BAC

Related Terms

Alcohol, alcohol tolerance, alcoholic beverage, alcoholic drink, BAC, blood alcohol level, blood alcohol concentration, blood alcohol test, Breathalyzer? test, driving under the influence, drunk driving, DUI, intoxicated, intoxication, urine alcohol test.

Background

Blood alcohol content (BAC), or blood alcohol concentration, is the concentration of alcohol in a person's blood. BAC can be mathematically estimated or measured with a blood, breath, or urine test. A blood alcohol level of 0.01 is considered low, while 0.4 and higher is toxic and potentially deadly.
When a person consumes alcohol it is absorbed into the bloodstream. Then the liver breaks down (metabolizes) the alcohol. Alcohol is metabolized more slowly than it is absorbed. Because it takes longer for alcohol to metabolized, consumption must be controlled in order to prevent alcohol from accumulating in the body and causing intoxication.
BAC is used to determine an individual's intoxication level, and it provides an estimate of his/her level of impairment. When individuals consume alcohol, their judgment, coordination, concentration, alertness, speech, and ability to feel sensations become impaired. The more an individual drinks, the more impaired these functions become.
Even though the degree of impairment varies among people with the same BAC, it is considered a reliable way to determine if it is safe for someone to drive or operate heavy machinery, including motor vehicles, boats, and aircrafts after drinking.
It is illegal for individuals to drive with BAC levels of 0.08 and higher in the United States. States may have stricter laws for individuals younger than 21 years old. Some states have zero tolerance laws for this age group that allow underage people to be convicted of driving under the influence with virtually any amount of alcohol in the bloodstream.
The amount of alcohol varies in different types of alcoholic drinks. In general, a 12-ounce beer, a five-ounce glass of wine, and 1.5-ounce shot of hard liquor are considered equivalent.
Counting the number of drinks consumed is not an accurate way to measure intoxication because individuals have different alcohol tolerances. An individual's tolerance to alcohol is dependent on many factors, including weight, age, gender, body fat percentage, genetics, synergistic effects of drugs, amount of food in the stomach, and how frequently the person drinks.
The length of time that elapses between drinks is also an important factor. The quicker an individual drinks alcoholic beverages, the more intoxicated he/she is going to become. This is because the body needs time to metabolize the alcohol. It is generally accepted that consuming two standard alcoholic beverages increases the average person's BAC by about 0.05% in about one hour. Limiting alcohol intake to one drink per hour after the first two drinks are consumed will keep the BAC near 0.05%.
Individuals should not consume any alcoholic beverages before driving or operating heavy machinery.

Theory / Evidence

Consuming alcohol has been shown to impair judgment, coordination, reflexes, reasoning, peripheral vision, depth perception, and motor control. For these reasons, alcohol can lead to risky behaviors that may lead to accidental injuries. For instance, driving under the influence has been shown to increase the likelihood of motor vehicle accidents.
In addition, intoxicated people are more likely to engage in aggressive behaviors. This is because alcohol causes changes in mood, exaggerates emotions, impairs judgment, and decreases inhibitions. Some studies suggest that the number of life-threatening assaults increase with an individual's blood alcohol content (BAC).
Law enforcement officials measure individuals' blood alcohol concentration (BAC) to prevent intoxicated people from driving or operating heavy machinery. In the United States, it is illegal to drive or operate heavy machinery, including a motor vehicle, boat, or aircraft, with a BAC of 0.08 or higher. Many states have zero tolerance laws for individuals who are younger than 21-years old. Several studies have shown that zero tolerance laws may help reduce the number of fatal crashes that involve intoxicated drivers. However, these studies are controversial, and it has been suggested that other strategies, such as prevention education, may be more or equally effective.
In the past, many states had laws that prohibited individuals from being publicly intoxicated. However, public intoxication is no longer considered a crime in any state in the United States.

