Enforcement of DUI laws often depends on the ability of an expert known as a toxicologist to show how alcohol interacts with your body and affects your ability to drive. This section explains in some detail what you would need to know about this subject should your case go to trial. It might also help you understand why you failed a breath test (even though you didn't think you had ingested enough alcohol to produce the high test result) and how you can tell what your BAC might be, should you be tempted to drive home from the bar or party.
Just as the amount of gasoline in your fuel tank depends on how often you fill it and how much you burn off as you drive, the amount of alcohol in your bloodstream is determined by a balance between how fast alcohol is absorbed into your blood and how fast it's eliminated from it. Elimination occurs when most of the alcohol is "burned" or "oxidized" in your body, while the rest of the alcohol is excreted in breath, urine, and perspiration. Since alcohol is eliminated from the bloodstream at a fairly steady rate, the degree of intoxication depends a lot on the rate of absorption. If alcohol is absorbed rapidly into the bloodstream, the blood alcohol level will get high fast—and so will you. If it is absorbed slowly enough to be eliminated before it builds up, you won't feel very high.
When you take a drink, the alcohol is absorbed into the blood through the mucous lining of the entire gastrointestinal tract: the mouth, the esophagus, the stomach, and the small intestine. The rate of absorption increases as the drink moves down the tract. Absorption from the stomach into the bloodstream (by way of blood-carrying capillaries in the stomach lining) is faster than from the esophagus or mouth. The street wisdom, which says that drinking on an empty stomach will get you higher, faster, is true because there is nothing else in your stomach to compete with the alcohol in terms of getting absorbed. The fastest rate of absorption is from the upper end of the small intestine.
For an "average individual," about 60% of the alcohol consumed at a given time will have been absorbed into the bloodstream a half-hour later. About 90% will have been absorbed in an hour, and all of it will have been absorbed in an hour and a half. However, this is just for an "average" individual with an "average" stomach food load, drinking "average" drinks. In fact, the rate of alcohol absorption depends on all sorts of things: the quantity of alcohol ingested, the concentration of alcohol in the drink, the rate of drinking, and the nature and amount of diluting material already in the stomach.
Alcohol is eliminated from the body in two ways. Ninety to ninety-five percent of it is oxidized, mostly in the liver, to form water and carbon dioxide (a gas that dissolves in the blood, goes to your lungs, and is exhaled). The rate of its oxidation is pretty much the same over time but varies depending on how well a person's liver functions. People who drink regularly burn alcohol faster than casual drinkers. Chronic alcoholics burn it even faster. The remaining 5% to 10% of the alcohol is eliminated unchanged by perspiration, in urine by way of the kidneys and bladder, and in the breath by way of the blood as it reaches the lungs.
Because driving with a .08 BAC is illegal, or at least a basis for being presumed under the influence, it can be helpful for you to be able to estimate your own blood alcohol level at any given time, based on the number of drinks you had and the time you had them. Although a person's exact blood alcohol level depends on a number of factors, there's a simple, reasonably accurate way you can figure what your highest possible blood alcohol level could be (for example, if you drank very fast on an empty stomach).
If you divide the number 3.8 by your body weight in pounds, you should obtain a number between .015 and .40. Call this your own personal "blood-alcohol maximum-per-drink" number. This is the maximum percentage alcohol that will be added to your blood with each "drink" you take. For the purposes of this calculation, a "drink" is a 12-ounce, 4% alcohol, bottle of beer, or a 4-ounce glass (a small wine glass) of 12% alcohol wine, or a one-ounce shot glass of 100 proof liquor (most bars's mixed drinks have this amount of alcohol). (Microbrewery beer, malt liquor, pint bottles of beer, large (6 oz.) wine glasses, 20% alcohol ("fortified") wines, and very stiff or large mixed drinks should be counted as "1