Why Breath Tests of Blood-Alcohol Don't Work

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Over the years, breath testing has become a widely used method for quantitative determination of the level of intoxication of individuals suspected of driving while under theinfluence of alcohol. After recognition of the need for quantitative assessment of intoxication, blood alcohol concentration was considered as the single most important variable. However concern about the invasiveness requirements of drawing a blood sample led to the development of the breath test as a non-invasive means of assessing level of intoxication. The breath test is an indirect test, but has been considered to be a good estimate of the blood alcohol concentration because of the assumption that an end-exhaled breath sample accurately reflects the alveolar (or
deep-lung) air which is in equilibrium with the blood. In spite of the great deal of effort that has gone into the studies attempting to validate the breath test, forensic scientists and toxicologists still have a very rudimentary understanding of the breath alcohol test and its limitations.


The lungs are located within the chest. This organ allows inspired air to come into close proximity with the blood so gases (such as oxygen and carbon dioxide) can exchange between the air and the blood. The lung is made up of over 300 million small air sacs called alveoli. Outside air comes to the alveoli from the mouth or nose via the airways. The major airway leading to the lungs from the throat is the trachea. The trachea divides into the left and right "mainstem bronchi" (going to the left and right lungs) which divide further into the "lobar bronchi". This division goes on about 23 times until the alveoli are reached. Actually, some alveoli begin to appear at about the
17th generation airways. Surrounding each alveolus are small blood vessels. The thinness (less than 0.001 millimeter) of the membrane separating blood from the air in the lungs allows oxygen and carbon dioxide to exchange readily between the blood and air. Because of the large number of very small alveoli, there is a very large surface area (70 square meters) for this gas exchange process. For more details regarding the basics of lung physiology, see Hlastala and Berger.


The evolution of scientific understanding depends on the continuous development of new ideas that form the bases for experimentation. This concept has been termed "scientific revolution" by Kuhn (Kuhn 1970), who sees science as the shift from one paradigm to
another (Figure 1). The term, "paradigm" refers to a set of universally recognized scientific achievements that for a time provide a model or conceptual framework for a phenomenon. This paradigm represents the core principles that define the scientific understanding.


The alcohol breath test was founded on “old science”. Progressive development of newer, more modern technologies show that it is impossible for the alcohol concentration in breath to remain unchanged during exhalation from the alveolar or “deep lung” regions to the mouth. Alcohol always exchanges between the respired air and the airway mucosa. Consequently, BrAC depends on a variety of factors including: lung volume, body temperature, breath temperature,blood hematocrit and body size, with smaller individuals having a greater BrAC.