You may have wondered how, if the legal blood alcohol content (BAC) limit is 0.08%, police can use a breathalyzer to get evidence you're driving drunk.

The answer is that the alcohol content on your blood is chemically related to your breath alcohol content (BrAC). However, there are crucial differences between your BAC and your BrAC, and these differences complicate cases of driving under the influence (DUI) in Arizona.

How Alcohol Gets Into Your Blood

When you drink alcohol, the liquid passes through your mouth, down your throat, and into your stomach. However, alcohol is neither digested nor chemically altered there. Instead, it escapes from the stomach or the small intestine and into your bloodstream through the process of diffusion.

How Alcohol Gets Into Your Breath

Once the alcohol is in your bloodstream, it gets carries throughout your body. This includes your lungs, where some of the alcohol in your bloodstream stays as the blood passes through. Every time you exhale, this alcohol in your lungs passes back up through your throat, into your mouth, and out into the world.

Similarities Between BAC and BrAC

Scientists have determined the rates at which alcohol enters your bloodstream and then from your bloodstream into your lungs. As a result, the legal limit of 0.08% refers to grams of alcohol per 100 milliliters of blood, and also to grams of alcohol per 210 liters of breath. Police rely on this ratio to find evidence of drunk driving and enforce the state's DUI laws using breath tests, which are easily portable and noninvasive, rather than blood tests, which can't really be done out on the road.

Differences Between BAC and BrAC

However, BrAC is actually very unreliable. Unlike BAC, the alcohol content on your breath can be altered by a huge variety of environmental factors. For example, if you've just taken a drink of alcohol and then provided a breath sample, the latent alcohol that's still in your mouth will drastically increase the BrAC reading. Your BrAC can be affected by a variety of other factors, as well, making it an adequate preliminary way of estimating your level of intoxication, but a terrible way of proving that you were at or above the legal limit.

Nevertheless, law enforcement still uses BrAC as a stand-in for BAC in many DUI cases, claiming that the two are exactly the same.

They're not.

Arizona DUI-Defense Attorneys at Arizona Criminal Traffic

The use of BrAC exploded with the invention of the breathalyzer because it was a cheap and convenient way of ballparking a DUI suspect's level of intoxication. However, in the years since, the use of BrAC in DUI cases has evolved: It's now seen as an infallible way of knowing exactly how inebriated someone is.

While this is incorrect, it often takes a DUI-defense attorney to explain the differences to a jury.

If you've been charged with a DUI in Arizona, call the Arizona Criminal Traffic law office at (888) 202-9222 or contact their attorneys online.