In Oregon, a person may be found guilty of driving under the influence of intoxicants (DUII) several ways. When it comes to alcohol, a person may be found guilty if a jury determines a person's blood alcohol content (BAC) was .08 or above at the time the person was driving. A jury also may find the person guilty of DUII involving alcohol if the jury believes the person was under the influence of intoxicating liquor at the time the person was driving.
To measure BAC, Oregon uses the Intoxilyzer 8000. The Intoxilyzer 8000 is a relatively large machine, about the size of a typewriter, that requires the person to provide three breath test samples in order to calculate a BAC. When used properly and when the correct procedures are followed, the Intoxilyzer provides a decently accurate estimate of a person's BAC at that time. The downside to the Intoxilyzer 8000 is that it is a large machine and cannot be used at the arrest scene. In order to obtain a BAC sample, an arresting officer must take the arrested person to the police station in order to get the person to provide a sample. Thus, BAC samples cannot be obtained instantaneous to the person driving or to the person's arrest. Obtaining a sample can take anywhere from 30 minutes to several hours after the person was driving. This leads to the obvious problem that the BAC sample does not tell us exactly what the person's BAC was at the time the person was driving.
Alcohol is not a constant in a person's body. When alcohol is consumed, it first must be absorbed by the body in order to get in to a person's blood system and affect their mental and physical faculties. Once the alcohol is absorbed, it will dissipate, or eliminate, from the blood system. Thus, alcohol works somewhat as a "bell-curve" in a person's body. The problem is, it is not a perfect bell-curve. The rate at which alcohol absorbs and stays in the blood system varies by the person. The rate and pattern of a person's alcohol consumption can affect how quickly the alcohol absorbs as well as if there is a plateau period, or time for which the alcohol content remains the same. The average dissipation rate ranges from .015-.020% per hour.
Because there are so many variables, merely knowing a person's BAC at a specific time does not tell us exactly what that person's BAC was at any specific time in the past. This obviously is important because it means, without more information, knowing a BAC, based upon a sample from the Intoxilyzer 8000, at some point in time after a person was driving, does not give us enough information to know what that person's BAC was at the time that person was driving. In fact, the Oregon Court of Appeals has held that "the fact that blood alcohol dissipates does not logically lead to any conclusion regarding a specific person's earlier BAC at a specific time. The fact that blood alcohol dissipates is notable for what it does not tell the court. It does not, for example, by itself inform the court whether at any given time, a person's blood alcohol is dissipating or increasing. After all, it is also a matter of common knowledge that, before a person's blood alcohol can dissipate, alcohol must accumulate in the blood."
This is all to say that, just because a person blows .08 or above, it does not necessarily mean that person is guilty of DUII. Much more goes in to it and the prosecutor must present more evidence to the jury to suggest what that specific person's BAC was at the time that person was driving.