How does law enforcement determine your blood alcohol content (BAC)?

Suppose a person has a few glasses of wine with dinner and then drives home. Right before getting home, he turns onto his block without signaling. A police officer then pulls him over and detects an odor of alcohol coming from his breath. The officer asks the person to step out of the car and the person complies. He is then given a field coordination test. The officer determines that the person may be intoxicated. So the person is taken to the precinct for further testing. Once at the precinct, the officer determines that the person’s blood alcohol content (“BAC” for short) is .09. This is over the legal limit in New York (where all this happens). The person is then placed under arrest and is charged with drunk driving. How did the police determine the blood alcohol content of the driver? How can you know the result was accurate?

There are usually two ways to determine the blood alcohol content of a person. The more accurate way is to physically take out the person’s blood and send it to a lab. The more common way, however, is to give them a breath test. There are numerous machines used to calculate a person’s BAC based on breath that are made by different companies. Most of them work in the same way. I am a New York lawyer so I will discuss the machine used in New York.

In New York, the machine that is used has the comically absurd name of “Intoxilyzer 5000.” This machine looks kind of like a computer built in the 1980’s. The way it works is relatively simple to understand. It basically shoots infrared light from one end of a chamber to the other end. Different molecules vibrate in the air in different ways. Ethyl alcohol (the kind of alcohol that people drink) molecules vibrate in a very specific way. The machine takes a sample reading when it knows there is no ethyl alcohol in the chamber and then it takes a sample from a person’s breath. It then compares the two samples by reading how much of the infrared light went through the chamber that wasn’t absorbed by those vibrating molecules of ethyl alcohol. The machine then does some math, applies some physics, and determines based on the amount of light that got through, what the person’s BAC is. If the same amount got through from when the machine knew it had no alcohol in it as when the person blew into it, it would determine that his BAC was .00 (assuming the machine is working accurately). The machine is designed to be idiot proof. If the machine is working correctly and if the person who runs the machine follows the proper protocol, then the results of the machine are usually accurate. The one variable that can affect the reading is the presence of “mouth alcohol.” Basically, the Intoxilyzer 5000 needs to measure “deep lung air” to get an accurate reading of a person’s BAC. Deep lung air is that stuff that comes out of your body when you take in a deep breath and then exhale. Mouth alcohol is the alcohol residue accumulated in a person’s mouth that he didn’t necessarily swallow or that was regurgitated from the stomach. For example, suppose I genuinely have not had any alcoholic drink and my BAC is .00. But suppose I took big mouthful of whiskey, gargled with it, swooshed it around my mouth, and spit it out. My deep lung air reading should still be .00 (as would my BAC), but the remnants of that whiskey that mixed in with my saliva could skew those results and the machine might think I’m intoxicated. There are two protocols that act as a failsafe that are designed to eliminate this situation. The first is a period of observation by the officer and the second is the use of something called a “slope detector.” Before giving someone a breath test, the police officer administering the test is required to watch the person for a period of time to ensure that he didn’t burp, vomit, hiccup, etc, before blowing into the machine. The reason for this observation is to ensure that no vapors or liquids from the person’s stomach came up into the person’s mouth, which could then give a false reading. Various jurisdictions require different periods of observation (in New York, it’s 20 minutes). The second failsafe is the slope detector. The slope detector is actually in the Intoxilyzer. The slope detector is designed to measure the rate of alcohol that the machine is detecting. If mouth alcohol exists, then a “drop-off” period occurs that can only happen when the machine is detecting alcohol that isn’t from deep lung air. The machine is then designed to give an “invalid sample.” The problem with these failsafes though is that officers don’t always follow the observation period and there is a growing body of research that suggests that the slope detector doesn’t always work. The machine needs deep lung air to accurately measure a person’s BAC. So if the machine is confusing deep lung air with something else, it may give a false result.

There are many complexities regarding the Intoxilyzer 5000 machine. Unfortunately, the legal profession is flooded with people who have no idea what they’re talking about who play themselves off as being DWI lawyers. They charge their clients thousands of dollars and give them horrible advice because they themselves have no idea what they’re doing. Only an experienced trial lawyer who is knowledgeable with regard to not only DWI laws, but with criminal law in general can help analyze whether the machine was working properly and and what the next steps should be.

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