Author information

This information has been edited and peer-reviewed by contributors to the Natural Standard Research Collaboration (www.naturalstandard.com).

Bibliography

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Technique

General: Blood alcohol content (BAC) can be measured at a hospital after a blood or urine test. However, BAC is more commonly measured with a Breathalyzer? test during law enforcement investigations. If it is suspected that an individual is under the influence of alcohol, a law enforcement official can indirectly measure BAC by measuring the alcohol concentration in the person's breath. A BAC test can only be performed if an individual consents to a test. However, refusing a BAC test often has legal consequences and may result in a suspended driver's license.
Many factors can influence an individual's BAC. For instance, cough medicines or herbal supplements that contain alcohol (e.g. kava or ginseng) may increase an individual's BAC. Also, babies, pregnant women, diabetics, and people who exercise, diet, or have physical trauma can have higher amounts of acetone in their bodies. Acetone may be falsely detected as alcohol by some Breathalyzer? tests.
Measuring BAC: There are three ways that BAC can be measured. It can be measured as a percentage of body mass per volume, by body mass per volume of blood, or a combination.
For instance, if an individual's BAC is 0.10%, it means that there is one gram of alcohol per 1,000 grams or 1,000 milliliters of blood.
Mathematical estimate: BAC can be mathematically estimated, although this method is not as accurate as a Breathalyzer? test. Therefore, this technique is rarely used to measure an individual's BAC. However, it may help predict how an individual's BAC will change as the body continues to absorb and metabolize more alcohol.
The U.S. Department of Transportation uses the following formula to calculate an individual's BAC. A person's BAC = [(The number of drinks consumed) x (ounces of alcohol consumed) x (23.36 grams of alcohol/oz.) x (0.806 ml water/ml blood)] / (body weight in pounds/2.2046 lb/kg) x (total body water volume) x (1,000g/kg)] x 100 - (time). It is important to note that males and females have different amounts of water in their bodies. In adult males, about 58% of the body weight is water. In females, about 49% of the body weight is water.
Breathalyzer? test: A Breathalyzer? test does not directly measure the BAC. Instead, these tests estimate the BAC by measuring the amount of alcohol in an individual's breath.
Once alcohol is absorbed into the bloodstream, the liver breaks down (metabolizes) the alcohol. Once alcohol has been completely metabolized, the concentration of alcohol is equal throughout the body, including in the breath. Breathalyzer? tests can only detect that alcohol that has already been metabolized by the body. In other words, if the body is still absorbing alcohol, a Breathalyzer? test may detect a lower BAC than the individual actually has.
During the test, the individual blows into a plastic tube that is connected to a BAC monitor. The Breathalyzer? then provides an estimate of the individual's BAC. Eating breath mints or gum will not affect the results of a Breathalyzer? test. This is because the test does not measure the smell of the breath but the alcohol content of the breath.
Law enforcement officials commonly use Breathalyzer? tests to determine if an individual is under the influence of alcohol. Individuals can also purchase disposable Breathalyzer? tests at their local pharmacies and drug stores.
Standardized field sobriety test: Not all police departments use Breathalyzer? tests in the field. In many cases, police officers will use the standardized field sobriety test if it is suspected that someone is driving under the influence. The standardized field sobriety test is a series of three tests that are used to evaluate an individual's level of impairment.
First, the police officer looks at the individual's eyes for an increase in the normal, involuntary jerking of the eye that occurs when the eyes move to the side, called horizontal gaze nystagmus. When an individual is impaired by alcohol, nystagmus is exaggerated.
The police officer will also evaluate the individual's ability to walk and turn. During this part of the test, the individual is asked to take nine steps, heel-to-toe, along a straight line. Then the individual must turn around and repeat the exercise in the opposite direction. If the individual loses his/her balance, starts before the instructions are finished, stops walking to regain balance, steps off the line, uses the arms to balance, does not touch heel-to-toe, makes an improper turn, or takes an incorrect number of steps, it indicates that the individual may be impaired. Evidence has shown that 79% of individuals who exhibit two or more of these indicators have a BAC of 0.08 or higher.
The next test is commonly called the one-leg stand test. The individual is asked to stand with one foot about six inches above the ground and count out loud until told to put the foot down. The police officer times the person for 30 seconds. During this exercise, the officer looks for four specific signs of impairment: using the arms to balance, hopping to maintain balance, swaying while trying to balance, and putting the foot down. Evidence suggests that 83% of individuals who exhibit two or more of these indictors have a BAC level of 0.08 or higher.
If the police officer concludes that the individual appears to be under the influence, he/she is arrested, and a Breathalyzer? test is performed at the police station. Some states allow the person to request a blood or urine test to confirm the results.
Blood test: A blood alcohol test is the most accurate way to measure an individual's BAC. Because alcohol is quickly absorbed into the bloodstream, this test can be performed just minutes after consuming an alcoholic beverage. However, this test is more expensive and invasive, and it cannot be performed on site. Therefore, it is performed less often than a Breathalyzer? test.
Urine test: A urine test may also be performed. This test assumes that there are 1.3 parts of alcohol in the urine for every one part of alcohol in the blood. However, this ratio can vary significantly among individuals. Also, it may take up to two hours after alcohol consumption for alcohol to be detectable in the blood.

