What Is Blood Alcohol Concentration (BAC)?

man blowing into breathalyzer test
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Blood alcohol concentration (BAC) is a measurement of the percent of alcohol is in a person's bloodstream. This, rather than the exact amount of alcohol the person has consumed, will determine the effects the alcohol will have on the person's brain, body and behavior. In all states, the legal limit is any number below .08.

There are many factors that affect BAC, including size, gender, physical condition; what has been eaten and when; how much sleep is logged; medications; and alcohol content of drinks.

 As BAC increases, so does the level of alcohol-related impairment a person will experience. 

How Is BAC Calculated?

BAC can be assessed accurately with a breath test or breathalyzer. To calculate a 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 Does BAC Affect 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.

BACTypical EffectsPredicted Effects on Driving
02%
About 2 alcoholic drinks**
  • Some loss of judgment
  • Relaxation
  • 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)
.05%
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
.08%
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
  • Concentration
  • Short-term memory loss
  • Speed control
  • Reduced information processing capability (e.g., signal detection, visual search)
  • Impaired perception
.10%
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
.15%
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)
  
   
   
   
   

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