Law enforcement uses the Breathalyzer, the breath analyzer, to estimate blood alcohol content (BAC) from the amount of alcohol in respiration. Despite arguments about its accuracy, it is used as evidence by courts in all 50 United States.
According to the Centers for Disease Control and Prevention (CDC), states are empowered to define their own limits. But all 50 states have adopted 0.08 per cent alcohol (80 mg/dl) as the legal limit for operating a motor vehicle.
The 0.08 per cent applies only to drivers over 21 years of age; drivers under 21 are not allowed to operate a motor vehicle with any alcohol whatsoever in their systems.
According to the National Highway Transportation Safety Administration (NHTSA), alcohol impairment begins at any number above 0.00.
A BAC calculator published by the state of Wisconsin estimates that a 77.1kg man who drinks a six-pack of beer over three hours has a BAC level of 0.10 per cent; a 125 pound woman who drinks the same has a BAC level of 0.192.
Because the breathalyser is an instrument that must be calibrated, and its accuracy can be skewed by alcohol-containing breath sprays and mouth washes, it is not a perfectly accurate measure. But it usually holds up in court.