How Blood Alcohol Concentration (BAC) Impairs Your Body and Your 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 is any number below .08.

There are many factors that affect BAC, including your size, gender, physical condition; what you've eaten and when, how much sleep you've had, medications, and the alcohol content of the drinks you've had. As BAC increases, so does the level of alcohol-related impairment a person 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 an individual's BAC, the amount of alcohol in the bloodstream is measured in mg. of alcohol per 100 ml. of blood. It is usually expressed as a decimal, such as 0.08 or 0.15.

For example, a BAC of .10 percent 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 conveys how alcohol consumption and BAC can impact behavior, judgment, physiology, and driving ability.

BAC* Typical Effects

Predicted Effects on Driving

About 2 alcoholic drinks**

Some loss of judgment


Slight body warmth

Altered Mood

Decline in visual functions (rapid tracking of a moving target)

Decline in ability to perform two tasks at the same time (divided attention)


About 3 alcoholic drinks**

Exaggerated behavior

May have loss of small-muscle control (e.g., focusing your eyes)

Impaired judgment

Usually good feeling

Lowered alertness

Release of inhibition

Reduced coordination

Reduced ability to track moving objects

Difficulty steering

Reduced response to emergency driving situations


About 4 alcoholic drinks

Muscle coordination becomes poor (e.g., balance, speech, vision, reaction time, and hearing)

Harder to detect danger

Judgment, self-control, reasoning, and memory are impaired


Short-term memory loss

Speed control

Reduced information processing capability (e.g., signal detection, visual search)

Impaired perception


About 5 alcoholic drinks**

Clear deterioration of reaction time and control

Slurred speech, poor coordination, and slowed thinking

Reduced ability to maintain lane position and brake appropriately


About 7 alcoholic drinks**

Far less muscle control than normal

Vomiting may occur (unless this level is reached slowly or a person has developed a tolerance for alcohol)

Major loss of balance

Substantial impairment in vehicle control, attention to driving task, and in necessary visual and auditory information processing

*Blood Alcohol Concentration Measurement

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 14.0 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 percent BAC
  • Legally intoxicated: .08 percent BAC
  • Very impaired: .08–0.40 percent BAC. You may have difficulty walking and speaking. Other symptoms may include confusion, nausea, and drowsiness.
  • At risk for serious complications: Above .40 percent 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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