The rate at which alcohol is removed from the body depends on the person's size, the age of the person, the person's sex, and their tolerance. That said, it is generally accepted that alcohol is removed from the body at a rate of 0.5 ounces per hour. It would take about an hour to eliminate the alcohol found in a beer.
Blood alcohol concentration is the most accurate source for determining alcohol in the body. However, because alcohol is processed rather quickly, alcohol can be detected only for short periods of time (about 12 hours) via this method.
You can find more information about testing for alcohol here: http://www.intox.com/about_alcohol.asp
In addition, the urine may contain metabolites (ethyl gluconeride) of alcohol for up to 3-5 days after drinking. There are some claims that hair analysis can detect the presence of alcohol for years.
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