Breathalyzer tests use blood alcohol content (BAC) points to show how much alcohol you have in your body. Alcohol breakdown is different from individual to individual, depending on things like age, gender and metabolic rate, but on average alcohol is broken down at a rate of 20 to 30mg/dl per hour. The average rate of alcohol excretion is 0.015 BAC points per hour.
Breathalyzers work by measuring the concentration of alcohol when a person exhales, by using electrochemical fuel cell sensors, semiconductor oxide sensors or infrared spectrophotometers to calculate the percentage of blood alcohol content.
Breathalyzers are able to detect even the smallest amount of alcohol, but for the majority of people, the concern is not that they have taken alcohol, but that they are under the legal limit. In the States, a person’s BAC needs to be under 0.04 to pass the test, while 0.08 is considered to be drunk.
How many hours that must elapse before you will pass a breathalyzer is different for each person, again due to the determining factors already outlined, but as a rule of thumb, take the reading and divide it by the standard (you can get breathalyzer testing kits for your own use). So, if the test gave a BAC reading of 0.150, divide that number by the standard 0.015, which equals 10. That means that it will take 10 hours for the alcohol to get out of your system.
Because they are measuring breath, rather than blood alcohol levels directly, there are sometimes inaccurate readings because of a variety of reasons. The most accurate breathalyzers have been found to be the ones that use semiconductor oxide sensors. Of course, if there is any real doubt, blood tests can be taken.