Blood Alcohol Concentration

Blood alcohol concentration (BAC) is the concentrated amount of alcohol within a person's bloodstream. One of three different methods is used to measure BAC: a proportion of a person's size, mass per quantity, or a combination of the two. For example, a BAC of .30% can mean 3 grams of alcohol per 1,000 grams of someone's blood, or it can mean .3 gram of alcohol per 100 milliliters (ml) of blood.

How many drinks someone has had is not the best method for gauging the level of inebriation, primarily because individuals differ in their alcohol tolerance. One drink increases a normal person's BAC by approximately .03%. However, this fluctuates due to the high level of individual differences concerning body mass, gender, and body fat percentage. Moreover, the amount of alcohol a person consumes, a person's BAC, or both, are not always precise guides for level of drunkenness. On average, the more frequently a person drinks, the more his or her body becomes tolerant, though this level depends upon genetics, personal habits, and the synergism of different drugs.

Alcohol is eliminated from the body by metabolism, excretion, and evaporation. While there are individual differences in the percentage of alcohol eliminated from the body through each function, about 90% to 98% is burned through metabolism, 1% to 3% is excreted by the kidneys, and about 1% to 5% evaporates through breathing. A minute amount (less than .5%) leaves the body by excretion in a person's tears and sweat. Approximately 40 to 50 minutes after a person begins drinking, excretion of alcohol by urination begins, while metabolism starts at the moment alcohol is absorbed, even before alcohol's effects have made it to the brain. This means that before a person feels "tipsy," his or her body has already started metabolizing alcohol.

Alcohol is eliminated from the bloodstream at a constant rate. This elimination differs from person to person, but an experienced male alcoholic with an elevated body mass may be able to deal with up to 30 grams (38 ml) per hour, whereas a more average amount is 10 grams (12.7 ml) per hour. Individuals under age 25, females, specific ethnic groups, and anyone with hepatic disease may metabolize alcohol more slowly. Other factors that influence BAC include

• The number of drinks consumed during a drinking episode
• The relative proof (alcohol content) of the alcohol consumed
• Whether or not food was eaten prior to drinking or while drinking
• The person's body fat (people with more body fat can drink more and maintain a lower BAC)

Law enforcement agencies use BAC as a method for defining intoxication levels and as an approximate guide of level of impairment. The level of alcoholic impairment varies from person to person, though they might have identical BAC levels. Even so, BAC can be calculated scientifically and, thus, is usually not easy to dispute in court. In most countries, it is illegal for someone with a high BAC level to operate motor vehicles or heavy machines. Likewise, watercraft and aircraft have restrictions regarding BAC level.

BAC can be calculated in any hospital by a blood test, which is the most accurate method. Most law enforcement agents use a breathalyzer, an instrument in which an individual breathes and receives a breath alcohol concentration. The assumption inherent in the breathalyzer is that the person tested is "average." For instance, with a typical individual, the relative amount of BAC to breath alcohol content is 2,100 to 1. That is, 2,100 parts of alcohol may be found in the average person's blood for every part in his or her inhalation. On the other hand, the precise amount in someone can differ as much as 1,300:1 up to 3,100:1. This percentage changes with each individual, as well as from one moment to the next. Hence, someone possessing a BAC of .08 with a ratio of 1,700:1 at the specific time of the test, would present a BAC of .10 when read by a breathalyzer due to its calibration of 2,100:1. Obviously, this could have serious ramifications for an individual suspected of drunk driving.

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