Breaking down the alcohol breath test
The alcohol breath test is the most commonly used form of alcohol testing evidence in drunk driving prosecutions. Many articles praise the breath test as a highly accurate and reliable means of testing the amount of alcohol in the alveolar air of a person at the time of the test, assuming certain safeguards are met. E.g., Kurt .M. Dubowski, Absorption, Distribution and Elimination of Alcohol: Highway Safety Aspects, 10 J. Stud. Alcohol Suppl., 98 (1985).
Forensic toxicologists have long assumed that a properly obtained breath alcohol sample contains alveolar air. However, in most statutes, both “blood” and “breath” are undefined. Blood can be venous, arterial, capillary, or from any other part of the body. Breath can be upper, lower respiratory, or mouth. “Blood” and “breath” were left undefined because state scientists did not want to legislate requirements that would be difficult to implement in practice, and difficult for the government to prove. Significantly, although breath statutes rest on the belief that the exchange of alcohol from the blood to the breath occurs in the alveoli, they do not require a sample of exclusively alveolar or deep lung air.
It is claimed that the failure to obtain a solely alveolar sample does not prejudice the defendant.
It should be noted that alveolar air is not required. Inasmuch as in an otherwise correctly performed breath test, failure to obtain air only of alveolar composition gives a nearly proportionately lower result and is without prejudice to the defendant, the question of the specimen being entirely alveolar in origin need not arise.
Morton F. Mason, & Kurt M. Dubowski, Breath as a Specimen for Analysis for Ethanol and Other Low-Molecular-Weight Alcohols,Medical-Legal Aspects of Alcohol 177, 178 (James C. Garriott ed., 4th ed. 2003). This is an example of scientists and lawyers using language in different ways. Although the scientists may have assumed that alveolar air is preferred for an accurate test, the law does not say that.
Instead, because “breath” is undefined, the only logical interpretation of the statute is that “breath” means the whole breath. A dictionary definition of breath is “air inhaled and exhaled in breathing.” Merriam-Webster’s Collegiate Dictionary, (11th ed. 2004). If the legal level is determined by reference to the entire breath, alveolar air is only a portion of that breath. If alveolar air contains a higher concentration of alcohol than the whole breath, then a tested sample of alveolar air will give a false high reading of the whole breath required by the statute.
Recent articles by Dr. Michael Hlastala of the University of Washington show that alveolar air does not accurately relate to the blood level because the exchange of alcohol from blood to the breath in the lung occurs in the upper airway, not the alveoli.
Calculations by Anderson show that, whereas gases with blood-air partition coefficients (λ) of <1 exchange entirely in the alveoli, gases with higher solubility (λ of > 10) also exchange within the airways. Gases with λ of >400 exchange entirely in the pulmonary airways, not within the alveoli. Exhaled BrAC originates entirely from the airway mucus and tissue (perfused by the systemic bronchial circulation).
Michael P. Hlastala, Invited Editorial on “The Alcohol Breath Test,“ 93 Journal of Applied Physiology 405, 405 (2002).
Hlastala also challenges the notion that breath test instruments will necessarily obtain an accurate reading of the alveolar air.
[T]he notion that a flat slope will always be obtained when expiratory flow rate approaches zero and that this represents alveolar air is incorrect. . . .At the end of exhalation, BrAC levels off when flow decreases, irrespective of the exhaled volume. The flat slope does not indicate the presence of air at alveolar concentration because BrAC is an increasing function with exhaled breath volume.
Michael P. Hlastala, The Alcohol Breath Test – A Review, 84 (2) Journal of Applied Physiology 401, 402-03 (1998).
He thus concludes that breath test instruments currently in use tend to favor individuals with larger lung size, because those individuals may stop breathing into the machine earlier, allowing the machine to be fooled into sampling a nonalveolar portion of the breath that is lower in alcohol concentration.
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