Alcohol and the body

Alcohol (also called ethanol) acts as a drug affecting the central nervous system (CNS). Its behavioral effects, such as slurred speech or stumbling, are a result of its influence on the response in the nervous tissue and not on the muscles or senses themselves. Alcohol is a depressant, and depending on the dose, it can be a mild tranquilizer or a general anesthetic.
At very low doses, alcohol can appear to be a stimulant by suppressing certain inhibitory brain functions. However, as the blood alcohol concentration increases, further suppression of nervous tissue functions produce the classic symptoms of intoxication, including slurred speech, unsteady gait, disturbed sensory perceptions, and inability to react quickly. At high concentrations, ethanol produces general anesthesia. A highly intoxicated person will be in a coma-like state and very difficult to wake. In extreme cases, if the alcohol concentration is high enough, it will inhibit basic involuntary bodily functions, such as breathing, and may cause death.
Alcohol is metabolized, or broken down, by the liver. Metabolism involves a number of processes, one of which is called as oxidation. Through oxidation in the liver, alcohol is detoxified. Alcohol and toxins are removed from the blood, preventing them from accumulating and destroying cells and organs. A small amount of alcohol escapes metabolism and is excreted unchanged in the breath, sweat, and urine. Until all the alcohol consumed has been metabolized, it is distributed throughout the body, affecting the brain and other tissues.
The liver can metabolize only a certain amount of alcohol per hour, regardless of the amount that has been consumed. The rate of alcohol metabolism depends, in part, on the amount of metabolizing enzymes in the liver, which varies among individuals. In general, after the consumption of one standard drink, the amount of alcohol in the drinker's blood peaks within 30-45 minutes. A standard drink is defined as 12 ounces of beer, five ounces of wine, or 1.5 ounces of 80-proof distilled spirits, all of which contain the same amount of alcohol. Alcohol is metabolized more slowly than it is absorbed. Therefore, consumption needs to be controlled in order to prevent alcohol from accumulating in the body and causing intoxication.
Some individuals may experience facial flushing in response to alcohol. Alcohol flush reaction occurs in individuals who have an inactive enzyme called aldehyde dehydrogenase 2 (ALDH2). This enzyme normally breaks down a byproduct of alcohol called acetaldehyde. Without ALDH2, acetaldehyde accumulates in the body and causes flushing. Other symptoms, such as dizziness, nausea, headache, and increased heartbeat, may also occur. Alcohol flush reaction is particularly common among Asian individuals. As many as 50% of Asians experience facial flushing in response to alcohol.