How Alcohol Can Lead to a DUI or DWI

Learn about blood alcohol concentration (BAC) and how it impairs your driving.

You can be charged with a DUI (driving under the influence) or DWI (driving while intoxicated) if your blood alcohol level (BAC) is above 0.08% or if your driving is impaired. Alcohol doesn't affect everybody in the same way, but the truth for everyone is that if you drive with enough alcohol in your system, the result can be lethal. And whether or not serious injuries result, you could be charged with very serious crimes. (To learn the basics of DUI and DWI law, see Nolo's article DUI and DWI Overview.)

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 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 the 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 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 1

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