The previous two posts on the physiological sources of inaccuracy in breath alcohol analysis have apparently caused considerable interest…and a number of queries. Let me be clear: Simpson is far from alone in his conclusions. Those involved in forensic alcohol analysis will generally recognize that the most recognized authorities in the field include Dubowski, Jones, Simpson and Hlastala. In the posts I quoted supporting conclusions from Dubowski and Hlastala; Jones has expressed somewhat similar views. To quote further from Professor Dubowski on physiological sources of error (as opposed to operator error or defects in the design or function of the machine itself — of which there are many):
First, not all blood and breath alcohol curves follow the Widmark pattern, nor is the elimination phase linear. Second, alcohol absorption is not always complete within 60 to 90 minutes as often claimed. Third, the peak alcohol concentration cannot be validly predicted or established in an individual instance without frequent and timely measurement of alcohol concentrations. Fourth, it is not possible to establish whether an individual is in the absorption or elimination phase, or to establish the mean overall rate of alcohol elimination from the blood or breath, from the results of two consecutive blood or breath alcohol measurements, however timed. Fifth, significantly large short-term fluctuations occur in some subjects and result in marked positive and negative departures from the alcohol concentration trend line. Sixth, short-term marked oscillation of the blood or breath alcohol concentration can occur at various points on the curve, resulting in repeated excursions of the alcohol concentration above and below a given concetration within a few minutes or for hours. Finally, no forensically valid forward or extrapolation of blood or breath alcohol concentrations is ordinarily possible in a given subject and occasion solely on the basis of time and individual analysis results.
Dubowski, "Absorption, Distribution and Elimination of Alcohol", 10 Journal of Studies on Alcohol, Suppl. 98 (1985). And those are just some of the physiological problems when trying to measure blood alcohol. We also have to consider: possible errors by the cop in operating the machine; malfunctions in the machine; design defects (there are many: see, for example, "Breathalyzers — and Why They Don’t Work" and "Why Breathalyzers Don’t Measure Alcohol"); maintenance and repair issues; calibration errors; and so on…. But, as they say in DUI law enforcement, "close enough for government work".