A breath analyser estimates a persons blood alcohol concentration. They do not directly measure a persons blood alcohol concentration. Direct measuring requires a blood sample. However, in the United States the National Highway Traffic Safety Administration (NHTSA) maintains a Conforming Products List of breath alcohol devices approved for evidentiary use, as well as for preliminary screening use. That approval makes the results admissible for legal purposes. Here is a link the Federal Register as support for my statement of the list.
Side Note: Blood Tests
Why do some people also get blood tests? It can be to verify the results of the breathalyser, it can be to look for additional intoxicants in your blood, and it can be because the results are abnormal. For instance, you have a high tolerance and you pass all the Field Sobriety Tests, but your breathalyser BAC is .28 (that was one of my DUII scores). Some juries may believe that the breathalyser was mistaken. It seems reasonable that if you can “walk and talk” you must be alright. A high tolerance may allow for proper “walking and talking”, it does not allow for increased reaction times in a motor vehicle. Any BAC over 0.08 is still against the law. Any BAC over 0.08 is dangerous, even for a person with a high tolerance.
I feel confident that some alcoholics pulled over for reasons other than those associated with drunk driving i.e. bad tail light, are spoken to and let go by the police, even with BACs over 0.08, because they just don’t seem impaired. If somebody looks alright, why subject them to a breathalyser test.
When alcohol enters the bloodstream, it is carried everywhere the blood goes, including the lungs. In the lungs it evaporates, that is why your breath smells like alcohol. Ethyl alcohol (ETOH) is the kind of alcohol people drink for fun and it is ETOH levels that are sampled. A breath analyser work by comparing known breath sample concentration samples of alcohol to their corresponding known blood alcohol concentrations. Here is a study from Forensic Science International. It will be 8 pages of boredom for most civilized people, don’t say I didn’t warn you.
From Wikipedia: Breath Analysers and False Readings:
… products such as mouthwash or breath spray can “fool” breath machines by significantly raising test results. Listerine mouthwash, for example, contains 27% alcohol. The breath machine is calibrated with the assumption that the alcohol is coming from alcohol in the blood diffusing into the lung rather than directly from the mouth, so it applies a partition ratio of 2100:1 in computing blood alcohol concentration—resulting in a false high test reading. To counter this, officers are not supposed to administer a preliminary breath test for 15 minutes after the subject eats, vomits, or puts anything in their mouth. In addition, most instruments require that the individual be tested twice at least two minutes apart. Mouthwash or other mouth alcohol will have somewhat dissipated after two minutes and cause the second reading to disagree with the first, requiring a retest