# Blood alcohol content

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Blood alcohol content, blood alcohol concentration, or blood ethanol concentration is the concentration of alcohol in a person's blood. Blood alcohol content, or BAC, is most commonly used as a metric of intoxication for legal or medical purposes. It is usually expressed as a fractional percentage in terms of volume of alcohol per volume of blood in the body. That is commonly expressed without units, as a decimal with 2-3 significant digits followed by a percentage sign, which means 1/100 of the previous number (e.g., 0.0008 expressed as a percentage = 0.08%). Each country/state may define BAC differently. For example, the U.S. state of California defines their BAC as a ratio of grams of alcohol per 100 milliliters of blood, which is equal to grams of alcohol per deciliter of blood. California's legal definition of BAC.

Since measurement must be accurate and inexpensive, several measurement techniques are used as proxies to approximate the true parts per million measure. Some of the most common are listed here: (1) Mass of alcohol per volume of exhaled breath (e.g. 0.38 mg/L), (2) Mass per volume of blood in the body (e.g.: 0.08 g/dL), and (3) Mass of alcohol per mass of the body (e.g.: 0.0013 g/Kg).

The number of drinks consumed is often a poor measure of blood alcohol content, largely because of variations in weight, sex, and body fat.

An ethanol level of 0.10% is equal to 22 mmol/l or 100 mg/dl of blood alcohol.[1][2] This same 0.10% BAC also equates to 0.10 g/dL of blood alcohol or 0.10 g/210L of exhaled breath alcohol or 0.476 mg/L of exhaled breath alcohol. Likewise, 0.10 mg/L of exhaled breath alcohol converts to 0.02% BAC, 0.022 g/dL of blood alcohol or 0.022 g/210L of exhaled breath alcohol.