There are three types of tests used by law enforcement officers in determining blood alcohol concentrations (BAC) of drivers. There is a blood test, a urine test and a breath test. Each test has its own strong and weak points. Under normal circumstances, drivers suspected of having high blood alcohol concentrations are asked to submit to a breath test. This test is the most commonly used because it provides the officer with an immediate result. Blood and urine samples must be submitted to a laboratory for testing.
In Minnesota, as of August 1, 2005, it is illegal to operate a motor vehicle with a BAC of .08 or more. It is also a crime to test with a blood alcohol concentration of .08 or more within two hours or driving conduct. Even if blood alcohol levels are below the .08 threshold, a driver may still be cited for "operating a motor vehicle under the influence of alcohol" if the officer observed deviant driving conduct (ie. weaving)
Blood alcohol concentrations are determined by testing the level of alcohol present in a driver's blood. Alcohol is testable because it is not processed like other food products. When alcohol is ingested it is absorbed into the bloodstream. This absorption is what causes the alcoholic effect we call intoxication.
The rate at which alcohol is absorbed into the blood stream is fairly constant. Generally, alcohol is absorbed into the blood stream within 50 minutes after it has been ingested. There are some factors which can affect the absorption rate. Food can slow the absorption rate by providing a place for alcohol to reside until the food product begins to digest in the body. Then, the alcohol is released and absorbed. Usually only a large quantity of food will have a noticeable effect on slowing absorption.
When alcohol is absorbed into the blood stream, it travels by way of the blood vessels and is distributed to cells throughout the body. The larger the individual, the more alcohol is needed to achieve a higher BAC. Given the standard rate of alcohol absorption, charts have been created to help determine how many ounces of alcohol it takes for persons of varying weights to reach specific blood alcohol concentrations.
Alcohol is also eliminated from the body at varying rates of speed. This elimination occurs when alcohol is metabolized by the liver and excreted, when it is exhaled or when it is expelled in our sweat.. The rate at which alcohol is removed from the body depends largely on a persons metabolism since metabolic processes are responsible for the vast majority of alcohol removed from the body. It is a fairly standard assumption that one shot, one 12 ounce beer or a glass of wine can be eliminated from the body in approximately one hour after it has been ingested. Although that may be true for most people, the elimination of alcohol may be slowed by different factors resulting in a higher BAC if consumption continues. Since metabolism requires the liver to process alcohol before it is expelled from the body, persons with liver problems may be unable to process alcohol at an ordinary rate. The alcohol is eliminated more slowly and the BAC rises as more alcohol is consumed.
Breath testing requires that a person provide a continuous sample of breath for an extended period of time (usually 30 seconds) in order to assure that air from the lungs is being tested. Machines that test air samples are testing alcohol levels in the air of the lungs. This is called "deep lung air." Scientific theories hold that concentrations of alcohol in breath will have constant relationships to alcohol levels in the blood beneath the surface. The formula that is used to compute blood alcohol levels from breath alcohol levels is 2100 to 1. This means that 2,100 milliliters of air will have the same alcohol content as one milliliter of blood. A BAC reading of .08 refers to the concentration of alcohol as it relates to 8 tenths of one milliliter of blood.
The equipment that is used to test BACs is subject to several problems. It is required that the operator of the machine use a test sample with a pretested reading. If the machine is working accurately, it will test the sample provided prior to each test of a driver's breath within the expected range. If the equipment malfunctions, an invalid reading is obtained. Equipment used to test breath samples may also be affected by radio waves. Operating the equipment in proximity to airports or in the presence of an operational radio may invalidate the test
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