One of the most important numbers in the law of driving under the influence (DUI) is a driver's blood alcohol content, or BAC. But if your BAC is the amount of alcohol that is in your blood, how can police get a BAC number without drawing a blood sample?
The reason for the confusion comes from the fact that police conflate BAC with BrAC, or your breath alcohol content. They're different, but police don't want you to know it.
Blood Alcohol Content (BAC)
The percentage of your blood that is alcohol is one of the most important pieces of information that can be used in a DUI case. 75 Pa. Cons. Stat. § 3802 makes it per se, or automatically, illegal to drive a car in Philadelphia or the rest of Pennsylvania with a BAC at or above 0.08%.
BAC is used as the standard for drunkenness because, when we drink alcohol, that alcohol seeps into our bloodstream at a known rate. This makes BAC a good indicator of how inebriated we are. It's not a perfect indicator, but that's a topic for another blog post.
The alcohol-laden blood acts just like normal blood, though: Its primary job is to bring oxygen from our lungs to our muscles, including our brain. Importantly, as the blood passes through the lungs to collect that oxygen to transport elsewhere, some of the alcohol in the blood gets left behind.
Just like with the amount of alcohol that gets into the bloodstream, the rate at which alcohol gets from the blood into the lungs does not change very much.
Breath Alcohol Content (BrAC)
The amount of alcohol that gets left in the lungs is very small – most of the alcohol that gets ingested ends up in the stomach, and most of the alcohol that does not end up in the stomach stays in the bloodstream. However, what little makes it to the lungs can be detected.
Because the alcohol gets from the bloodstream into the lungs at a known rate, what little there is in the lungs can be used to determine how much is in the blood.
To detect the amount of alcohol in someone's lungs, that person has to provide an air sample by exhaling. Police use breathalyzers that capture a breath sample when a driver exhales into a tube. Those breath tests then analyze the air sample and provide a reading.
Police claim that the reading is a driver's blood alcohol content. It isn't. The reading is the driver's breath alcohol content.
As we'll discuss in our next blog post, the difference is important. Simply put, a lot of things can happen between the lungs and the breath testing machine that can make a BrAC reading unreliable.