What Is a Breath Alcohol Test?

Medically Reviewed by Carol DerSarkissian, MD on February 20, 2024
3 min read

When you drink alcohol, it goes into your stomach and small intestine. It gets absorbed into your blood, which carries it through your body and into your brain and lungs. You exhale it when you breathe.

A breath alcohol test measures how much alcohol is in the air you breathe out. The device uses that measurement to estimate how much alcohol is in your blood. That number is known as your BAC, or blood alcohol content.

It may go up as soon as 15 minutes after drinking. BAC is usually highest about an hour after you drink.

As your BAC rises, you can get clumsy and take longer to react. You may not make good choices, either. These things make driving dangerous.

In every state but one, it’s illegal for a driver over the age of 21 to have a BAC above 0.08%. As of December 2018, Utah’s BAC level will be 0.05%. All states have zero tolerance laws for drivers under 21.

If you’re speeding, in an accident, or weaving on the road, local police may suspect you of driving under the influence, or DUI. They can use a device known as a Breathalyzer to test your BAC right at the scene of an accident or on the side of the road if they pull you over.

Tests can also be manual or electronic. Most police use an electronic device about the size of a walkie-talkie. You blow into a mouthpiece, and it gives an immediate reading. You may be asked to repeat this a few times so the officer can get an average reading. It takes about a minute, and it doesn’t hurt.

The most common manual test includes a balloon and a glass tube filled with yellow crystals. You blow into the balloon and release the air into the tube. The bands of crystals in the tube change color from yellow to green depending on how much alcohol is in your system.

Check the instructions included with the device to read the results. Generally, one green band means your BAC is under 0.05%, which is within the legal limit to drive. Two green bands indicate that your BAC is between 0.05% and 0.10%, and three bands mean it’s over 0.10%.

You can buy either type of test for yourself if you want to make sure you’re safe before you get behind the wheel. The manual ones are less expensive.

Not always. There are a few things that could cause an error in the reading.

If you had a drink 15 minutes before the test, trace amounts of alcohol in your mouth could lead to an inaccurate result. Smoking can also affect results. So can products that contain alcohol, like mouthwash and breath fresheners.

Sometimes the machines need to be recalibrated or have batteries replaced. These possibly could affect the reading.

Some tests have software that needs to be updated occasionally and can cause glitches.

Professional breath alcohol tests, like the ones police officers carry, use fuel cell technology. They’re the most accurate. But no breath test is as accurate as a blood or urine test.

How fast your BAC rises and how long it stays that way depend on several things:

Your weight. The heavier you are, the more water is in your body. The more water, the more the alcohol gets diluted.

Your gender. Alcohol doesn’t affect men and women the same. Men have higher levels of a stomach enzyme that helps break down alcohol, so they process it faster. Women typically have less water and more fat. Hormonal changes in women also can affect the BAC.

How many drinks you had, how strong they were, and how fast you drank them. The more you drink each hour, the faster your BAC rises.

How much you ate. A full belly, especially high-protein foods, will slow the processing of alcohol.

If a police officer gives you a breath alcohol test and your BAC is over the legal limit of 0.08%, you may be arrested and charged with driving under the influence.

You also may be asked to provide a blood or urine sample for further testing to determine a more accurate BAC.