Fighting a DUI: Understanding Alcohol

Find out how alcohol interacts with your body, and how this science plays a role in a drunk driving case.

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.

How Alcohol Interacts With Your Body

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.

More about Absorption

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.

Elimination From the Body

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.

Calculating Approximate Blood Alcohol Levels

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 "1½" drinks.) For each such "drink," your blood alcohol concentration will be increased by about the percentages in the chart below.

Example: Linda Light, weighing a petite 100 pounds, could possibly have had a blood alcohol level of up to .038 from just one drink and up to .076 from two drinks. Three drinks could put her over the .08 blood alcohol level, especially if she drank them quickly on an empty stomach. Hans Heavy, on the other hand, weighs in at 240 pounds, and his maximum blood alcohol increase per drink is only .016. He's barely feeling the effects of the first one. To get past .08 blood alcohol, he'd have to down at least six drinks in an hour.

Now let's look at how long the alcohol elimination process takes. After about 40 minutes have passed, your body will begin eliminating alcohol from the bloodstream at the rate of about .01% for each additional 40 minutes. So, once you multiply the number of drinks you've had by your blood alcohol maximum per drink, subtract .01% from that number for each 40 minutes that have passed since you began drinking—but don't count the first 40 minutes.

Note: If you are so addled by alcohol that you cannot do the math, you are probably too drunk to drive. For those of you who like mathematical shorthand:

Example: One hundred-pound Linda Light's blood alcohol level after two drinks gulped down rather quickly could be as high as .076. But if she drank them over a period of an hour and 20 minutes (or 40 minutes beyond the first 40 minutes) her blood alcohol would be about 0.010% less, or .066. Forty minutes later, it would be down to about .056, and so on. (Keep in mind that these are only approximate calculations.)

Finally, for those of you who prefer bar graphs over numbers and formulas, a reproduction of a set of graphs printed by the California Department of Motor Vehicles is provided below.

Additional Information

Here are a few additional references dedicated to specific subjects:

  • Medical-Legal Aspects of Alcohol, edited by James C. Garriott (Lawyers & Judges Publishing Company).
  • Intoxication Test Evidence, by Edward F. Fitzgerald (Thompson). This book is a treatment of breath, blood, and urine tests. It takes you even deeper than the other general-purpose books.
  • Innovative DUI Trial Tools, by Bruck Kapsack. For more information about this book, see

You may find these books in a law library but more likely you'd have to purchase them (try Amazon). These books for lawyers typically cost $100 or more, but $100 may be a steal if it can help you (or your lawyer) win your DUI case.

In addition to the resources mentioned here, you will learn more about your situation by researching your state's DUI laws. See How to Research Traffic Laws

 Find free legal information at Nolo offers free legal information on a wide range of legal topics, including DUIs and other traffic offenses. Just go to, click "Free Legal Information," then choose your topic.

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