Three male and three female volunteers, of varying ages and weights, trialled the breathalysers. After drinking no alcohol for 24 hours they first rated each breathalyser for its instructions and ease of use — and then had lunch and started drinking.
To keep the trial realistic, we let them drink as much as they wanted, pouring the amounts they’d normally pour, because most people don’t drink strictly according to standard drink servings. However, we did measure how many standard drinks each trialist consumed, in order to give a useful comparison to government advertised safe drinking guidelines (see How many drinks?).
After about three hours each trialist waited for 20 minutes to allow any mouth alcohol to disappear, then took a BAC reading from each device. They also had blood samples taken at the same time. After another hour of drinking, a second round of readings and blood samples were taken. The blood samples were analysed for BAC at a professional pathology lab, and we used the results as a benchmark for the breathalysers.
Breathalysers measure breath alcohol rather than blood alcohol, and while there’s an accepted ratio between the two, it can vary slightly from person to person. Breathalysers are therefore generally calibrated to err on the side of caution. Because of this we decided it was OK for a BAC of 0.045 or more to give a reading of 0.05 or more, or ‘over the limit’, depending on the type of breathalyser.
How many drinks?
Blood alcohol concentration, or BAC, is usually expressed as the number of grams of alcohol per 100 millilitres of blood. It's just a measurement: different people will have a different BAC after drinking the same amount, and will be affected differently by it. But for legal purposes it's useful for setting limits.
In Australia, it's illegal to drive with a BAC of 0.05 or over. (That's one twentieth of a gram of alcohol in each 100 mL of blood.)
Most states and territories have further limitations as well, such as zero or 0.02 limits for learners, probationary and commercial vehicle drivers. These vary from state to state so if you're not sure, check with your state government.
Staying under 0.05
A commonly accepted guide for staying under 0.05 BAC is:
- Men: Two standard drinks in the first hour and one per hour after that.
- Women: One standard drink per hour. (But see below for more on standard drinks.)
Generally, it takes one hour for your body to completely process the alcohol from one standard drink. Exercise, drinking coffee or eating food won't speed this up. This is only a general guide, as many factors affect your individual BAC:
Body size and build. Lean muscle tissue contains more water (and can therefore take up more alcohol) than fatty tissue, so the more lean muscle you have, the less concentrated the alcohol in your blood is and the lower your BAC. So for the same amount of alcohol consumed, a larger person will tend to have a lower BAC than a smaller person, and a lean person will have a lower BAC than someone of the same weight but with more body fat.
Gender. A woman who drinks the same amount as a man of the same size will usually have a higher BAC. Women tend to have a higher proportion of body fat than men, plus have fewer of certain enzymes that break down alcohol.
Food consumption. Food in your stomach slows alcohol absorption and reduces your peak BAC. However, most of the alcohol will reach your bloodstream eventually.
General health. If you're in poor health, your liver will be less able to deal with the alcohol you drink and it will remain in your blood for longer. Some medicines can also increase the effects of alcohol, so be careful about drinking if you're on any medication.
What's a standard drink?
A standard drink is a measure used to define safe drinking levels. It's basically any drink that contains approximately 10 grams of alcohol: the amount the average body can process in one hour. Common standard drink sizes include:
425 mL of light beer (2.7% alcohol/vol).
285 mL of full-strength beer (4.9% alcohol/vol).
Small glass of wine (100 mL, 12% alcohol/vol).
Nip of spirits (30 mL, 40% alcohol/vol).
Most people don't in fact pour standard drinks. Many people's idea of a glass of wine, for example, is more like 200 mL. That's two standard drinks in one glass; even more if the wine is stronger than 12%, as many are. If you're drinking anything other than precise bar measures you're likely to be drinking more than a standard drink.