The concentration of ethanol in a person’s blood, the definitive measure of intoxication used in medicine and law enforcement. It is expressed variously as a percentage, as grams per liter, or as milliliters per liter depending on jurisdiction, with 0.08 percent, or 80 milligrams per 100 milliliters, the common legal driving limit in many countries. The level rises as alcohol absorbs from the gut and falls as the liver metabolizes it at roughly one standard drink per hour, with food, body weight, and sex shifting the curve. Effects scale predictably: impaired judgment at low levels, loss of coordination higher, unconsciousness and death at extremes. Breathalyzers estimate it from exhaled air, but blood analysis remains the reference standard.