This post continues the Breath, Blood, and Bull series, which explores how science, technology, and human judgment shape DWI
The first article examined the limits of field sobriety testing. This installment turns to the machines that translate breath into evidence, using “breathalyzers.”
By unpacking how they measure alcohol, where they can fail, and how lawyers challenge their results, you’ll see why science is not necessarily as simple as a number on a printout.
If you were charged with DWI in the Charlotte metro region, including Monroe, Iredell, Gaston, or Lincoln counties, the defense lawyers at Powers Law Firm would like to help. Call or text 704-342-4357 to speak with Bill Powers, a North Carolina DWI defense attorney and author of the North Carolina DWI Quick Reference Guide who has spent more than 30 years analyzing and challenging the science behind breath and blood testing.
How Modern Breath Testing Technology Measures Alcohol
Chemical tests of breath are a cornerstone of DUI enforcement technology. Breath-testing devices (often colloquially called “Breathalyzers,” though that term technically refers to a specific brand) determine the alcohol concentration in a person’s breath (BrAC), which is then used to estimate blood alcohol concentration (BAC).
The Science Behind Breath Alcohol Concentration (BrAC)
Modern evidentiary breath testers use relatively reliable scientific principles, chiefly infrared (IR) spectroscopy and/or electrochemical oxidation (fuel cell technology), to detect and quantify alcohol vapor in exhaled breath.
Infrared Spectroscopy: Many stationary breath test machines (such as the Intoxilyzer, Intoximeter, and Datamaster models) employ some form of infrared spectroscopic analysis. In simple terms, an IR breathalyzer passes an infrared light beam through the suspect’s breath sample in a sample chamber. Ethanol molecules absorb light at specific wavelengths. As the infrared beam goes through, some of the light is absorbed by any alcohol present; the more alcohol, the more light is absorbed (and the less detected on the other side). The device measures the amount of light that makes it through the sample. Because the absorption at particular wavelengths is proportional to the concentration of ethanol, the machine’s sensor can calculate the alcohol concentration in the breath from the degree of light attenuation.
Why Wavelength Filters and Interference Matter
In practice, some modern IR breathalyzers use filters to select the wavelengths of light that ethanol absorbs, thereby focusing only on certain areas of the spectrum.
The result is a reading of breath alcohol, typically reported in terms of grams of alcohol per 210 liters of breath (the units equivalent to blood alcohol grams per 100 milliliters, by the standard 2100:1 breath-to-blood partition ratio).
IR technology is valued for its rapid analysis and non-invasive nature. However, it’s not without issues. Substances with molecular structures similar to ethanol might also absorb IR light at nearby wavelengths, potentially causing interference. Manufacturers mitigate this by using multiple wavelength detectors (to distinguish ethanol’s signature from other compounds with similar molecular structures) and by software algorithms. Still, the science behind IR breath testing can provide the defense several angles of attack, such as whether the machine’s wavelength filters adequately exclude chemicals like acetone (which can be present in the breath of diabetics or fasting individuals and absorb IR, potentially yielding a false BAC reading). It’s a notoriously complicated and somewhat controversial topic.
How Fuel Cell Breathalyzers Generate Results
Fuel Cell Sensors: A fair number of portable breath testers (PBT) and some tabletop, evidentiary units incorporate fuel cell technology either as the primary sensor (such as the EC/IR II in North Carolina) or as a dual-system alongside IR. A fuel cell sensor contains two platinum electrodes with a porous medium between them. When exhaled breath is blown over the fuel cell, any ethanol present undergoes a chemical reaction (oxidation) at the surface of the platinum. In essence, the alcohol is converted to acetic acid and electrons are released in the process. This generates an electrical current proportional to the amount of alcohol. The device measures this current. A stronger current indicates more alcohol was oxidized, hence a higher alcohol concentration in the sample.
Fuel cells can be highly specific for alcohol (they react primarily to alcohols and not many other substances) and are valued for their simplicity and relative durability. They do, however, require calibration and can be overwhelmed by very high alcohol concentrations (or poisoned by certain chemicals over time, such as cigarette smoke).
Differences Between Portable and Evidentiary Breath Testers
In real-world DUI scenarios, preliminary breath testers (PBTs) used in the field are usually small fuel cell devices. These are typically used for probable cause determinations, and their results are generally not be admissible at trial (due to less stringent calibration and reliability issues).
DUI First Defense in North Carolina
Modern “breathalyzers” are interesting, yet imperfect technology that utilizes light absorption and electrochemical reactions to estimate the BAC. They produce quick results that law enforcement and courts may treat as strong evidence.
Yet, they operate on assumptions and are subject to limitations. A savvy DUI defense attorney should become conversant in how these devices function in order to identify when a result might be untrustworthy or when the device’s limitations have been exceeded.
Potential Sources of Error in Breath Testing
Beyond mechanical and calibration issues, physiological and environmental factors can introduce errors in breath alcohol measurements. A DUI attorney needs to be well-versed in these factors to effectively question the validity of a breath test result. Sources of error may include residual mouth alcohol, gastroesophageal reflux disease (GERD) or acid reflux, and volatile interferents (substances or conditions that mimic alcohol in the breath sample).
