Why is Anyone Still Using Breath Testing for DUI? The Case for Banning Breath Testing

Breath testing for blood alcohol content (BrAC) has been a staple in law enforcement for decades. From roadside stops to DUI checkpoints, the technology has been widely utilized to help determine whether someone is under the influence of alcohol. However, as scientific advancements and innovations continue to develop, it becomes increasingly clear that breath testing is an ancient and outmoded method for determining BAC. In this blog post, we will explore the reasons why breath testing should be banned in favor of blood testing for BAC, which offers a more accurate and reliable assessment of intoxication.


Breath Alcohol Concentration (BrAC) and Blood Alcohol Concentration (BAC) are both measurements used to determine the level of alcohol in an individual’s system. Understanding the differences between BrAC and BAC is crucial for interpreting alcohol testing results accurately. Some states pin their culpability for their per se DUI crime on BAC. Others pin it to BrAC.

For the states pin their culpability to BrAC, such a scheme is arguably not logical for the reasons we will see throughout this blog post. It is hypothesized that BrAC-based per se crime puts ease of prosecution and cost as the motivating factors as opposed to punishing the true issue of DUI. The harm that DUI crimes are designed to avoid is people who are too intoxicated to safely drive. The best measurement for a per se based analysis is BAC over BrAC as we will see. Whether or not in this day and age that pinning criminal culpability to a number (BAC) versus the true harm sought to be avoided (too intoxicated to safely drive) is a policy discussion for another post, but it should be noted, nonetheless.

Breath Alcohol Concentration (BrAC)

BrAC is the measurement of alcohol concentration in a person’s breath. It is typically determined using a breathalyzer, a portable device that analyzes a breath sample by measuring the amount of ethanol present in the exhaled air (but please see our section on “Is a breath test machine even measuring ethanol and only ethanol?” that will prove this basic tenant is not true necessarily). Breathalyzers work by employing various technologies such as infrared spectrometry or fuel cell sensors to arrive at a BrAC. BrAC measurement is not necessarily a true BAC as we will see.

Blood Alcohol Concentration (BAC)

BAC, on the other hand, is a direct measurement of alcohol concentration in an individual’s bloodstream. It is usually expressed as a percentage, which represents the grams of alcohol per 100 milliliters of blood. BAC can be measured through blood tests, which are considered the most accurate method for determining alcohol levels in the body, if they are conducted and analyzed and interpreted correctly.

How a BrAC may not be a BAC

A Breath Alcohol Concentration (BrAC) result, obtained through a breathalyzer test, may not accurately reflect the true Blood Alcohol Concentration (BAC) of an individual due to several reasons. BrAC is an indirect measurement, relying on the assumption that there is a correlation between result of the anlaysis of the breath and the blood. However, this correlation can be influenced by various factors that can cause discrepancies between the BrAC result and the actual BAC:

  1. Breath-to-blood ratio variability: The conversion factor, known as the breath-to-blood ratio, is commonly set at 1:2100. However, this ratio can vary between individuals and even within the same person at different times. Factors such as hematocrit levels, body temperature, and alcohol absorption rates can cause this ratio to differ, leading to inaccuracies when estimating the BAC based on BrAC measurements.
  2. External factors: Breath testing can be influenced by factors such as failure to observe strict pre-test deprivation period, mouth alcohol, temperature, and the presence of other volatile compounds in the breath. Residual alcohol in the mouth, dental work, or certain medical conditions like acid reflux can affect the accuracy of BrAC measurements, resulting in an inaccurate estimation of the true BAC.
  3. Timing: BAC levels change over time as alcohol is absorbed, distributed, and eliminated by the body. If a breathalyzer test is administered while an individual is still in the absorptive phase, the BrAC result might overestimate the BAC, as alcohol concentration in the breath could be higher than that in the blood.
  4. Non-specific detection: Infrared spectrometry, a common technology used in breathalyzers, relies on non-specific wavelengths to detect ethanol (more on this later). This lack of specificity can lead to the detection of other compounds (other VOCs) with similar absorption characteristics, causing false positives or inflated BrAC readings that do not accurately reflect the true BAC.
  5. Device calibration and maintenance: Breathalyzers require regular calibration and maintenance to ensure accurate readings. This is to be distinguished from regular calibration check which is what most users of breathalyzers do. A calibration check does not adjust for any bias discovered in the machine. If a device is not truly routinely calibrated properly AND THEN RE-CALIBRATED to fix any systemic bias observed, it can produce incorrect BrAC results that do not correspond to the actual BAC.

