The Anatomy of a DUI – Alcohol and the Dose Response Relationship

Dose-Response Relationships

People often ask the questions, "how 'high' is 'drunk'?" What is the "legal limit" for "drunk driving"? How much can a person drink before becoming "impaired" or “under the influence”? There is no simple answer to these or similar questions, except to say that any amount of alcohol may affect a person's ability to drive to some degree.

It is true that California has established a blood alcohol concentration (BAC) limit of 0.08% (grams of alcohol in every 100 milliliters of blood) which makes it explicitly unlawful to drive a vehicle at the .08% BAC level (Vehicle Code §23152(b)). But California also makes it unlawful to drive when "under the influence" of alcohol, (Vehicle Code §23152(a)), thus the law admits the possibility that a person may be under the influence at much lower BAC than 0.08%.

How much alcohol does someone have to drink to reach these levels of blood alcohol concentrations? Obviously, it depends on how much time the person spends drinking, on whether the person is a man or a woman, on how large the person is, on whether the drinking takes place on an empty stomach, has a full meal before drinking, and on certain other factors.

For example, if an average 175-pound man drinks two beers, or two shots of whiskey in quick succession on an empty stomach, his BAC will climb to slightly above 0.04% on average. Two more beers could boost him above 0.08%. In one respect, then, it doesn't take very much alcohol to reach the illegal BAC level or to reach a level that impairs someone - a couple of beers can do it.

Keep in mind that although the above example assumes an average 175-pound man, there really is no average person as each individual metabolizes alcohol at different rates at differing times.

Practical Application

It is important to understand the information provided in this article (and the previous articles) because it sets out the factors that affect a person’s blood alcohol concentration in relation to their alcohol consumption. By understanding these factors, a person can better control his/her own behavior relating to alcohol consumption and their resulting BAC. For example, if a person sips two drinks (one drink per hour) after eating a high protein meal, the digestion process will break down some of the alcohol while in the stomach thus lessening the amount of alcohol that enters the blood stream. In addition, absorption of alcohol into the blood stream will take longer because of the food in the stomach, thus allowing the body to burn off the alcohol that has already entered the blood stream.   Assuming a 0.015% BAC burn off rate per hour, if a person absorbs 0.015% BAC per hour (two-thirds of a drink per hour for males, one-half a drink per hour for females) then the person would theoretically be burning off the alcohol at the same rate it is being absorbed into the blood stream, thereby resulting in a flat BAC curve. It is when a person absorbs more alcohol than they burn off, which causes a person’s BAC to climb.

Using a bell curve, the more alcohol absorbed that exceeds the burn off rate, the higher the BAC will be over time.   Once drinking stops and all alcohol is absorbed into the blood stream the BAC will have reached its peak (represented by the peak on the bell curve), and will then start to decrease because the burn off rate (0.015 per hour) will burn off the alcohol in the body until it is all metabolized and BAC reaches 0.00%. A person that peaks at .15% BAC will return to 0.00 BAC in about 10 hours (on average).