Residual Mouth Alcohol and Observation Period Violations
- Residual Mouth Alcohol: Breath testing is predicated on measuring deep lung air (alveolar air), which estimates the amount of alcohol in the blood. The assumption is that the subject blows out a representative sample from deep in the lungs. However, alcohol residue in the mouth or throat can contaminate the sample with extra alcohol that did not come from the blood, causing an erroneously high reading. Mouth alcohol can come from recent consumption (if a suspect hastily took a last gulp before being stopped), from belching or regurgitating stomach contents, or from substances like mouthwash, breath spray, or even alcohol-based medicines (like cough syrup or oral anesthetic). Recognizing this risk, the standard protocol in many jurisdictions is to observe the suspect for 15 minutes prior to the breath test, ensuring they do not eat, drink, vomit, or place anything in their mouth. If the officer or test operator fails to strictly adhere to this observation period, the defense could have an argument that the result may be contaminated by mouth alcohol. For example, if video shows the suspect burping and the officer not resetting the observation clock, that could prove important. Residual alcohol in the mouth dissipates over time (usually within a few minutes), but without a proper deprivation period, a mouth alcohol artifact can significantly inflate BAC. A classic defense scenario is the “mouth alcohol defense,” which involves pointing out that an elevated BAC could be due to mouth alcohol rather than true blood alcohol. Indeed, raw, undigested alcohol could result in an extremely high reported BAC.
GERD, Reflux, and Other Physiological Variables
- GERD and Acid Reflux: A special subset of the mouth alcohol problem is when the alcohol vapors originate not from recent drinking or mouthwash, but from the person’s own stomach due to acid reflux or belching. Gastroesophageal reflux disease (GERD), acid reflux, or heartburn can cause the contents of the stomach (including any alcohol) to flow back up the esophagus and even into the mouth or throat. This means that a person with GERD might have intermittent (and even constant, consistent) alcohol vapor in their esophagus/mouth unrelated to deep lung air. If a suspect has GERD, the defense might raise this issue in appropriate circumstances.
Defense counsel might introduce medical records or testimony to establish the defendant’s condition. On cross-examination, ask the state’s witness if they are aware that GERD can trap alcohol in the esophagus.
If the observation period was done correctly, it mitigates (but may not entirely eliminate) the concern, since even during observation, GERD can both intermittently and consistently bring up alcohol.
In contrast, the state may argue the slope detector greatly reduces the likelihood of unreliable results due to residual mouth alcohol, esophageal and/or GERD-related false-positive results.
Scientific defenses like GERD or mouth alcohol only work when the surrounding evidence leaves room for them. If jurors see a client who looks impaired, the science will not save the case.
Medical and Environmental Interference Factors
Other Physiological Factors: Beyond GERD, other health conditions may affect breath tests. Uncontrolled diabetes can lead to high levels of acetone in the breath (acetone is a ketone byproduct in diabetics with ketoacidosis or “false ketosis”). Some older breath machines without dual filters could mistake acetone for ethanol because acetone also absorbs infrared light at certain wavelengths. Modern devices largely have safeguards, but (when appropriate) raising the question could show the jury the machine isn’t perfect. Dietary factors like being on a ketogenic (low-carb) diet or fasting can similarly raise breath acetone to remarkable levels. Another factor is body temperature. A person with a fever will have a higher breath-blood partition ratio (meaning more alcohol in breath per given BAC) than a normal 34°C breath temperature, potentially causing a slight overestimation of BAC. Again, modern devices largely remove such errors by heating breath tubes and/or confirming temperatures in the testing process. While certain factors (acetone, temperature) might not make or break a case unless extreme, they could paint a picture that individual variability can matter.
- Interfering Substances & Environmental Contaminants: The defense may also want to consider if any volatile substances in the testing environment or in the suspect’s exposure to volatile substances could cause error. Inhalation of certain solvents (paint fumes, gasoline) might, in theory, register on some machines. Police stations are usually free from such fumes, but strange cases have arisen. Most modern breath testing devices perform an “air blank” test to zero out any ambient alcohol, but significant environmental alcohol (like testing in a small room where hand sanitizer was just used heavily or numerous DUI suspects are exhaling alcohol-infused breath) could conceivably affect the baseline. Defense counsel should not overlook unique facts. Sometimes the best defenses are born from peculiar details in the police report or client’s story. At a minimum, these are questions that may be worth asking an expert, “Could that affect the result?”
How Defense Attorneys Challenge Breathalyzer Results in Court
In challenging a breath test, it can make sense to advance a multi-faceted attack: (1) Did the machine function correctly, and was it maintained? (2) Was the test administered according to protocol? (3) Are there physiological or external factors that nonetheless could have yielded an inaccurate reading?
Machine, Method, and Subject — Three Lines of Attack
By covering all these bases, machine, method, and subject, an attorney may have a chance of casting doubt on the validity of the BAC result.
Even in cases with higher BACs, certain scientifically centered arguments may erode the confidence in the test, perhaps leading a jury to conclude that the defendant was not, in fact, impaired.
Why Understanding the Science Matters in DWI Defense
Breath testing seems simple: you blow into a machine, it prints a number, and that number decides everything. But every stage of that process, from breath sample collection to infrared absorption, fuel cell oxidation, and temperature correction, depends on assumptions that can be tested, challenged, and, in some cases, disproven.
What is a DWI Civil Revocation?
For a defense lawyer, understanding those assumptions is not a matter of curiosity; it is part of the job. You need to know what the technology measures, what it assumes, and what it simply cannot do. The line between science and speculation is not always obvious, and courts can mistake the precision of a digital printout for proof. When the science is misunderstood, or misrepresented, it becomes your role to show that technology is only as reliable as the method, maintenance, and human judgment behind it.
The next article in the Breath, Blood, and Bull series examines blood testing in DWI cases, how samples are collected, preserved, and analyzed, and what can compromise their integrity long before the courtroom debate begins.
If you are facing a DWI charge in North Carolina or are a lawyer seeking to strengthen your command of forensic evidence, contact the Powers Law Firm in Charlotte. Call or text Bill Powers at 704-342-4357. Preparation and scientific literacy can redefine how DWI cases are understood and defended.