BAC explained: Everything you need to know

BAC explained: Everything you need to know

What is BAC?

Blood alcohol concentration, or BAC, is a measure of how much alcohol is in the bloodstream. In Australia, the legal BAC limit for driving – or operating any motorised vehicle – is 0.05, which means there is 0.05g of alcohol in every 100ml of blood. 

How does your body process alcohol?

For an average adult, the body will typically metabolise 0.01 BAC per hour - roughly one [standard] drink per hour, but how many drinks one can have before reaching 0.05 varies from person to person.

Your body is effective at processing small amounts of alcohol to maintain a low BAC, Dr Benson says, but over the course of a day, the system for processing alcohol becomes weaker as it gets depleted.

If you're the designated driver, don't risk it. Stick to mocktails and soft drinks. 

What can affect BAC?

BAC is a relatively fluid concept (pardon the pun). “A number of things can impact your BAC reading,” Dr Benson explains. “From your height, weight and genetics to how much you’ve eaten – there is so much individual variation in how people react to alcohol. This is why it is such a difficult thing to understand."

As a general rule, the heavier someone is, the more body mass they have to absorb alcohol, which means they can drink more and still have the same BAC as someone who is shorter, less heavy or has less muscle.  

“It all comes down to the amount of fat and muscle a person has,” Dr Benson explains. “Fatty tissue has low water content, while muscle is high in water content. Alcohol is water soluble so the more muscle a person has, the more alcohol can be absorbed, which results in a lower BAC.” 

Why is it so important to eat if you’re drinking?

Dr Benson says the amount of food in your stomach “drastically influences your BAC”, which is why it’s never a good idea to drink on an empty stomach. “If you’re drinking on an empty stomach, alcohol will be absorbed into the bloodstream much quicker, which will result in a much higher BAC,” she says.

“The majority of alcohol you drink is absorbed in the stomach. Therefore, if you eat a substantial meal before drinking, alcohol will be absorbed a lot slower so you won’t get as drunk.” 

Having a chicken parma before you have a few pints of beer won't stop you from getting drunk - it will just delay how long it takes before you feel the effects. If you're planning on drinking, Dr Benson says, it's safest to avoid driving altogether. 

BAC myths, debunked

Eating fatty food will reduce BAC

There is not a lot of evidence showing that the type of food affects alcohol absorption. More important, Dr Benson says, is eating a substantial meal.  

This why many people often crave a late-night snack.

"If all the alcohol hasn’t been absorbed in the stomach, [eating before bed] may slow that down, and if you haven’t had dinner and you’ve been drinking, eating before bed will probably help your blood sugar,” Dr Benson says. “But it won’t guarantee you’ll wake up hangover free.” 

You can speed up how quickly your body processes a drink

Put simply, no, you can’t speed up how quickly your body processes alcohol. Some people think that exercising will help them sweat it out, or that going for a run will get their blood flowing, which will help them metabolise the alcohol quicker. But, Dr Benson assures, it won’t.

And sleeping on it isn't always the answer: depending on how many drinks you've had, you could still have a BAC reading of above 0.05 the following day.

Say you're having a dinner party and you drank two bottles of red wine with around 13.5 per cent of alcohol by volume (ABV) each. Over four hours, you will have consumed 16 standard drinks. If you started drinking at 7pm on Friday, you would still have alcohol in your system until at least 11am the following day. 

Staying hydrated will reduce your BAC

While being hydrated is really important while drinking, Dr Benson says skolling a bottle of water before a big day will have no bearing on your BAC. “BAC comes down to how much alcohol you’re drinking,” she says. “Being hydrated might help with the hangover, but it won’t change your BAC.” 

Drinking coffee will reduce BAC

Urban legend has it that there are hacks you can use to reduce your blood alcohol concentration, such as having a cold shower, drinking coffee or taking a nap. But, Dr Benson warns, none of it works. “Once alcohol is in the bloodstream, there is nothing you can do to lower it. You just have to wait it out. Some drugs might change your subjective feeling of intoxication, but nothing will change your actual BAC.”

