Breath Alcohol Basics
How it works, and what can go wrong
Counterpoint Volume 4: Issue 1 - Article 4 (September 2019)
An article in the Core Skills I-4 module
Jan Semenoff, BA, EMA
Forensic Criminalist
Article information:
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1500 words (approximately 5-8 minutes)
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Breath alcohol testing
Breath Alcohol Concentration (BrAC) results are created when a test subject provides a sample that is suitable for analysis into a breath testing device. BrAC testing is predicated on the notion that standardized act, practices, and conditions are correctly followed in order to obtain accurate and reliable results. This article provides a quick introduction to the testing process, and some of the issues surrounding the reliability of reported results.
What happens when you consume alcohol?
Before we get into breath testing, let's take a quick peek at the metabolic processes surrounding alcohol consumption. They play an important role in the reliability of the reported readings.
When a person consumes alcohol (ethanol, actually – let’s get this right), it follows a specific pathway in the human body as it is Absorbed, Distributed, Metabolized, and Eliminated from the body. This is sometimes referred to as the ADME of ethanol. We’ve covered individual articles on each of these separate, but concurrent, physiological mechanisms. I’m going to leave the mental and physiological effects of alcohol consumption as a separate article. For now, we are only concerned with how it moves through your body, and how we measure its presence.
Ultimately, that ethanol becomes resident in your body, forming a level in your blood that can be empirically measured as a concentration of grams of alcohol per millilitre of blood. This is referred to as a Blood Alcohol Concentration, or BAC.
As your blood courses through your arteries and veins, it will make its way to your lungs. When you exhale, a small portion of that ethanol will also be expelled from your body with each exhalation. This exhaled alcohol is measured as a Breath Alcohol Concentration, or BrAC.
We’ve talked previously about the difference between a Blood and Breath alcohol concentration. Try right off the bat to separate the two types of readings. BAC reports Blood Alcohol Concentrations, and BrAC Breath Alcohol Concentrations. They are not congruent readings and are based on different measurements. These terms are often, and incorrectly, used interchangeably.
We’ve also talked about that exchange of ethanol and gases in your lungs. For now, just know that the Blood alcohol reading is an extrapolated reading produced based on the Breath alcohol measurement.
How is the reading produced?
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In general terms, the test subject provides a breath sample by exhaling into a breath testing device. The device captures a portion (depending on the type of device used, maybe an extremely small portion – less than one thousandth [1/1000] of the entire exhalation may be sampled) of the breath sample and analyses it for alcohol concentration. Then, based on that reading, the unit is calibrated to report back on an extrapolated Blood Alcohol Concentration (BAC). Basically, it takes the breath reading produced, and multiplies it by the so-called Partition Ratio (the Blood to Breath Ratio) to produce a reported BAC measurement.
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What types of devices can produce readings?
So, in essence, the ethanol in the exhaled breath is measured by the breath test device. There are three main technologies that can be used to produce that measurement:
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1. By measuring the reaction with a chemical compound.
The original Breathalyzer ™ (the actual device, not the generic reference to “breathalyzers”) utilized a chemical reaction between ethanol and a sulfuric acid compound. The antiquated Breathalyzer is not used in any jurisdictions in North America anymore. You may be interested in looking at the entry on the Breathalyzer in the Forensic Encyclopedia. |
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2. By measuring the electrical current produced by the reaction of the ethanol in a fuel cell.
Modern “roadside testers” use fuel cells to determine the presence and concentration of ethanol in a breath sample. This technology has specific limitations. You should visit the articles on Fuel Cells and Fuel Cell Devices for an introduction to that technology. |
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3. By measuring the absorption of the ethanol in an infrared measuring device.
Modern evidentiary instruments use the principal of infrared absorption to identify the presence of ethanol and determine its concentration. We’ve covered a lot of ground on how these devices work, and what limitation exist in the application of this technology in breath testing. |
Are these measurements accurate and reliable?
Well, of course, that depends on who you talk to. For the most part, I’d guess that the overall majority of the Breath readings taken by these devices provide a pretty close estimate of the Blood alcohol concentration of the subject. But, as with any technology, there are limitations to the accuracy and reliability of the measurements. Again, we’ve covered issues with accuracy and reliability, and you should understand how these terms are used by forensic scientists.
