The Problem of Breath Alcohol Concentration and Exhaled Volume

The Flaw in Breath Alcohol Tests: Why Exhaled Volume Matters

In forensic breath alcohol testing, there persists a naive belief that as a subject blows into a breath alcohol analyzer, breath alcohol concentration (BrAC) will quickly rise and reach a plateau, at which point the subject’s breath alcohol concentration will no longer increase [1]. This old paradigm of alcohol diffusion into the breath is frequently touted by forensic scientists in the courtroom and promulgated in training materials related to breath alcohol testing [1–4].

Figure 1. The old model of BrAC

In reality, alveolar concentration is never reached, and the BrAC continues to rise throughout exhalation [5,6]. Jones has pointed out that a mechanistic understanding of alcohol exchange in the lungs using only Henry’s law is insufficient to explain alcohol exchange in breath [7]. In 2010, Hlastala called the simplistic model of alcohol exchange in the lungs the “old paradigm” [5]. Other mechanisms such as diffusion, perfusion, ventilation, and differential solubilities must be accounted for when describing alcohol exchange in the lungs and airways.

While Henry’s law accurately describes the exchange of alcohol in a breath alcohol simulator when used to calibrate a breath analyzer, it does not capture the complexities of the alcohol exchange in the lungs [8,9]. In particular, Henry’s law does not consider the complex dynamic interaction of alcohol diffusion in the lungs and mucous membranes of the airways leading to the lungs in vivo [10–14].

The difference between a simulated breath and an actual human breath is so pronounced that the U.S. National Bureau of Standards built a specialized three-chamber breath alcohol simulator to try to mimic the dynamic nature of human breath when testing breath alcohol analyzers [15].

Figure 2 shows Hlastala’s new paradigm of breath alcohol, which contrasts with the old paradigm. The new paradigm shows how BrAC continues to rise as the volume of exhaled air increases. Indeed, Hlastala and Anderson showed that for BrAC to reach a true plateau, a subject with a 5-liter vital capacity would need to exhale ~50 liters (a physical impossibility) [6].

Figure 2. The new paradigm of BrAC.

The Sensitivity of BrAC to Exhaled Volume

In 2019, Anderson and Hlastala looked at 115 breath alcohol expirograms from evidentiary breath alcohol tests administered in the State of Washington to determine the influence of volume on BrAC [16]. They found that for every liter of breath provided beyond the minimum volume required of 1.5 liters, the BrAC continued to rise by 9.2% per liter.

Similarly, in 2011, Gullberg and Polissar found that BrAC was sensitive to exhalation time and volume [17]. In addition, Simpson et al. and Clatterbuck have looked at short vs. long exhalations and found long exhalations consistently exceeded short exhalations in BrAC [18,19].

The sensitivity of BrAC to volume raises the legal issue of fairness in breath alcohol testing. A subject that follows an officer's commands to “keep blowing, keep blowing…” is at a disadvantage, while the person who exhales just beyond the minimum volume will gain an advantage.

In addition, one-size-fits-all minimum volume requirements show bias against people groups with smaller forced vital capacities, such as women, Asians, and Blacks [20], who must exhale more of their available breath into the instrument, resulting in higher readings [21,22].

Attempts to Standardize Breath Alcohol Sampling Parameters

Many investigations have been conducted to standardize the sampling parameters for the breath alcohol test (BAT). Early studies looked at the effect of having subjects rebreathe into a heated bag to homogenize the breath before analyzing it [23–28]. Other efforts have included using exhaled carbon dioxide or water vapor as an internal standard to gauge when a suitable sample of breath has entered the analyzer for analysis [29–38].

Some scientists have considered using mathematical modeling to standardize BrAC [10,39–41]. For example, Anderson and Hlastala proposed a mathematical model that normalized BrAC based on the subject’s forced vital capacity [16]. By mathematically normalizing BrAC to ~50% of forced vital capacity, no subject gained or lost an advantage based on exhaled volume [42].

While manufacturers have focused on the minimum exhaled breath volume in the BAT, little attention has been paid to the maximum volume. This crucial parameter of breath alcohol sampling must receive more thought and attention to ensure an unbiased sample is analyzed [43].

Conclusion

Standardization of breath sampling parameters is needed to avoid the pitfalls of uncontrolled exhaled volume in breath alcohol testing. A forensic test should not be determined by the arbitrary length of a person’s exhalation or the capricious instructions of the officer administering the test.

In addition, a one-size-fits-all approach is biased against people with smaller lungs, as they must contribute a greater fraction of their vital capacity to the instrument.

Manufacturers and forensic scientists must consider the impact of exhaled volume in breath alcohol testing. Without consistent exhaled volume sampling parameters, reliable BrAC comparisons between instruments or jurisdictions cannot be made.

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