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Blood alcohol concentration (BAC)

Definition

Factors affecting the BAC

Safety considerations

Measuring BAC

How can I keep my BAC below 0.05%?

Legal Disclaimer:
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.

Definition

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.

Body size

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.

Empty stomach

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.

Body fat

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.

Gender

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.

Inexperience

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.

Safety considerations

Even very small amounts of alcohol can affect concentration, judgement and performance. This may be important where a high degree of skill is needed, or if the safety of others is involved, such as with recreational and occupational activities like driving, flying, water sports, skiing, using complex or heavy machinery, or farm machinery.

Blood alcohol limits are set by legislation and/or company policy.

Driving

In Australia, the legal maximum BAC is 0.05% for licenced drivers in private vehicles.

Learner drivers, provisional or probationary drivers (regardless of age), truck and bus drivers, driving instructors and drivers convicted of "driving under the influence" (DUI) have a limit of 0.00% BAC in most Australian states or territories. This is the case for probationary and learner drivers in Victoria.

In Victoria and most Australian states and territories, it is illegal to drive with a BAC greater than 0.05%, regardless of age or experience. However, judgement and performance may be affected at blood alcohol concentrations lower than 0.05%. Research shows that at a BAC of 0.05%, the risk of crashing is twice as high as when no alcohol is consumed.

Working

Blood alcohol limits apply in occupational settings such as flying aircraft (e.g. no alcohol within 24 hours of flying); driving public or heavy vehicles or operating commercial vessels, machinery or mobile plant. In each case the BAC limit is zero (0.00%).

Measuring BAC

Blood alcohol concentration may be measured with a breathalyser, or by analysing a sample of blood, saliva or urine.

Each testing method has its limitations and the most suitable method is dependent upon the pupose of testing.

Workers

Workplace alcohol and other drug (AOD) testing has become a component in some workplaces in Australia. Testing can be used for pre-employment screening or may be random or following an incident in the workplace.

Workplace alcohol testing is generally only undertaken where there are existing legislative provisions or where employers, health and safety representatives, workers and unions formally agree that it is warranted, or where it might be held to be reasonable in the circumstances.

If testing is implemented it should be as part of a comprehensive occupational health and safety program with appropriate safeguards, clear policy and procedures, and provision of education and counselling.

Drivers

Police generally breath test drivers suspected of being impaired due to alcohol, or if the driver is subject to a random breath test.

If the result of a preliminary breath test indicates that a driver is over the legal BAC limit, they are required to have a second, evidentiary, test. If this proves that the driver is over the legal limit, they will be penalised.

In Victoria, a BAC between 0.05% and 0.07% may result in demerit points and a fine. A BAC of over 0.07% may result in a fine and the cancellation of the driver's licence.

Read more about roadside drug testing.

How can I keep my BAC below 0.05%?

For men of average size, blood alcohol concentration should remain below 0.05% if no more than two standard drinks are consumed in the first hour and no more than one per hour after that.

For women of average size, no more than one standard drink per hour should be consumed in order to stay under the limit.

However, the effects of alcohol can vary significantly from person to person, so the above formula can only be regarded as a guide.

Last updated: 21 May 2014

 


Information you heard is intended as a general guide only. This audio is copyrighted by the Australian Drug Foundation. Visit www.DrugInfo.ADF.org.au for more