Overview of Blood Alcohol Concentration (BAC) in Drunk Driving Cases

All states and the District of Columbia have drunk driving statutes. Every statute contains a legal definition of intoxication, but the legal definition does vary between the states. In order for someone to be convicted of driving while under the influence (DUI), the prosecution must prove that the defendant was so affected by the consumption of alcohol that the defendant’s faculties were impaired. It is not always necessary to show that the defendant’s driving ability was impaired.

Due to numerous problems in demonstrating that a defendant was driving while under the influence, each state has created a statutory offense of per se intoxication that is based on the defendant’s blood-alcohol content level (BAC). The definition of blood alcohol content is the concentration of alcohol in the defendant’s bloodstream, measured as a percentage. In the United States, BAC is measured by dividing grams of alcohol by 100 milliliters of blood. For example, a BAC of .10 percent means that for every 100 mL of blood, there is 8 grams of alcohol in the blood stream.

How much alcohol is required to cause acute alcohol intoxication varies from person to person. Variables that will affect BAC include body weight, body fat, stomach contents, speed of drinking, general health, and individual tolerance for alcohol. A BAC of 0.10% produces signs of intoxication in about half the population. However, a BAC as low as 0.02% or 0.03% will produce noticeable effects in some individuals. Many people will appear to be intoxicated at a lower BAC than 0.10%.In essence, determining an individual’s BAC is an indirect attempt to measure the amount of alcohol present in brain tissue as alcohol consumption affects the brain. As it would be impossible to sample brain tissue in living people, blood, breath, saliva, or urine samples are typically used to measure the presence of alcohol in the live body. However, the tests do have an inherent margin of error. The margin of error is small, but it may become an issue if the test result is at or very close to an illegal level.

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