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

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 .08 (at or above that number is considered impaired).

A person's BAC level impacts their brain and behavior, with higher blood alcohol levels resulting in a higher degree of impairment. There are many factors that influence BAC.

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

BAC can be measured using a breathalyzer test or a blood test. 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 the 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 a person's blood supply contains one part alcohol for every 1,000 parts blood.

The ratio of breath alcohol to blood alcohol is 2,100 to 1. Because of this, a breathalyzer measures BAC as grams per 210 liters of breath.

How BAC Affects Impairment

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

People who use alcohol chronically may show minimal behavioral changes on simple observation due to alcohol tolerance. A breath or blood test would still show a high BAC level.

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 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)

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 healthcare provider and/or a lawyer.


Effects and impairments become gradually more serious as BAC levels go up. At the 0.08% level of legal intoxication, people experience poor coordination, impaired judgment, and problems with perception.


Your BAC level indicates how much alcohol is in your bloodstream. It can indicate how alcohol might impact your brain and behavior and can be affected by a variety of factors including your biological sex, body size, and physical condition. Higher BAC levels are associated with greater impairment, although this can vary from person to person. If your BAC level is 0.08% or above, then you are considered legally intoxicated.

A Word From Verywell

Blood alcohol concentration can be a measure of impairment and can indicate how alcohol affects your brain and behavior. It is important to remember that this number does not indicate how your alcohol use might be affecting your life, relationships, and ability to function.

If you are concerned about your alcohol use, consider talking to your healthcare provider. If your drinking is making it difficult to function in your daily life, impairing your relationships, causing distress, and creating other negative consequences in your life, there are treatments available that can help you reduce or stop your alcohol use.

If you or a loved one are struggling with substance use or addiction, contact the Substance Abuse and Mental Health Services Administration (SAMHSA) National Helpline at 1-800-662-4357 for information on support and treatment facilities in your area.

For more mental health resources, see our National Helpline Database.

5 Sources
Verywell Mind uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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  2. Kriikku P, Wilhelm L, Jenckel S, et al. Comparison of breath-alcohol screening test results with venous blood alcohol concentration in suspected drunken drivers. Forensic Sci Int. 2014;239:57-61. doi:10.1016/j.forsciint.2014.03.019

  3. The University of Toledo. Blood alcohol content.

  4. Centers for Disease Control and Prevention. Impaired driving: Get the facts.

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