How Alcohol Effects People's Ability to Drive, Leading to DUI or DWI

by Attorney David Brown

How can people who think they're just happy after a drink or two get arrested for a DUI or DWI?

In health terms, a little bit of alcohol isn't always a bad thing -- some studies for example, suggest that a daily glass of wine can improve your health. But when people with alcohol in their system try to drive, the results can be lethal.

Alcohol reaches the brain within seconds after entering the bloodstream. Once there, it has a depressant effect, which results in all of the following:

  • Less efficient vision and hearing.
  • Lack of muscular coordination (clumsiness).
  • Deterioration of judgment and self-control (euphoria and loss of inhibitions).

The extent of impairment will vary from person to person. Some people, particularly regular drinkers, build up a sort of immunity to alcohol -- even when their blood alcohol level (BAC) reaches 0.08% (the standard legal limit) or more.

At the other extreme, people who normally abstain from alcohol begin to suffer slight impairment at a level as low as 0.02%! Moderate drinkers begin to show mild symptoms at 0.04% to 0.07%, while some heavy drinkers require 0.07% to 0.09% to suffer any impairment at all.

The amount of alcohol in your bloodstream depends first on how much the person drinks (which we'll discuss below) and then on the following:

  • How fast alcohol is absorbed into the bloodstream.
  • How fast it's eliminated from the bloodstream.

Absorption Into the Bloodstream

When someone takes 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, faster and faster as the drink moves down the tract. The common wisdom saying that drinking on an empty stomach will get you higher faster is true, because there's nothing else in your stomach to compete with the alcohol in getting absorbed. In addition, water and citrus juice in the stomach can slow down the absorption of alcohol, while the presence of carbonation in a drink (as in champagne) can speed up the absorption of alcohol.

For the average person, 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 taken in, 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 leaves the body in two ways:

  • Oxidation, or metabolism.
  • Excretion.

Ninety to ninety-five percent of the alcohol is oxidized, or metabolized, 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 oxidation is pretty much the same over time, but depends 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 excreted or eliminated without any chemical change. It leaves the body via sweat, urine, and in the breath, as a gas (which is what the breath test, or breathalyzer, measures).

Now let's look at how long this two-pronged alcohol-elimination process takes. After about 40 minutes have passed, the average person's body will begin eliminating alcohol from the bloodstream at a rate of about 0.01% for each additional 40 minutes.

Calculating Individual Limits

At this point, you may be wondering, how much can you drink and stay safely within the blood-alcohol level (BAC) limits. Here's a simple way to calculate that:

If you divide the number 3.8 by your body weight in pounds, you should get a number between 0.015 and 0.040. 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 purposes of this calculation, a drink is a 12-ounce, 4% alcohol, bottle of beer, a four-ounce glass (a small wine glass) of 12% alcohol wine, or a one-ounce shot glass of 100-proof liquor (most bars' mixed drinks contain 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 example, if you weigh a petite 100 pounds, you'd run this calculation and get .038, meaning you could possibly have had a blood alcohol level of up to 0.038% from just one drink. That would rise to 0.076% from two drinks, and three drinks could put you over the 0.08% blood alcohol level, especially if you drank them quickly on an empty stomach.

If, by contrast, you weighed 240 pounds, your maximum blood alcohol increase per drink would be only 0.016%. You'd barely feel the effects of the first one. To get past 0.08% blood alcohol, you'd have to down six drinks in an hour. Of course, these are averages, and different people process and absorb alcohol at different rates.

In all states, 0.08% BAC is now the legal limit for driving; anything over that can get you arrested and convicted for a DUI (driving under the influence) or DWI (driving while intoxicated).

Reprinted with the permission of Nolo Press.  Copyright 2010 Nolo,