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: How reliable is a breathalyzer?

On Behalf of | Jun 5, 2015 | Drunk Driving

Whenever a motorist in Pennsylvania is stopped on suspicion of drunk driving, the arresting officer must request that the motorist submit to a test to determine his or her blood alcohol content (“BAC”). The most common method of administering such a test is the use of a “breathalyzer,” a device that measures the amount of alcohol in a person’s lungs and then determines the amount of alcohol in the person’s blood. (The request for the test can be refused, and the consequences of a refusal were explored in this blog on May 13, 2015.) Most people, including the police who use them, assume that the breathalyzer test is both accurate and reliable, but this assumption is not necessarily correct.

The breathalyzer does not measure BAC directly. Instead, it measures the percentage of alcohol in a person’s lungs and uses a ratio (1/2,100) to determine BAC. However, this ratio may vary from person to person and even, over time, for the same person. Thus, an accurate measurement of BAC for one person may be significantly inaccurate for another person. Some harmless substances in a person’s mouth, such as mouthwash and breath fresheners, may inflate the amount of alcohol in the breathalyzer sample. Using the breathalyzer requires following an exact protocol, and any departure from this protocol can cause a false measurement.

Breathalyzers can malfunction. They must be carefully calibrated using air samples containing known amounts of alcohol. Each police department should keep records of when each breathalyzer has been calibrated.

A skilled criminal defense attorney can use this knowledge to impeach the breathalyzer test results if they are introduced as evidence of the defendant’s intoxication. This impeachment can have a decisive effect on the verdict, especially if the prosecution has not offered any other measurement of BAC. Any person who has failed a breathalyzer test and faces drunk driving charges may wish to consult an attorney who specializes in defending drunk driving cases for an evaluation of the facts of the case and an estimate of probable outcomes, including the chances of obtaining an acquittal.

Source:, “BAC Test FAQs,” Accessed June 4, 2015