When a police officer stops a driver because his behavior on the road may indicate that he has been drinking, he will ask him to do a breathalyzer test. The breathalyzer test is rather quick and easy: the police officer will turn on the device and tell the driver to blow into it until the device signals to stop. The breathalyzer will display the result after several seconds. The officer will immediately know if the driver has been drinking.
The most common legal limits in the US are 0.05 and 0.08. The limits are lower for new and/or young drivers and persons operating commercial vehicles.
But how does the breathalyzer work really? How can it tell the content of alcohol in the blood from one's breath? Well, the breathalyzer is able to display the results because the blood alcohol level and the amount of alcohol in someone's breath are related. By knowing how much alcohol a person has in their breath, it will easily calculate a quite accurate estimate of alcohol in their blood.
When a person drinks alcohol, the alcohol will dissolve in the person's blood. The liver will do most of the job when getting rid of it, but as the blood courses through the body, it will inevitably reach the lungs as well. Once the alcohol in that blood comes into contact with the air that we breathe in, it will evaporate and exit our lungs as we breathe out. The more alcohol a person ingests, the more alcohol will evaporate through the lungs, leading to higher results.
This is the exact alcohol that the breathalyzer analyzes. When a person who drank alcohol blows into the device, the alcohol in the breath causes a chemical reaction that generates an electrical current, which is then registered by the device. After the current is measured, the breathalyzer will display a result.
The result is an approximate value only - the only accurate way to measure blood alcohol level is through a blood test. The breathalyzer results are only approximates because there are some factors that contribute to their inaccuracy. These factors include a person's body temperature and cellular composition of blood. The quality and calibration of the breathalyzer also play an important role.
High quality breathalyzers will distinguish the difference between alcohol and similar substances that can be found in dietary supplements or in people who suffer from diabetes. Imagine a scenario where you get pulled over, take a test and get positive results even though you were not drinking - that would be a real nightmare. This could happen because you have used a strong mouthwash not so long ago.
Elevated body temperature is another issue - a person with elevated body temperature will have higher results because the alcohol will evaporate more rapidly in environments with higher temperature. People suffering from diabetes may have issues as well because there may be ketones present in their breath - a lower quality breathalyzer will not be able to distinguish between ketones and ethanol present in the breath of a person who drank alcohol, and it will display positive results. The percentage of white and red blood cells in the blood also differs from person to person - the variability in the ratio adds to accuracy issues, but the difference isn't high enough to be of any higher significance.
The breathalyzers the police uses are high quality and they will be able to distinguish between mouthwash and alcohol - you won't be able to fool them with the common "it's a strong gum" or "it could be my mouthwash" excuses.