How Blood Alcohol Concentration (BAC) Impairs Your Body and Brain

By Elizabeth Hartney, PhD
Updated September 19, 2019
man blowing into breathalyzer test

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Blood alcohol concentration (BAC) is a measurement of the percentage of alcohol that's in your bloodstream after you've been drinking. Your BAC, rather than the exact amount of alcohol you've consumed, will determine the effects the alcohol will have on you. In all states, the legal limit to drive is any number below .08.

Factors That Affect BAC

  • Body size
  • Biological sex
  • Physical condition
  • Medications you take
  • What you've eaten (and when)
  • How much sleep you've had
  • Alcohol content of your drinks

As BAC increases, so does the level of alcohol-related impairment you will experience. While a breathalyzer gives fast results and is often used by police officers who suspect that an individual has been driving while drunk, it is not as accurate as measuring the amount of alcohol in your blood.

How BAC Is Calculated

To calculate BAC, the amount of alcohol in the bloodstream is measured in milligrams (mg) of alcohol per 100 milliliters (ml) of blood. It is usually expressed as a decimal such as 0.08 or 0.15.

For example, a BAC of 0.10% means that an individual’s blood supply contains one part alcohol for every 1,000 parts blood.

How BAC Affects Impairment

The Centers for Disease Control and Prevention provides the following chart which illustrates how alcohol consumption and BAC can impact behavior, judgment, physiology, and driving ability.

BAC Standard Drink Equivalent Typical Effects Predicted Effects on Driving
0.02% ~2 alcoholic drinks Some loss of judgment, relaxation, slight body warmth, altered mood Decline in visual functions (rapid tracking of moving target) and ability to perform two tasks at the same time (divided attention)
0.05% ~3 alcoholic drinks Exaggerated behavior, loss of small-muscle control (e.g., focusing your eyes), impaired judgment, lowered alertness, release of inhibition Reduced coordination, reduced ability to track moving objects, difficulty steering, reduced response to emergency driving situations
0.08% ~4 alcoholic drinks Poor muscle coordination (e.g., balance, speech, vision, reaction time, hearing), difficulty detecting danger, impaired judgment, self-control, reasoning, and memory Loss of concentration, short-term memory loss, impaired speed control, reduced information processing capability (e.g., signal detection, visual search), impaired perception
0.10% ~4 alcoholic drinks Clear deterioration of reaction time and control, slurred speech, poor coordination, slowed thinking Reduced ability to maintain lane position and brake appropriately
0.15% ~7 alcoholic drinks Far less muscle control than normal, potential for vomiting, major loss of balance Substantial impairment in vehicle control, attention to driving task, and visual and auditory processing
The number of drinks listed represents the approximate amount of alcohol that a 160-pound man would need to drink in one hour to reach the listed BAC in each category.

A standard drink is equal to 14 grams (0.6 ounces) of pure alcohol. Generally, this amount of pure alcohol is found in:

  • 12 ounces of beer (5% alcohol content)
  • 8 ounces of malt liquor (7% alcohol content)
  • 5 ounces of wine (12% alcohol content)
  • 1.5 ounces or a “shot” of 80-proof (40% alcohol content) distilled spirits or liquor (e.g., gin, rum, vodka, whiskey)

Your blood alcohol level results may be given in different ways, including the percentage of blood alcohol content (BAC). Typical results include:

  • Sober: 0.0% BAC
  • Legally intoxicated: .08% BAC
  • Very impaired: .08–0.40% BAC. You may have difficulty walking and speaking. Other symptoms may include confusion, nausea, and drowsiness.
  • At risk for serious complications: Above .40% BAC. An individual with this blood alcohol level may be at risk for coma or death.

A blood alcohol test is only accurate within 6 to 12 hours after your last drink. If you have concerns about your results, you may want to talk to a health care provider and/or a lawyer.

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