In conclusion, while BrAC can provide a quick and non-invasive method for estimating BAC, it is important to recognize that a BrAC result may not always accurately represent the true BAC. So, in other words, it is faster and cheaper. Is it fair that those two concerns (price and quick results) override trueness? Factors such as breath-to-blood ratio variability, external influences, timing, non-specific detection, and device calibration can all contribute to discrepancies between BrAC and BAC results. Blood testing, when preformed correctly, does not suffer from these for determining BAC, as it directly measures alcohol concentration in the bloodstream and is less susceptible to the inaccuracies associated with breath testing as we will read.

Is a breath test machine even measuring ethanol and only ethanol?

Breath testing for blood alcohol content (BrAC) using breathalyzers is primarily based on infrared spectrometry, a technique that measures the absorption of infrared light by molecules present in a person’s breath. While this method has been widely used for decades, it is not without its limitations, particularly when it comes to the specificity of the wavelengths used to detect ethanol.

Infrared Spectrometry in Breath Testing

Infrared spectrometry operates on the principle that molecules absorb infrared light at specific wavelengths. When a breath sample is introduced into a breathalyzer, the device directs an infrared light through the sample. As the light passes through, molecules present in the breath, including ethanol, absorb specific wavelengths of the light. The device then measures the amount of light absorbed, which is used to estimate the breath alcohol concentration (BrAC).

Non-specific Wavelengths and Ethanol Detection

One critical issue with infrared spectrometry in breath testing is that the entire wavelength is not analyzed versus an ethanol standard unlike FTIR machines. Instead, they use filters that are often designed to detect a non-unique part of the output wavelength (certain micrometers), rather than a multiple, specific wavelengths unique to ethanol. This lack of specificity can lead to the detection of other compounds that share similar absorption characteristics at the set filter detector settings, resulting in false positives or inflated BrAC readings.

Depending on the device, the scientific literature is clear that the list of substances that may cause interference in breath testing is long an include common and abundantly present VOCs include acetone, methanol, isopropanol, and certain volatile compounds found in everyday products like paint or cleaning agents. These substances can be present in a person’s breath or the environment, leading to inaccurate BrAC readings that do not truly reflect the individual’s BAC.

This is where the scientist stops in me. If you do not have a specific (or a very highly highly selective) measurement, you cannot quantitate. That is basic. This flaw alone should entirely disqualify its use where proof beyond a reasonable doubt is required.

Moreover, factors like the presence of residual alcohol in the mouth (failure to adhere to strict pre-test deprivation period), burping, LERD, GERD, dental work, and even certain medical conditions such as diabetes can further complicate the accuracy of the readings.

But unfortunately there is more…

Breath Testing: An Indirect Measurement

One major issue with breath testing is that it only provides an indirect measurement of BAC. Instead of directly measuring the amount of alcohol in the bloodstream, it estimates BAC by analyzing the ethanol concentration (or other VOCs that trigger the filters) in a person’s breath. This indirect method introduces several variables that can lead to inaccuracies and false results.

Breath Testing and Its Influential Variables

Breath testing can be influenced by a range of variables, such as hematocrit levels, temperature, and other factors. Let’s take a closer look at these variables and how they impact breath test results:

  1. Hematocrit: Hematocrit refers to the percentage of red blood cells in a person’s blood. Since alcohol is more soluble in water than in red blood cells, individuals with higher hematocrit levels will have a higher breath alcohol reading than their actual BAC. This discrepancy can lead to false results, especially in people with naturally high hematocrit levels. Women and men have different hematocrit. Also the use of several medications can influence hematocrit. Donating blood too can change hematocrit. Finally something is simple is hydration can influence hematocrit.
  2. Temperature: The temperature of the breath sample and the surrounding environment can also impact breath test results. Warmer breath samples may produce higher alcohol readings, while colder samples may result in lower readings. These temperature fluctuations can occur due to weather conditions, the subject’s body temperature, amount of saliva in the oral cavity, prolonged closing or opening of the oral cavity prior to testing, or even the presence of a fever, all of which can lead to inaccurate results.
  3. Other Variables: Factors such as mouth alcohol, gastric reflux, and the presence of certain substances in the mouth can also impact breath test results. These factors can lead to higher alcohol readings, even if the person’s BAC is within legal limits.