You can cheat a breathalyser

Despite popular belief, when time comes to blow into the breathalyser there’s nothing you can do to mask your reading. When alcohol is absorbed into the bloodstream, Dr Benson explains, it goes through to the lungs, so you’re exhaling a bit of alcohol as well.

“There are so many stories I have heard of people trying to use tactics to trick the breathalyser,” she laughs. “Some people talk about using mouthwash, stuffing their mouths with cotton balls, breathing through the nose and exhaling through the mouth – not sure how this one is meant to work as the breath is still coming from your lungs – and even eating foods to mask the smell of alcohol in the mouth. But the reality is, the only way to get a lower BAC reading is to drink less alcohol.” 

How do breathalysers work?

Breathalysers are not measuring the alcohol in your mouth, Dr Benson clarifies, they’re measuring the alcohol that is on your breath when you exhale from the lungs. This is why a breathalyser can’t be used accurately on someone within 15 to 20 minutes of them having finished a drink, as there will still be residual alcohol in the mouth. 

Why shouldn’t you drink if you’re on antibiotics?

The reason doctors tell you to avoid alcohol if you’re taking medication – particularly prescription medication – is because the alcohol may interfere with the medication’s effectiveness, induce adverse symptoms. The medication may also enhance the effects of the alcohol. Drinking alcohol while taking antidepressants, for example, can make you feel sick and exacerbate feelings of intoxication.

But when it comes to most antibiotics, Dr Benson says, avoiding alcohol is advised for different reasons. “When antibiotics were first designed, they were used to treat STIs,” she explains. “So it’s been speculated that they told people not to drink because they didn’t want people to have sex and either pass on an STI or make their own worse.” 

If you’re taking any medication, Dr Benson says to always consult your doctor before drinking. 

What effect does even one drink have on your neurocognition?

When it comes to the effects of alcohol, impairment is directly related to the level of intoxication. 

“At very low BACs, say one or two drinks, most people will actually appear normal,” Dr Benson says. “There are very subtle side effects, but they can be hard to pick up on. Once someone is between 0.05-0.1, they may have blunted feelings or feel more disinhibited. They will also experience some level of cognitive impairment. They might have trouble with their psychomotor skills, have delayed reaction times, impaired reflexes, slurred speech, among other things. A BAC reading above 0.4-0.5 can be deadly. "

If you're worried you might be over the limit and, sometimes, even if you're not), the best option is to play it safe and not get behind the wheel.

What is a standard drink?

Any alcoholic drink that contains 10 grams of alcohol is called a standard drink. Drinks come in different sizes and strengths so, depending on the alcohol content of a particular drink, it can take varying amounts of any particular beverage to make up a standard drink. 

Standard drinks in Australia

• 30ml of spirits (40% alc/vol)
• 60ml of fortified wine (18% alc/vol)
• 100ml of champagne (12% alc/vol)
• 100ml of wine (12% alc/vol)
• 150ml of light wine (8% alc/vol)
• 280ml of standard strength beer (4.9% alc/vol)
•  375ml of low-alcohol beer (2.8% alc/vol)

Standard drinks v standard serving sizes

Confusingly, a standard drink is not always the same as a restaurant serving of wine or beer so standard drink sizes are a guide only. Many beers and wines have higher ABV than listed below, which can greatly impact the number of standard drinks in a single serve. For example, a standard restaurant pour of red wine with an ABV of 15.5 per cent (which is not uncommon for many Australian shiraz wines), could contain 1.8 standard drinks. It's important to always read the label to find out how many standard drinks are in any given alcoholic beverage.

How can I calculate standard drinks?

The formula to work out the number of standard drinks in any alcohol beverage is:
Drink in litres (ie. 150ml) x ABV (ie. 14.5%) x 0.789 (density of ethanol at room temperature)
= Standard drinks

Standard drinks in Australia

*Drink sizes and ABV measurements taken from the Department of Health's Standard Drink Guide. 

Australian Drinking Guidelines

The Australian Guidelines recommend healthy adults should drink no more than two standard drinks on any day to cut the lifetime risk of harm from alcohol-related disease or injury. They also recommend consuming a maximum of four standard drinks on a single occasion to reduce the risk of alcohol-related injury. Visit health.gov.au for more information.