A breathalyzer, or breath testing machine, analyzes a sample of your breath and produces a number that estimates your blood alcohol concentration (BAC). Breath alcohol testing machines are commonly used by police stopping people on suspicion of drunk driving since they offer an easy way to estimate whether a driver’s BAC is over the legal limit of 0.08 percent or not. In order to understand how these machines work, it’s important to know how the alcohol you drink winds up on your breath.
When you take a drink of alcohol, the liquid enters your stomach where it is temporarily stopped by the valve at the bottom of the stomach known as the pyloric sphincter. Some alcohol is absorbed into your blood through the walls of the stomach, but the majority of it is absorbed in the part of the small intestine that hooks up to the pyloric sphincter, known as the duodenum. This is one reason that eating while drinking alcohol makes the alcohol work more slowly: as you eat, you encourage the pyloric sphincter to stay closed, keeping most of the alcohol from being absorbed in the intestines.
As the alcohol enters the small intestine and is absorbed into the bloodstream, it travels throughout the body, including into the lungs. The lungs contain membranes called alveoli which spread the blood out so that it can release carbon dioxide for you to breathe out and soak up oxygen taken in when you breathe in. When the blood drops off the carbon dioxide, it also drops off a certain amount of alcohol molecules. This alcohol is what’s measured by a breath testing machine.