Skip to main content

How a breathalizer works

Posted by attorney Mitchell Sexner

At some point in almost every DUI investigation the officer will ask the driver to take a breath test in order to determine the level of blood alcohol concentration (BAC) in that person’s body. In Illinois, the “legal limit" to drive is a BAC of 0.08 or less. But this is not to say that you can’t get convicted of a DUI with a lower BAC though, because everyone’s tolerance for alcohol is different. This concept is discussed in a different section. A BAC of 0.08 means that 0.08%, or an eighth of one percent, of a person’s blood is alcohol. How this gets determined and how the breathalyzer works requires quite a bit of science.

The word “breathalyzer," is a global term that most people attribute to all different types of breath alcohol testing machines. In fact, there are several kinds of machines, and each determines alcohol levels using different methods of detection. The basic idea, however, is that the amount of alcohol in the body can be detected with a breath sample. The reason for this is that alcohol does not change from what it is as it gets absorbed into the bloodstream. As blood goes to the lungs, some measurable amount of alcohol passes through the lungs into the air. A breath machine then measures the amount of alcohol in the air, through a breath sample provided by an individual, and converts that into the amount of alcohol in the blood. There is a scientifically accepted ratio for this of 2,100:1; meaning, the amount of alcohol in the blood is 2,100 times higher than that in the air sample.

As stated above, there are several types of breath testing devices. They each measure BAC using different technology, including how the molecules absorb infrared light and even using fuel cells. These machines are typically kept at police stations and are the ones that give the BAC level used at court in the prosecution of DUI cases.

There are also portable machines called preliminary (or portable) breath screening devices, often referred to as PBTs. These are the machines that police officers keep in their cars and use as part of the field sobriety testing process on the street. The reliability of these portable devices has often been placed into question and the results may not be admissible in all stages of a DUI case. Ultimately, consulting with an attorney can help determine if and how the results of the breath test will affect a DUI case.

Additional resources provided by the author

If you, a family member or friend has been charged with any criminal or traffic matter, we urge you to contact an experienced attorney. Such an attorney will carefully evaluate your case and explain the legal options available to you at no charge.

Author of this guide:

Was this guide helpful?

Avvo DUI email series

Sign up to receive a 3-part series of useful information and legal advice about DUIs.

Recommended articles about DUI

Can't find what you're looking for?

Post a free question on our public forum.

Ask a Question

- or -

Search for lawyers by reviews and ratings.

Find a Lawyer