Understanding Blood Alcohol Content (BAC)

Blood alcohol content (BAC), also known as blood alcohol concentration, is a term used to describe the amount of alcohol (ethyl alcohol or ethanol) in your bloodstream at any given time. Law enforcement authorities use this measurement to determine whether or not a person is legally intoxicated, and medical professionals use it to determine health risks related to alcohol poisoning. An understanding of BAC requires an understanding of the nature of ethyl alcohol. It also requires an understanding of how this substance affects the body when BAC levels start to rise.

Ethyl Alcohol Basics

Ethyl alcohol comes from a chemical process called fermentation, which occurs when the sugar content of sugar cane and various types of grains and fruits get broken down in the presence of microorganisms called yeasts. Alcohol is a waste product made by yeasts during their breakdown of sugar. There are four main types of alcoholic beverages—beer, malt liquor, wine, and distilled liquor—each of which comes from a variation on the basic fermentation process.

Each specific fermentation method produces ethyl alcohol at a different rate; this means that each type of alcoholic beverage has a different basic alcohol content. In order to avoid confusion and make useful comparisons between different types of beverages, scientists use a measurement called a standard drink, which equals 0.6 ounces of ethyl alcohol. Beer has the lowest amount of alcohol, with 0.6 ounces in every 12-ounce serving. Malt liquor contains 0.6 ounces of ethyl alcohol in every 8-ounce serving, while wine contains 0.6 ounces of alcohol in every 5-ounce serving. Distilled liquor that is 80-proof contains 0.6 ounces of alcohol in every 1.5-ounce serving. Each of these serving sizes equals one drink.

Intoxication and Blood Alcohol Content

Although we don’t usually think of it in this way, alcohol intoxication is a form of poisoning. Your body deals with this poisoning by passing the ethyl alcohol in your bloodstream to your liver. Cells within this organ then gradually break alcohol down and render it harmless. However, your liver processes ethyl alcohol quite slowly, and if you consume even one drink per hour, you will start to overwhelm its limited abilities. Intoxication occurs when processing in your liver backs up and alcohol begins accumulating in your bloodstream.

Blood alcohol content is measured by the weight of ethyl alcohol contained in a given volume of blood. To make this measurement easier to understand, doctors and scientists commonly convert it into percentages. Typically, the first effects of intoxication—including mild relaxation and some lightheadedness—appear when BAC reaches a level of 0.02 to 0.04 percent. When BAC reaches 0.06 percent, common effects include increased relaxation, euphoria, increased talkativeness, intensification of your existing mood and some degree of judgment impairment. A BAC of 0.08 percent is the standard for legal intoxication in all 50 US states, as well as in federal statutes. Common effects of this level of intoxication include clear impairment of normal judgment and muscle coordination, as well as impaired sight, hearing, and self-control.

Blood alcohol concentrations from 0.10 percent to 0.20 percent produce increasing degrees of drunkenness, accompanied by highly impaired muscle function, body balance, memory and judgment.

As BAC nears 0.20 percent, the true poisonous nature of ethyl alcohol starts to become apparent; effects associated with this degree of intoxication include vomiting, blacking out and failing to notice or respond to injuries. When BAC reaches 0.30 percent, common effects include passing out completely and failing to wake up when roused; people with this much alcohol in their blood can also easily die from alcohol poisoning. When BAC reaches 0.35 percent, the affected person may stop breathing. Percentages of 0.40 percent or higher commonly produce coma and death.

Factors That Affect BAC

Apart from the amount of alcohol you drink and the speed of alcohol consumption, several additional factors can affect the blood alcohol concentration in your body. They include your gender, your weight, and the amount and type of food in your stomach. Generally speaking, women have higher BAC than men for any given level or rate of alcohol consumption. Part of the equation here is body fat, not size. The presence of fat gives alcohol easier access to the bloodstream, and people with higher body fat levels get intoxicated relatively quickly. Women typically have higher body fat percentages than men. Also, when alcohol use is the same, people with a relatively low body weight typically have a higher BAC than people with a relatively high body weight. The presence of food in your stomach will keep BAC lower by delaying the entry of alcohol into your bloodstream; protein, in particular, helps delay alcohol absorption.