Blood alcohol content or BAC refers to the amount of alcohol contained in a person's blood. It is measured as weight per unit of volume. A breathalyzer or alcohol detector are scientific instruments that can measure BAC accurately. Typically this measurement is converted to a percentage such as 0.10%, which indicates that one-tenth of a percent of a person's blood is alcohol. Alcohol in the blood travels directly to the brain and affects cognitive functions such as motor skills, resulting in increased risk of injury. The most common and often times significant injury is the risk of a motor vehicle crash when a person drives with too great a concentration of alcohol in his or her system.

In the United States it is illegal to drive with a BAC of 0.08% or higher. On October 1, 1993 the legal BAC limit was reduced from 0.10%. All States have now lowered the legal limit to 0.08%.

Research on the effects of alcohol has found that many important cognitive functions involved in driving (e.g., information processing, decision making, eye-hand coordination) are at least somewhat degraded in many individuals at BACs as low as 0.04%. Although it is not known how much any of these functions contribute to driving, other evidence obtained from carefully designed, case-control studies of drivers involved in crashes indicates that at BACs above 0.08%, the risk of a crash begins to increase sharply