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Blood Alcohol Concentration

By Gerry Kupferschmidt, B.Sc., M.Sc.

Blood alcohol concentration (BAC) reflects the amount of alcohol in the body. Food, alcohol content and quantity of beverage, weight, sex, and rate of elimination determine the BAC after the consumption of alcohol.

To estimate blood alcohol level at a given time, knowledge of other factors is required. These factors may include age and height of the subject; consumption start and stop times, pattern of drinking; times when meals were eaten; disease states; and any medications that may have been taken.

Alcohol absorption

Alcohol is absorbed from the stomach and small intestine. Most absorption occurs from the small intestine due to its large surface area and rich blood supply. The rate of absorption varies with the emptying time of the stomach. Generally, the higher the alcohol concentration of the beverage, the faster the rate of absorption. However, above a certain concentration, the rate of absorption may decrease due to the delayed passage of alcohol from the stomach into the small intestine.

The maximum absorption rate is obtained with the consumption of an alcoholic beverage containing approximately 20-25% (by volume or v/v) alcohol solution on an empty stomach. The absorption rate may be less when alcohol is consumed with food or when a 40% (v/v) alcohol solution is consumed on an empty stomach. The rate may also slow down when high fluid volume/low alcohol content beverages, such as beer, are consumed.

Normal social drinking

For normal social-type drinking, the highest BAC is usually achieved within 30 minutes after completion of consumption, though it could take as long as 60 minutes. When large amounts of alcohol are consumed over a short time, or when a large quantity of food is eaten with the alcohol, the absorption phase may continue for up to two hours after last consumption.

Weight and sex affect BAC

A person's weight and sex determine the total volume of body water and consequently the BAC obtained upon consumption of a particular quantity of alcohol. Generally, the more a person weighs, the larger the volume of body water and the lower the BAC obtained from the consumption of a given amount of alcohol.

A female may have more fat tissue than a male of the same weight and therefore a smaller volume of body water. As a result, a female may obtain a slightly higher BAC upon consumption of the same quantity of alcohol as a male, all other factors being equal.

Elimination of alcohol

Alcohol is eliminated from the body by excretion and metabolism. Most alcohol is metabolized or burned in a manner similar to food, yielding carbon dioxide and water. A small portion of alcohol is excreted, such as through the breath, leaving the body as alcohol, unchanged. It is this latter process that allows for breath alcohol testing.

Average rate of elimination

Elimination occurs at a constant rate for a given individual. The median rate of decrease in BAC is considered to be 15 milligrams per cent (mg%) per hour. The range of decrease in BAC is 10-20 mg% per hour. This range represents the extreme ends of the rate encountered in a normal population. Most people eliminate at a rate of between 13 and 18 mg% per hour. Of these, the majority eliminates at the higher end. Very few people eliminate at as low a rate as 10 mg% per hour.

Mr. Kupferschmidt is a forensic toxicologist. For a longer version of this article and information on related topics visit his website, www.forcon.ca. He can be reached at services@forcon.ca.

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