Each state in the country has laws regarding drinking and driving. Those laws state that someone who has a blood alcohol content of .08 percent or more is legally intoxicated and can be charged with a DUI or a DWI.
How is someone's BAC determined? Most police departments across the nation have three ways that someone's BAC is tested. The most accurate is a blood test. This is an invasive test that is inconvenient and must be done at the hospital or other clinical setting. Preliminary breath testers are often used by police because they are easy to use, portable and lightweight. However, many states do not allow the results of a PBT in court. A breath analyzer is a machine that is usually in the police station. It requires a great deal of calibration and the operators must be trained and certified.
There are many factors that can have an influence on the results of a breath test. Some of those factors are:
-- Calibration: As stated before, calibration is required periodically, and the batteries will need to be replaced so the machine maintains its accuracy.
-- Software: Because a Breathalyzer has special software that occasionally has glitches and bugs.
-- Foreign substances: If a suspected drunk driver has substances in his or her mouth, then the machine could give false positives. Some everyday substances like toothache medicines, breath fresheners and mouthwashes can contain alcohol.
-- Human error: As simple as it is to use a Breathalyzer, it will require attention to detail by the officer using it.
-- Consistency: For a Breathalyzer to give accurate results, repeated tests are necessary.
-- Environmental factors: Some false results may be given if there are things like chemicals, paint fumes or varnishes in the room.
As you can see, there are a number of factors that can skew the results of a Breathalyzer. Many times, a person facing DUI or DWI charges may present a successful defense by challenging the Breathalyzer results.
Source: BACtrack, "Are Breathalyzers Accurate?," accessed July 02, 2015