbike after even a single drink. For those permitted to drive with a BAC less than 0.05, it gets a bit more complicated. Your BAC at any given time is determined by a number of factors, including:
- the quantity and concentration of the alcoholic beverage consumed;
- your health, sex, body weight and body water;
- the rate at which your liver metabolises alcohol (which may depend on its size, condition and ‘tolerance’ of alcohol);
- time after drinking;
- whether or not any alcohol is metabolised in your stomach; and
- the amount of food in your stomach.
There are other factors: for example, drinks with a lot of fizz – such as champagne – are absorbed into the bloodstream more quickly than, say, wine. This is because the fizz irritates the stomach’s sphincter, which opens to allow the liquid into the small intestine where the rate of absorption is much higher. (The expression ‘champagne goes straight to my head’ is only partially incorrect. In fact, it goes straight to the lower intestine, from where it goes straight to your head.)
This all means that calculating your BAC, and therefore your fitness to drive, is difficult. Nevertheless, various agencies have come up with approaches that give good rules of thumb for deciding how many drinks you can safely drink over a given time and how long you should wait after drinking before driving.
You can also make use of breath-testers, which are coin-operated machines available in many pubs and clubs. You can even do an experiment: drink about as much as you normally would and then monitor your BAC as it rises, peaks and slowly declines. How long do you need to wait before it is safe to drive? Conducting the experiment on different occasions or with different beverages will show how different factors affect results.
Plan ahead
By planning ahead you can avoid having to calculate your BAC at all. For example, when you go out with a group of friends, choose a 'designated driver'. The designated driver does not consume any alcohol. Or leave your car at home and use public transport or taxis.
Clear thinking?
The chances of becoming a crash victim increase dramatically when a driver is under the influence of alcohol, so it makes sense to avoid situations where you drink and then drive. But here’s a catch-22: once you start drinking, your ability to think clearly and act wisely diminishes. Recognising this while sober could be the first step towards safeguarding your own life, and the lives of others.
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Posted October 1999.