How Stuff Works

Telltale Breath

Alcohol consumed is absorbed into the bloodstream, but not immediately digested. As blood moves through the lungs, some alcohol passes through membranes in the lungs into the air. The concentration of alcohol in the air sacs, or alveoli, indicates the concentration of alcohol in the blood. Testing devices measure a sample of exhaled air and determine the amount of alcohol in an individual's bloodstream. In California, the legal blood-alcohol limit is 0.08%--equivalent to 0.08 grams of alcohol per 100 milliliters of blood.


Breath Alcohol Testing Devices

Breath-alcohol testing devices measure the amount of alcohol an individual has consumed without requiring a sample of bodily fluid. Common testing devices analyze an individual's exhaled air for the presence of alcohol. Here's how one, the Intoxilizer, works:



An infrared lamp generates a broadband infrared beam, which passes through a lens into the breath sample chamber.

The exhaled air passes through the sample chamber.

The beam is focused onto a spinning wheel containing band filters for the wavelengths of ethanol bonds. As light passes through the filters, a photocell converts it into an electrical pulse.



Breathalyzers use a chemical reaction to test for alcohol. Breath is blown into a glass vial containing sulfuric acid, potassium dichromate, silver nitrate and water. If alcohol is present, the mix will change color. Photocells on the outside of the vials measure the light that passes through, and a result is displayed.



As the subject exhales into an Alcosensor, the breath flows over a fuel cell--a porous acid electrolyte material bordered by two platinum electrodes. The platinum oxidizes alcohol in the air, producing acetic acid, protons and electrons. Electrons flowing through the fuel cell create a measurable current.



Researched by CHRISTINE FREY / For the Times

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