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BAC (Blood Alcohol Concentration) Definition

The definition of BAC (blood alcohol concentration) is a measurement of the alcohol (ethyl alcohol or ethanol) present in an individual’s bloodstream. Presented as a percentage, BAC reflects the speed by which the body absorbs, distributes, metabolizes, and excretes the substance. 

For example, if a person has a BAC of 0.08%, this means that there is .08 grams of alcohol per every 100 milliliters of blood. 

A BAC of just 0.02% can result in partial loss of judgement, relaxation, modest body warmth, and changes in mood. In driving terms, this could translate into a decrease in visual function, such as following a rapid, moving target, or in performing two tasks at once. 

The effects of alcohol on the body will depend on the blood alcohol level. The liver can only process approximately one drink per hour. In most cases, one drink is: 

  • 12 ounces (oz.) of beer
  • 5 oz. of wine
  • 1.5 oz. of whiskey 

If an individual consumes more than one beverage in less than an hour, BAC will be higher than in an individual who consumes only a drink per hour. 

By the same token, when an individual stops consuming alcohol or is unconscious, it does not mean that BAC will remain as a steady figure or even decrease. The body will continue to release alcohol into the bloodstream and circulate throughout. This can increase BAC and result in more dangerous health consequences. 

The effects of alcohol are dependent on BAC. The higher the blood alcohol concentration, the stronger (and sometimes more harmful) effects that an individual may experience. 

In most cases, that is, in those who do not suffer from chronic drinking, signs of intoxication will be visible in correlation with BAC. Otherwise, those who suffer from chronic alcohol abuse will have built tolerance that masks outward signs of intoxication.

To determine BAC, a breathalyzer (a device that tests alcohol supply in a person’s breath) or blood test may be used. 

Factors That Affect Blood Alcohol Content

As BAC relates to the extent of the body’s ability to absorb and break down the substance, it is important to consider factors that may influence such absorption and metabolism. 

Amount of Alcohol (Number of Standard Drinks) 

The amount of alcohol in a person’s blood will contribute to BAC. The liver is capable of metabolizing one standard drink (14 grams of pure alcohol) in an hour. If a person consumes more than one alcoholic beverage in the same time frame, BAC will increase. 

This is especially the case for those who partake in binge drinking or heavy drinking: 

  • In binge drinking, men consume 5 or more drinks within 2 hours, while women consume 4 or more drinks. 
  • In heavy drinking, men consume 4 or more drinks on any given day, while women consume 3 or more drinks. 
  • In both scenarios, the large amount of alcohol in the body overwhelms metabolic processes and results in elevated BACs in little time. 


Women will face more difficulty in metabolizing alcohol than men. As a result, women will have higher BACS in less time than when compared to men. 

This occurs as a result of three primary reasons:

  • Body composition Women have a higher percentage of body fat and a lower percentage of water in comparison to men. Alcohol cannot be dissolved in fat. This means that there will be a build-up of alcohol in the bloodstream. 
  • Stomach ADH ADH (alcohol dehydrogenase) is an alcoholic-metabolizing enzyme that is not prevalent in a woman's stomach. As a consequence, women do not metabolize as much alcohol as men before it enters into the bloodstream. 
  • Liver ADH ADH in a woman’s liver does not perform as efficiently in breaking down alcohol as that in a man’s liver. This too contributes to a rise in BAC. 

Men face a higher probability than women to be driving under the influence in fatal crashes. In 2018, 21% of men were under the influence in such types of crashes, whereas for women, only 14%. 

Finally, hormones can affect a person’s ability to metabolize alcohol. When a woman has her period, she may have a higher BAC due to fluctuations in hormones. 

Body Weight

An individual’s weight can determine the quantity of space by which alcohol diffuses in the body. For example, a person who weighs 130 pounds can consume two beers and have a lower BAC than an individual who consumes the same amount of beverages but weighs less. 

Similarly, an individual with more muscle mass will be able to absorb and metabolize alcohol better than an individual with a higher body fat percentage. Again, alcohol does not dissolve in fat. 

Body Size

In relation to body weight, body size can also determine how quickly alcohol is absorbed and metabolized. Those with a smaller body frame have higher BACs than their bigger counterparts, even when alcohol consumption is the same. 

Stomach Contents

Drinking on an empty stomach can contribute to higher BACs. It is recommended to eat before drinking, especially food high in protein, to decrease the processing of alcohol in the body. This is important, so as to not overwhelm the liver and manage a safe BAC. 


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BAC Levels While Driving: The Legal Limit 

In all 50 US states, it is illegal to drive with a BAC of 0.08% or higher. The risk of alcohol-related traffic crashes grows exponentially when BAC is 0.08% or higher. 

When an individual has a BAC of 0.08%, different effects of alcohol may occur. These include:

  • Poor muscle coordination, such as balance, speech, or reaction team 
  • Impaired judgement, self-control, and reasoning
  • Short-term memory loss
  • Difficulty in detecting danger 

In terms of driving, these effects translate into issues of speed control and limited information processing capability like signal detection. 

Additionally, it is also illegal for those under the age of 21 to be caught driving with a BAC of at least .01%. 

Approximately a quarter of younger drivers aged 15 to 20 who died in crashes had BACs of .01% or higher in 2018. 

It is important to remember that even if an individual does not exceed the legal alcohol limit, there is still a risk of alcohol-related traffic accidents. With a BAC of .05%, an individual can begin having trouble steering the wheel or responding quickly and appropriately to unexpected emergency situations. 

Risks of Driving Under The Influence (DUIs)

When individuals decide to drive under the influence, they run the risk of suffering undesirable, possibly life-threatening events. These events include:

  • Alcohol-related traffic accidents
  • Unintentional physical or emotional harm to others
  • Manslaughter 
  • Jail 
  • Hefty fines 
  • Suspended or revoked driver’s license
  • Sudden spike in car insurance fees

To avoid any of the negative circumstances above, it is important to have a plan of action in advance. If you believe that you will consume alcohol later in the day, seek different ways of moving about without the need to start your own car. Nowadays, there are convenient, economical transport services available that can take you where you want to go. 

Similarly, if you believe that you suffer from alcohol dependence or abuse, it is important to seek professional medical help. Healthcare specialists can provide you with a series of steps and actions to guide you through withdrawal, minimize your risk of overdose, resource recommendations and set you on your path to recovery. 


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“Module 1: Gender Matters.” The Alcohol Pharmacology Education Partnership, Duke University , sites.duke.edu/apep/module-1-gender-matters/explore-more/summary-of-differences-between-girls-n-guys/.

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