One of the most common methods of identifying drunk drivers – probably the most common, in fact – is the use of breathalyzers. But how do they work? How is it that a machine can tell how inebriated you are just by taking a sample of your breath? The short answer – witchcraft.

Okay, not really. Here’s the scientific explanation.

The Science of Breathalyzers

A breathalyzer is never going to tell you your exact blood alcohol concentration (BAC) – for that, you would need a blood sample analysis. But through the magic of science, breathalyzers can give an accurate approximation of BAC.

When alcohol is ingested, it quickly enters the bloodstream and is carried throughout the body. When that blood reaches the lungs, some of the alcohol is left behind on the various structures inside the lungs. Some of that leftover alcohol is expelled from the body through respiration.

Inside the breathalyzer is a thing called an anode. When you blow into the breathalyzer, the leftover alcohol on your breath is converted into acetic acid at the anode. When acetic acid meets oxygen and water (from your breath) inside the breathalyzer, it creates a measurable electric charge. This electric charge is what will determine your BAC reading.

Breathalyzers are not foolproof. Some say that the margin of error for breathalyzer readings can be as high as 15 percent. There are so many factors that can throw off a breathalyzer. Breathalyzer calibration is regularly required, and if a breathalyzer is not maintained it can give falsely inflated readings. If you have elevated levels of acetone in your system (diabetics, for example, or even people with certain diets) your breathalyzer reading might be wrong. Because the science of breathalyzers is so dicey, if you are ever arrested for DUI after giving a breath sample, you should always have an attorney examine your case.

Drivers accused of DUI require the help of a legal professional to identify wrongdoing on the part of police to help have your sentence reduced or dismissed.