In general terms, breath testing is predicated on the notion that the subject is average. Average as in:
- Average height and weight,
- Average body mass index,
- Average lung volume and function,
- Ordinary (average) medical history (meaning no medical issues that would affect the test, such as);
- GERD
- Diabetes
- Ketogenic diet
- Any dental work that could trap alcohol
- Average alcohol consumption pattern, with
- Average physiological functions in ADME.
- More importantly, average in terms of their specific relationship between Blood and Breath concentrations.
The “average” list could go on for quite a while longer, but you get the drift. Just remember that the average person will produce average, or “standardized” breath test results.
But, of course, if the test subject is NOT average, their BrAC breath test results may be skewed, either higher or lower than their true Blood Alcohol Concentration. We have, in general, one of three circumstances that can produce these skewed results:
- The acts of the individual breath test operator or breath test device are non-standardized.
- The practices of the organization using these devices deviate from standardized practice.
- The conditions under which the test was performed was less than optimal, either in the device itself, the environment that the testing occurred, or variables in the test subjects themselves.
Practice Tip: |
Keep in mind that the breath reading produced (BrAC) is indicative of the blood alcohol concentration (BAC) only when the reading was taken, given the assumptions made by the devices. Due to variables in the ADME of alcohol in the person being tested, it is virtually impossible to predict the test subject's true BAC at another point in time - either before or after - when the test was taken. Are they absorbing alcohol? Eliminating alcohol? Without knowing where your client was on the absorption-elimination profile, the reading is merely a snapshot in time.
Final thoughts:
You need to know a few important points:
- Broken or compromised breath test devices can produce numerical results that are flat out wrong. They often do so without raising any alarms as to their performance. In this regard, the historical performance of a device is relevant to the readings it produces.
- Operators can adversely influence the results obtained, often employing a variety of strategies to get readings before the courts. Sometimes this is based on intentional malfeasance, and other times, the product of poor training, poor working knowledge of the devices, or sub-standard local protocols.
- Calibration Checks can be in range, indicating the unit is in proper working order, but the unit can still accept an unsuitable sample for analysis.
- How that breath sample was physically obtained influences the readings produced. You should know and understand the difference between a suitable sample, and one that is sub-standard and therefore unreliable.
- Duplicate breath (BrAC) readings taken within minutes that fall within 0.02 g/210L of one another (sometimes referred to as the ".02 agreement") do not necessarily mean they represent the true BAC of the test subject. Samples can be influenced externally (Burps, mouth alcohol contamination, GERD, etc) and produce two results that agree with one another, and still be wrong.
- The overall design of the devices may not necessarily lead to accurate and reliable readings. Some instruments, as an example, seem to be influenced by chemicals your client was exposed to in their workplace, while other devices seem immune to the effects of these Volatile Organic Compounds (VOCs). This is referred to as Specificity and is something that you should be on the look-out for.
- The software used in modern devices can have an overall influence on their performance.
- A number produced by the device is just that – a number. It must be taken in context in order to establish its reliability, and its influence on impairing the test subject.
We could go on and on in this regard, and indeed we have tackled many of these issues (and will continue to tackle more) as they arise. Don’t let a number flummox you…. I hear all the time from lawyers who feel that the number from a reported reading means to end of your client’s hopes, and nothing could be further from the truth. Many times, the number is irrefutable, but under a variety of circumstances, comes laden with baggage in its production that deserves further scrutiny.
I consulted on one case where the unit (an Intoxilyzer 5000EN) seemed to produce an accurate and reliable reading, with no error messages indicated. However, upon further examination of the unit's logbook, we learned that it was prone to error messages. In fact, the unit produced an error message requiring re-testing in 68% of the entries in its logbook. The test subject in this case had provided a BrAC reading of 0.081 g/210mL. Given that this reading was only one of 32% of the readings issued by this device WITHOUT an associated error message, how reliable can it be considered? The case was a Vehicular Homicide matter, and the client was facing serious consequences. Understanding the limitations of the technology, and the variables in how breath readings are produced, is key to examining the accuracy and reliability of numbers presented to the courts.
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