Blood alcohol content (BAC) is the amount of alcohol present in a 100 milliliter (mL) volume of blood. In 1999, sixteen states in the US defined intoxicated drivers as those having at least a 0.08 BAC. The remaining 34 states and the District of Columbia used a 0.10 BAC legal limit. Recent Federal legislation intends to lower the level to 0.08 in all states.

Compliance with these standards, in part to reduce the more than 17,000 alcohol-related traffic fatalities per year in the US, has engendered a variety of alcohol detection devices.

Gas chromatography, used for blood alcohol analysis in controlled laboratory situations, has yielded to the simpler and quicker analysis of breath samples required by routine police work. The first breath alcohol analyzer was developed by Glenn Forrester in 1937. This device worked by allowing expired breath to pass through and react with the dichromate crystals. Alcohol in the breath sample reduces the chromium.

Silver nitrate is present as a catalyst to bring the reaction to completion. The amount of green crystals produced is proportional to the amount of alcohol in the breath sample. Current technology has replaced this apparatus with a device housing either a fuel cell, an infrared spectrometer, or a semiconductor. A comparison of these methods is available. An onboard computer calculates the BAC, and displays the result on a digital readout. Breath analysis is subject to measurement errors, and usually are used for screening.