In the case of alcohol, a drunk driver's level of intoxication is typically determined by a measurement of blood alcohol content or BAC. A BAC measurement in excess of a specific threshold level, such as 0.05% or 0.08%, defines the criminal offense with no need to prove impairment. In some jurisdictions, there is an aggravated category of the offense at a higher BAC level, such as 0.12%. In most countries, anyone who is convicted of injuring or killing someone while under the influence of alcohol or other drugs can be heavily fined in addition to being given a lengthy prison sentence. In the United States, DUI and alcohol-related crashes produce an estimated $45 billion in damages every year.
In some U.S. and German studies BAC level 0.01-0.03% predicted a lower crash risk than BAC 0%,
Blood alcohol content
With the advent of a scientific test for blood alcohol content, enforcement regimes moved to pinning culpability for the offense to strict liability based on driving while having more than a prescribed amount of blood alcohol, although this does not preclude the simultaneous existence of the older subjective tests. BAC is most conveniently measured as a simple percent of alcohol in the blood by weight. Research shows an exponential increase of the relative risk for a crash with a linear increase of BAC as shown in the illustration. BAC does not depend on any units of measurement. In Europe it is usually expressed as milligrams of alcohol per 100 milliliters of blood. However, 100 milliliters of blood weighs essentially the same as 100 milliliters of water, which weighs precisely 100 grams. Thus, for all practical purposes, this is the same as the simple dimensionless BAC