Breath Testing and the Absorptive Phase

Breath testing is particularly problematic when it comes to estimating BAC during the absorptive phase. The absorptive phase occurs when alcohol is still being absorbed into the bloodstream, and its concentration is not yet uniform throughout the body. During this phase, breath testing can significantly overestimate a person’s BAC, leading to unjust penalties and legal consequences.

Blood Testing: A More Accurate and Reliable Alternative

In contrast to breath testing, blood testing directly measures the amount of alcohol in a person’s bloodstream, providing a much more accurate and reliable assessment of BAC. Blood tests are not influenced by all of the variables that can affect breath tests, and they are not subject to the same inaccuracies during the absorptive phase. There are still some variables that can invalidate a BAC test, but the list is less. A poorly conducted blood test is just that poor. Let’s look at the difference.

  1. Direct measurement: Unlike breath testing, which indirectly estimates BAC based on the alcohol content in a person’s breath, blood testing directly assesses the amount of alcohol present in the bloodstream. This eliminates the inaccuracies and inconsistencies associated with indirect measurements, providing a more precise assessment of an individual’s level of intoxication.
  2. No breath-to-blood ratio variability: Breath testing relies on the assumption that there is a correlation between the amount of substance detected in the breath and the blood, which is represented by the breath-to-blood ratio. However, this ratio can vary between individuals and even within the same person at different times. Blood testing eliminates the need for this conversion, as it directly measures the ethanol concentration in the blood, avoiding potential inaccuracies related to breath-to-blood ratio variability.
  3. Less susceptibility to external factors: Breath testing can be influenced by various factors, such as mouth alcohol, temperature, and the presence of other volatile compounds in the breath. Blood testing, when conducted properly with sufficient specificity proven, on the other hand, is less susceptible to these external influences, as it directly measures the ethanol concentration in the blood, providing a more accurate reflection of an individual’s BAC.
  4. No interference from other substances: Breath testing devices that use infrared spectrometry are prone to detecting other compounds with similar absorption characteristics as ethanol, resulting in false positives or inflated BrAC readings. Blood testing, when conducted properly with sufficient specificity proven, however, specifically measures the ethanol concentration in the blood, eliminating the risk of interference from other substances.
  5. Accurate assessment of absorptive and elimination phases: BAC levels change over time as ethanol is absorbed, distributed, and eliminated by the body. Breath testing might not accurately reflect BAC levels at the time of testing, especially if the individual is still in the absorptive phase. Blood testing provides a more accurate measurement of BAC at the time the sample is collected, regardless of the phase of alcohol metabolism the person is in.
  6. Blood testing can also used for determination of drugged driving and BrAC testing cannot. BrAC testing, at best, tests only for ethanol in breath. It will not catch a DUID driver. With collected blood, using a separate means of analysis, it can be testing for ethanol as well as drugs. A Governors Highway Safety Association (GHSA) report from 2018 found that, among fatally injured drivers with known drug test results in 2016, 43.6% tested positive for drugs, while 37.9% tested positive for alcohol. Of the drivers testing positive for drugs, 50.5% had used marijuana, 50.5% had used opioids, and 49.4% had used a combination of both. According to the National Highway Traffic Safety Administration (NHTSA), in their 2013-2014 National Roadside Survey, 22.5% of nighttime weekend drivers tested positive for illegal, prescription, or over-the-counter drugs. This was an increase from 16.3% in the 2007 survey. The same study found that the number of drivers with alcohol in their system had decreased from 30.0% in 2007 to 25.0% in 2014.


The first breathalyzer, known as the Drunkometer, was invented in 1938 by Dr. Rolla Harger, a professor of biochemistry and toxicology at Indiana University. This early version of the breathalyzer used a chemical reaction to estimate alcohol concentration in a person’s breath. The more modern version of the breathalyzer, called the Breathalyzer, was invented by Robert F. Borkenstein in 1954. This makes the technology ancient. It did serve a purpose at one point in time. But like all technology, it must be replaced with better and more modern technology.

Sadly, out of all of the statistics that are kept for DUI, the number of prosecutions that are based on blood testing versus breath testing is not collected. This would be really good to know from an epidemiological point of view to examine any trends in moving from the ancient to the modern. Sadly, this does not exist.

In conclusion, blood testing for BAC, when conducted properly and with sufficient specificity, offers a more accurate and reliable method for measuring BAC, addressing many of the limitations associated with breath testing. By directly assessing the amount of ethanol present in the bloodstream, blood tests provide a more precise foundation for legal decisions related to alcohol intoxication. In an institution where proof beyond a reasonable doubt is required, using BrAC unnecessarily invites the doubt in every single case.

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