Blood Alcohol Decay

Alcohol elimination formula

Alcohol behaves differently from most drugs. Once the enzyme ADH is saturated, the body clears it at a more or less constant rate (zero-order kinetics): BAC(t) = BAC₀ - β·t, where the Widmark β sits around 0.015 g/dL/h (~0.15 g/L/h) on average. One standard drink (14 g of pure alcohol — a 350 mL beer at 5%, 150 mL wine at 12%, 45 mL spirits at 40%) takes about 1 hour to clear. BAC peaks somewhere between 30 and 90 min after the last drink, and how much you've eaten shifts that window. Example: a BAC of 0.08 g/dL falls to 0.035 g/dL after 3 h. In Brazil, the Lei Seca (Law 9.503/1997, art. 306) makes BAC ≥ 0.6 g/L a crime, with a R$ 2,934.70 fine and a 12-month license suspension; it's verified by breathalyzer, blood test, or clinical exam.

Applications

Figuring out a safe gap between the last drink and getting behind the wheel. Professional drivers checking themselves the morning after a social event. Estimating when a breathalyzer will read clean. Event organizers spacing out food and drinks across the evening.

FAQ

Can coffee or cold showers speed it up? No. The only things that bring BAC down are time and a healthy liver. Caffeine hides the drowsiness, but it leaves the elimination rate exactly where it was.

Does β vary? Yes. It runs anywhere from 0.010 to 0.025 g/dL/h depending on chronic use, your genetics (ADH/ALDH variants) and how well your liver is working. Stick with 0.015 as a conservative average.

Why is the curve flat, not exponential? ADH saturates even at low BAC, so the enzyme is already working at full speed and keeps that pace no matter how much alcohol is left.

Is "zero BAC" the legal limit in Brazil? For all practical purposes, yes. The administrative tolerance is 0.05 mg/L on the breathalyzer (≈ 0.1 g/L blood). Go over that and you face a fine and license suspension; hit ≥ 0.34 mg/L breath (0.6 g/L blood) and it becomes a criminal offense.