Blood Alcohol Content, or Blood Alcohol Concentration (BAC), is the concentration of alcohol in the blood. It is measured as mass per volume, which means it determines how many grams of alcohol are present in 100 milliliters of blood.
A BAC of .01% means that there are .01 grams of alcohol per 100 grams of an individual’s blood, or .1 grams of alcohol per 1000 grams of blood. In some countries, BAC is measured in grams per liter of blood (g/L).
Blood Alcohol Content measurements were first introduced in Norway in 1936. Scientists were eager to find a chemically-based method of determining an individual’s level of intoxication for two reasons. First, it would allow for drunk driving charges to be assigned on an objective basis, rather than relying on the subjective analysis of a law enforcement official. Second, due to differences in our individual physiologies and alcohol tolerance, BAC is a much more accurate measure of intoxication than the number of drinks consumed.
The legal BAC in all of the United States has been .08% since 2002. As a general rule, the consumption of two beverages containing 20 grams of alcohol will raise the BAC of the average individual to approximately .05%. If a single beverage containing 20 grams of alcohol is consumed every hour after the first two, the BAC will remain at a level of .05%.
While BAC does determine the level of an individual’s intoxication from a chemical perspective, it does not necessarily determine a person’s level of impairment. Alcohol tolerance varies widely among individuals and is affected by race, age, gender, genetics, adaptation, and the effects of other simultaneous intoxicants.
Unless a person’s tolerance is very high, a BAC of .2% means the person is seriously intoxicated while a BAC of .35% indicates alcohol poisoning, which could be fatal.
The measurement of an individual’s blood alcohol concentration is not a perfect science. For one, breath analysis tests assume that the alcohol has already been absorbed and distributed throughout the body. If alcohol has been consumed very recently, then the BAC will report a lower percentage.
The BAC also assumes that there are 2,100 parts of alcohol in the blood for every part in the breath. While this may be true for many people, there are a number of exceptions. The actual ratio of parts of alcohol in the blood for every part in the breath can fall anywhere between 1,300:1 and 3,100:1.