DWI - Driving While Intoxicated.
DUI - Driving Under the Influence (of alcohol or drugs).
BAC - Blood Alcohol Concentration (or Content).
These three acronyms form a deadly combination. The main links between the three are alcohol and driving. It is well documented that drinking alcohol impairs judgment, slows reaction time, and decreases coordination. When a person is DWI or DUI, he or she is piloting a one-ton time bomb with little control over it. Is it speeding towards you?
These three acronyms have legal ramifications too. DUI and DWI are criminal offenses. To determine if a DWI or DUI offense has been committed, it is necessary to measure how much alcohol was consumed by the suspect driver (the BAC).
In the field, police officers utilize tests to determine the amount of a driver's impairment due to alcohol. Such tests include, but are not limited to, breath alcohol tests and field sobriety tests. The breath test is further discussed in this article.
A person's reaction to alcohol varies widely. Some factors that affect alcohol impairment include gender, body weight, time of last meal, time of day, alcohol tolerance, medications, genetics, rate of alcohol consumption, and many more.
A police officer stopping a suspected drunk driver has visual, olfactory (smell), and breath tests to judge a person's impairment from alcohol. The officer must make some kind of quantitative determination of the amount of alcohol impairment at the time of the stop. All the factors mentioned above do not matter to the officer. All he or she wants to do is to create a "snapshot" of the degree of impairment.
Blood alcohol concentration (BAC) is a measure of the amount of alcohol in the blood, expressed as a percentage by volume. This can be measured precisely by taking a sample of the driver's blood for analysis. In the U.S., a BAC level of 0.08% (commonly written as .08) or more is considered illegal. However, the officer on patrol certainly doesn't have a medical lab in the car, and must rely on a breath testing device.
The brand name Breathalyzer is often used generically to describe devices that measure blood alcohol concentration. This type of breath analyzer does not actually measure the concentration of alcohol in the blood. Instead, the reading on the breath analyzer is only an estimate of the alcohol in the blood.
Correlations between the amount of alcohol in the blood and the resulting amount of alcohol in the breath have been made. The tests show the ratio to be approximately 2100 (in the blood) to about 1 (in the breath), or 2100:1. Based on factors listed above, a person's blood:breath alcohol ratio can vary from 1700:1 to 2400:1.
In simpler terms, if an average person has 2100 drops of alcohol in the blood, the breath tester will show a reading of 1 drop. Doctors have measured the blood and breath alcohol contents of test subjects simultaneously, allowing them to calculate the relationship between alcohol in the blood and alcohol in the breath.
When a drunk driving suspect blows into the breath alcohol tester, the machine measures the alcohol in the breath. The machine then calculates the amount of alcohol in the blood by using a blood:breath alcohol ratio. This calculation of BAC is only an estimate. However, that estimate is what goes on the police report and is what the judge will see.
The next time you have consumed alcohol and feel like you are not impaired to drive, remember these points. First, your confidence may be simply alcohol-induced euphoria. Second, your life and the lives of those you drive near may all depend on what the little breath machine thinks is normal. Are you willing to bet your freedom on the opinion of some fancy calculator? Me neither. Be safe!
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