Blood alcohol concentration (BAC)
Please note: This information does not constitute legal advice and should not be relied upon in this way. The information is correct at the time of publication. For information specific to your situation contact a legal aid service in your state or territory.
Blood alcohol concentration (BAC) refers to the amount of alcohol present in the bloodstream.
A BAC of 0.05% (point 0 five) means that there is 0.05 grams of alcohol in every 100 millilitres of blood.
Factors affecting the BAC
The more a person drinks, the higher their BAC, however, two people who drink the same amount of alcohol might register quite different BACs. This is due to a variety of factors including body size, whether or not they have eaten recently, percentage of body fat, whether they are male or female and whether or not they have drunk before.
A smaller person will have a higher BAC than a larger person for the same amount of alcohol consumed, because the alcohol is concentrated in a smaller body mass.
A person with an empty stomach will reach a higher BAC sooner than someone who has just eaten a meal, because food in the stomach slows the rate at which alcohol passes into the bloodstream. However, eating before drinking does not prevent intoxication.
Alcohol is not absorbed into fatty tissue, therefore people with a greater proportion of body fat will develop a high BAC more quickly than those who have lower proportions of fatty tissue. This occurs because the alcohol becomes concentrated in the smaller mass of non-fatty tissue.
Women’s bodies, being generally smaller than those of men and with a higher ratio of fatty tissue to lean muscle, will absorb alcohol more quickly than men’s bodies do. This means that a woman drinking the same amount of alcohol as a man, will develop a high BAC more quickly than the man will.
Less experienced drinkers have a lower tolerance to alcohol and so their BAC is likely to rise more quickly than it will in more experienced drinkers.
Alcohol is a depressant drug. It slows down the activity of the central nervous system, including the brain.
Alcohol could affect your driving by causing:
- impaired vision
- reduced reaction times
- reduced concentration and vigilance
- feeling more relaxed and drowsy, which may cause a driver to fall asleep at the wheel
- difficulty in understanding sensory information
- difficulty doing several tasks at once (e.g. keep in the lane and in the right direction, while concentrating on other traffic)
- failure to obey road rules
- over confidence, which may lead to risk taking.
The hangover effects of alcohol, the next day, can make it hard to concentrate and drive safely, and might cause you to fall asleep while driving.
A person who has been drinking alcohol may think that if they are especially careful, they will be able to drive safely. However, the alcohol may have affected their view and experience of reality. Their actions and responses may be quite different to what is actually needed, but they may be unaware of how much their driving skills have been affected.
Tips for driving safely
If you intend to drive, the safest option is not to drink.
Keep track of how much you drink
If you do decide to drink, keep track of how much you drink, by monitoring the number of standard drinks you consume each hour.
Some people may need to drink less to keep their blood alcohol concentration (BAC) under 0.05 and drive safely. Find out more about BAC.
Limit your drinking
- Start with a non-alcoholic drink, and have a non-alcoholic drink (a “spacer”) every second or third drink.
- Avoid topping up your glass, as it makes it difficult to keep an accurate track of how much you’ve had.
- Drink low-alcohol drinks, and avoid mixed drinks, like cocktails, as it is difficult to tell how much alcohol they contain.
- Avoid drinking in shouts or rounds, so you don’t feel pressured to keep up with your friends.
- Sip drinks, and avoid salty snacks or other food that increase thirst.
Wait for your BAC to drop before driving
It is important to remember that BAC can continue to rise up up to 3 hours after the last drink was consumed.
The only way to remove alcohol from your system is to allow the body time to process it. Showers, coffee or fresh air will not reduce BAC.
On average, the liver breaks down a little less than one standard drink per hour. Before driving, you should wait at least an hour for each standard drink you’ve had.
Have a back-up plan
If you have too much to drink, be prepared to make other arrangements so you don’t have to drive to get home.
For more information, please click on the Alcohol and Drug Foundation’s (previously Australian Drug Foundation) logo below.
Last Reviewed: 08/06/2011
Reproduced with kind permission from the Australian Drug Foundation.
1. Australian Drug Foundation. Blood Alcohol Concentration (BAC). Last updated 8 June 2011. http://www.druginfo.adf.org.au/topics/bac (accessed Feb 2013).
2. Australian Drug Foundation. How does alcohol affect driving? Last updated 27 April 2012. http://www.druginfo.adf.org.au/topics/how-does-alcohol-affect-driving (accessed Jan 2013).