February 14, 2017

Blood alcohol levels

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The level of alcohol in your bloodstream is called blood alcohol concentration. (BAC). BAC is what police test for in roadside alcohol breath tests.

A BAC of 0.05% (point 0 five) means that there is 0.05g of alcohol in every 100ml of blood. This is the legal limit for driving in Australia. To avoid a driving under the influence conviction, your BAC must be under 0.05%.

How do I stay below 0.05?

To stay under the limit:

  • For men of average size: no more than 2 standard drinks in the first hour and 1 drink per hour after that
  • For women of average size: no more than 1 standard drink per hour.

This is only a guide. You might still register above 0.05% even if you follow this.

What affects my BAC?

Your blood alcohol levels can be influenced by:

  • your body size
  • whether you have eaten or not
  • your body fat
  • how fast your body processes alcohol
  • your gender
  • how often you drink.


In general, the smaller you are, the less you can drink to stay under the limit.

Empty stomach

A person with an empty stomach will get drunk sooner than someone who has just eaten a meal. Alcohol can absorb into the stomach and bloodstream quicker when there’s no food. Eating won’t stop someone from getting drunk though.

Body fat

People who have more body fat will get drunk quicker.


Women's bodies, which are generally smaller than men’s and with a higher ratio of fatty tissue to lean muscle, will absorb alcohol quicker than men's bodies. This means that a woman drinking the same amount of alcohol as a man, will have a high BAC quicker than the man will.


Less experienced drinkers have a lower tolerance to alcohol and so their BAC is likely to rise faster than more experienced drinkers.

Why does my BAC matter?

Even very small amounts of alcohol can affect your concentration, judgement and performance. You may feel like you’re fine, but your reaction times will be slower than normal, and your concentration will be reduced.  This makes it difficult to perform activities that require skill and concentration, or where the safety of others is involved. For example, driving, flying, water sports, skiing, using complex or heavy machinery, or farm machinery.

Blood alcohol limits are set by legislation or company policy.


If you choose to drink, consider public transport, rideshare or organising a friend to be designated driver. In Australia, it’s illegal to drive if your blood alcohol level is over 0.05%. Even below this, your judgement, reaction times and driving skill are not as good as you think they are. For example, you are twice as likely to have a crash when driving with a BAC of 0.05%, versus no alcohol.

If you’re a learner driver, provisional or probationary driver (regardless of age), truck and bus driver, driving instructor or convicted of driving under the influence (DUI), you must have a 0.00% BAC in most Australian states or territories.


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

Some workplaces implement alcohol and drug testing as a protocol. You can be tested as part of pre-employment screening, at random or following an incident in the workplace.

Workplaces usually enforce alcohol testing, only where they are legally obligated or where employers, workers and unions agree that it’s necessary.

Measuring BAC

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


The police do breathalyser tests at random breath testing (RBT) roadblocks or if they think you’re drink driving.

If you blow 0.05% or over, they’ll need a second, evidentiary test at a police station. If your second result is over the legal limit, you’ll be charged with driving under the influence (DUI).

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