Why do Virginia DUI / DWI laws deal with the amount of alcohol in my breath anyway? Isn't the amount of alcohol in my blood what is important?
It's really not possible (and certainly not practical) to directly measure the amount of alcohol in the brain, so we have to find another way to try and figure out how much alcohol may be in the brain. The best way to do this is to measure the amount of alcohol in the blood since it is blood that carries alcohol to the brain. The traditional drawing of blood and sending it to the lab for testing is possible, but it is both time-consuming and costly, and lawmakers would obviously prefer a quicker less expensive method of determining blood alcohol content.
Enter breath alcohol testing.
Breath alcohol testing is based on the premise that there is a known, constant ratio between the amount of alcohol in your blood and the amount of alcohol in your breath - so if we can accurately measure the amount of alcohol in a person's breath we can calculate the amount of alcohol in that person's blood. Interestingly, breath alcohol testing is not only easy to administer and incredibly quick, but also dirt cheap in comparison to blood testing.