The best way to assess intoxication is to observe or test a person’s behavior. Every person will show different intoxication symptoms for the same number of drinks he/she consumes. The law enforcement community observes behavior, but must rely on a legally-defined measure of intoxication. This measure is the blood alcohol concentration (BAC), which is normally obtained by breathing into a Breathalyzer™ device. The BAC is calculated from the concentration of alcohol in the breath. In most states, the legally-defined level of alcohol intoxication is a BAC of 0.08%, although Utah is the first state to lower the legal limit for driving to a BAC of 0.05%.  In many states the BAC limit is even lower (0.02%) for teen drivers. [The BAC can also be determined from taking a blood sample, but this procedure takes too much time.]

Learn more more about alcohol and the Breathalyzer™ test.

However, the BAC does not necessarily reflect the actual degree of intoxication. In fact, for the same number of drinks, the degree of intoxication and the BAC can vary significantly among individuals depending on the pattern of drinking, body weight, gender, and genetics (discussed in the next section).

Learn more about alcohol and gender differences.

In addition, the rate at which someone eliminates alcohol from his/her body is important. Because alcohol is eliminated slowly from the body (by metabolism), the BAC will remain elevated well after drinking has ended. If someone is tolerant to alcohol, the rate of elimination by metabolism is most likely faster (see the previous section).

A general association between BAC ranges and the effects of alcohol in the average person can be shown in the table below (Figure 2.7). Note that impairment of alertness, judgment, and coordination can occur at BACs well below the legal limit for driving a vehicle. This has prompted some states, like Utah, to even consider a BAC of 0.05% as the legal limit for driving while impaired from alcohol.

* a drink is considered a 12 oz beer, a glass of wine (8 oz), or 1 shot (1.5 oz) of liquor (e.g., vodka, gin); each has about the same amount of alcohol

Figure 2.7 Estimation of how the BAC and effects of alcohol correspond to the number of drinks consumed.