Understanding Blood Alcohol Content (BAC) Levels

Blood and Urine Test Measure BAC

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Blood alcohol content (BAC) is a measure of the amount of alcohol circulating in the bloodstream. It is expressed in terms of weight (milligrams) per unit of volume (milliliters); BAC levels are usually shown as a percentage. Blood alcohol content is used for legal and medical purposes to indicate a person's level of intoxication

Synonyms for BAC include blood alcohol level, blood alcohol concentration, and blood ethanol concentration.

What Are Blood Alcohol Content (BAC) Levels?

Blood alcohol content is the amount of alcohol present in 100 milliliters (mL) or its equivalent of 1 deciliter (dL) of blood. For example:

  • 80 mg is 0.08 grams
  • 0.08 grams of alcohol in 100 mL is 0.08%
  • This can also be expressed as 80 mg/dL or a BAC of 0.08

A blood-alcohol content of 0.1 (0.1% or one-tenth of 1%) means that there are 0.10 grams of alcohol for every deciliter of blood in the person's body at the time of the test.

In 49 of 50 states and the District of Columbia, the legal limit for driving under the influence of alcohol is 0.08. In Utah, the legal BAC limit is 0.05. Commercial drivers have a limit of 0.04 (this is a federal standard). Any detectable blood alcohol content is a violation for individuals under the age of 21.

Effects of BAC Levels

When alcohol is consumed and absorbed into the bloodstream, it travels throughout the body and to the brain, affecting many cognitive functions and the ability to perform physical tasks. Driving skills can be impaired long before someone reaches the legal limit, but at 0.08 the risk of having a vehicle crash increases dramatically.

The National Highway Traffic Safety Administration lists these effects on driving at different blood alcohol content levels:

  • 0.02 BAC: You are likely to feel relaxed and have some loss of judgment. You aren't able to quickly track the movements of other vehicles, pedestrians, or animals. You lose some of your ability to do two things at once, so you are more likely to be distracted.
  • 0.05 BAC: You begin to exhibit loss of small-muscle control, such as being able to focus your eyes, and you can have lowered alertness. You have even worse ability to track moving objects. Your ability to steer is degraded. If an emergency situation develops, such as needing to brake quickly or maneuver around an unexpected obstacle, you are likely to have a poorer response.
  • 0.08 BAC: You will usually exhibit poor muscle coordination, loss of balance, slower reaction time, slurred speech, loss of acuity in vision and hearing, difficulty in detecting danger, and impaired judgment, self-control, reasoning, and memory. When driving, you have difficulty with speed control and recognizing and reacting to signals and emergency situations. You have an increased risk of injuries in general, and particularly those related to driving a vehicle.
  • 0.10 BAC: At this level, you will have further deterioration of your abilities. It will be hard to maintain lane position and to brake when needed.
  • 0.15 BAC: You will have poor muscle control and ability to balance. You are likely to vomit. You will have significant problems in controlling your vehicle and paying attention to your driving and what is happening around you.

A 2018 report from the National Academies of Sciences, Engineering, and Medicine calls for the lowering the BAC limit for driving to 0.05% from the current legal limit (in most states) of 0.08%.

Factors That Affect BAC Levels

Many physical and situational factors determine blood alcohol content and the level of impairment you may experience. These can vary from person to person and also from situation to situation.

  • Individual differences: Your weight, body fat percentage, hydration level, digestion, and how alcohol typically affects your body chemistry all play a role.
  • Alcohol consumption: The alcohol content of the drinks you have consumed, the spacing of the drinks, and the amount of time that has passed since consuming the drinks all influence BAC.
  • Other substances: Use of medications and other drugs alongside lcohol can affect how much impairment the alcohol causes. 

Managing Your Alcohol Intake

It is safest not to drink at all if you will be driving, since driving skill is impaired before reaching illegal BAC levels. If you do drink, it is important to limit your intake, space your drinks out over a longer period of time, and drink plenty of water. "Moderate" alcohol intake is defined as two or fewer drinks per day for men and one or fewer for women.

Research published in 2015 showed that people are not good at estimating their own BAC or level of impairment, leading to making poor decisions.

There are many simple weight/gender charts that estimate BAC levels. For example, a 120-pound woman can reach a 0.08 BAC level after only two drinks within one hour. A 180-pound man can be at 0.08 after four drinks. A "drink" is either one shot of liquor, a five-ounce glass of wine, or one beer.

These estimates should always be used with caution because of the variation in measures and alcoholic content of drinks within different classes of alcohol. For example, some craft beers have twice the alcohol as a typical can of beer, and the serving size is often a full pint (16 ounces), compared to 12 ounces in a can.

A Word From Verywell

Enforcing the legal blood alcohol content limit is important for public safety, but you must use caution anytime you consume alcohol and then drive. You will have some impairment from the first drink and it is always best to avoid driving after you have taken a drink.

Frequently Asked Questions

  • What reduces BAC levels?

    Time—waiting it out—is the only way to reduce BAC levels once they are elevated. You cannot drink water or coffee, eat greasy food, or take a cold shower to speed up the metabolism of alcohol. Alcohol can be detected in your blood for up to 12 hours after you consume it.

  • Why do women tend to have higher BAC levels?

    Differences in body chemistry and structure mean that women tend to absorb more alcohol than men, and also take longer to metabolize it. This means that women's BAC levels tend to be higher. In most cases the effects of alcohol occur sooner and last longer in women.

  • How long does it take BAC levels to decrease?

    The half-life of alcohol is between 4 and 5 hours. So 4 to 5 hours after your last drink, half of the alcohol you consumed is still in your bloodstream. Depending on the type of test, alcohol use is detectable for as long as 90 days:

    • Blood: Up to 12 hours
    • Breath: Up to 24 hours
    • Urine: Up to 12 hours, or up to 3 to 5 days, depending on test method
    • Saliva: Up to 48 hours
    • Hair: Up to 90 days
  • What BAC level is lethal?

    Although alcohol affects everyone a bit differently, a BAC level of 0.40 and above can be lethal. At BAC levels approaching 0.40, health consequences including coma and death are possible.

    Symptoms of acute alcohol poisoning include:

    • Confusion
    • Loss of consciousness
    • Seizures
    • Hypothermia
    • Slow heart rate
    • Irregular breathing
    • Vomiting

    These are all signs that emergency medical attention is necessary.

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10 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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