BACs

Blood alcohol concentration (BAC) is a measure of the amount of ethanol in a person’s bloodstream, expressed as a percentage or grams of alcohol per deciliter of blood. BACs describe individual measurements of intoxication and are used in medicine, forensic contexts, and law enforcement. BAC rises with the amount and rate of alcohol consumed and is influenced by body weight, sex, age, metabolism, liver health, medications, and food intake. In many jurisdictions a legal driving limit is 0.08% BAC, with lower limits for some drivers; some places enforce zero tolerance or lower thresholds.

Measurement and interpretation: BAC can be estimated with breath tests (breathalyzers) that measure alcohol in the breath and infer BAC, or determined directly by blood testing. Blood tests are more accurate, while breath tests are commonly used for roadside purposes. After drinking, BAC increases as alcohol is absorbed and then declines as the liver metabolizes it, typically at about 0.01–0.02% per hour, though rates vary widely.

Factors and effects: Individual differences in body composition and drinking patterns affect BAC. Women may reach higher BAC than men with the same amount of alcohol due to distribution differences. Effects at common BAC levels range from mild relaxation and impaired judgment around the 0.04–0.08% region to serious motor impairment and risk of loss of consciousness at higher levels; BAC above about 0.40% can be life-threatening.

Calculation and context: Researchers sometimes estimate BAC using models that relate the dose of ethanol to body weight, time since drinking, and physiological distribution ratios. BACs are snapshots in time and do not predict impairment for every person; testing results should be interpreted cautiously.