Blood Alcohol Level Defining Intoxication
The legal and clinical threshold for intoxication is a blood alcohol concentration (BAC) of 0.08 g/dL (0.08%), which is the standard used by the National Institute on Alcohol Abuse and Alcoholism (NIAAA) for defining binge drinking and represents the level at which driving impairment is legally recognized. 1
Legal and Clinical Definitions
The 0.08% BAC threshold is the established standard in the United States for:
- Legal intoxication for driving purposes 1
- The level typically reached after consuming 5 drinks for men or 4 drinks for women within approximately 2 hours 1
- The point at which binge drinking is defined by NIAAA 1
Important Context: Impairment Begins Well Below Legal Intoxication
While 0.08% represents the legal threshold, cognitive and motor impairment begins at much lower BAC levels:
- At 0.03% BAC, specific cognitive functions relying on visual information processing are significantly impaired, particularly in complex and urgent tasks 2
- At 0.05% BAC, driving performance shows clear degradation including increased variability in lane keeping, average speed, and overall lateral and longitudinal control 3
- The relative risk of fatal crash involvement is 7-21 times higher at BACs of 0.05-0.079% compared to 0.00% 4
Clinical Considerations
For medical assessment purposes, the context matters:
- In trauma settings (such as mild traumatic brain injury evaluation), studies have used mean BACs of 143-182 mg/dL (0.143-0.182%) to define the "intoxicated" group 1
- Many countries have adopted 0.05% BAC as their legal limit for driving, recognizing that impairment occurs below 0.08% 4
Common Pitfall
Do not assume patients are unimpaired below 0.08% - functional impairment affecting safety-sensitive activities begins at BAC levels as low as 0.03%, even though legal intoxication is defined at 0.08% 2, 3, 4. The 0.08% threshold represents a legal standard for prosecution, not a physiological threshold below which impairment is absent.