Most New Yorkers know two things about being stopped on suspicion of drinking and driving: The police will ask the driver to take a breathalyzer test to determine his (or hers) blood alcohol content (“BAC”), and the legal limit for intoxication for persons over 21 years of age is a BAC of 0.08{e7aaffd771c1d72ed73aa4f2b2654701d87584ba154d7edc68255dac3d0c1e84}. But these same people have no idea how a breathalyzer measures BAC and what, exactly, does the BAC percentage represent.

BAC is the amount of alcohol in the blood stream expressed as a percentage of the alcohol in the blood per unit of volume measured by weight. But how can a breath test measure the amount of alcohol in the blood? The answer depends upon the way in which the body handles blood. After a drink is consumed, it is absorbed into the blood stream, where it eventually reaches the lungs. As the blood passes through the lungs, a small amount passes through the lung tissue into the air in the lungs; this air is called alveolar air because it has escaped through the lungs’ air sacs, or alveoli. Scientists have determined that the amount of alcohol in alveolar air is 1/2,100 of the amount in the same volume of blood. In other words, 2,100 milliliters of alveolar air in the lungs will contain the same amount of alcohol as 1 milliliter of blood.

A breathalyzer captures a sample of alveolar air and infuses it into two separate liquid solutions. One solution is then subjected to a chemical reaction that removes the alcohol from the sample and changes the color of the sample. Photocells in the breathalyzer measure the degree of color change in the sample containing alcohol and produce an electric current that allows the operator to determine the BAC of the person providing the breath sample.

The scientific principles behind the breathalyzer are not complex, but the test itself can easily be carried out incorrectly. A person who has received a breathalyzer test that measures BAC in excess of the legal limit may benefit from consulting a criminal defense attorney who handles cases that involve breathalyzer tests. Such a lawyer will know how to challenge the results of the breathalyzer test and thereby create the “reasonable doubt” that can be the difference between conviction and acquittal.

Source: How stuff works, “How Breathalyzers Work,” Craig Freudenrich, Ph. D., accessed April 27, 2015