[Blog editor;s note: Joshua Goldberg of Pittsburgh, PA writes into us to share information about Breath Alcohol Content machines.]

THE HISTORY OF BREATH TESTING:  TWO ASSUMPTIONS AND A QUESTIONABLE SCIENTIFIC PEDIGREE. Part 2

Assumption 2:  Temperature of Breath Sample

The second startling assumption occurs when the government couples the partition ratio with an assumed 34° C breath temperature.

Henry’s law requires a “fixed temperature” for its analysis.  Modern breath testing uses:

“. . . 34 degrees Centigrade  (93.2 degrees Fahrenheit).  This temperature is essentially the temperature of expired alveolar air when body temperature is “normal” – 98.6 degrees . . . In reality, however, normal body temperature varies over a range of several degrees in the course of a day . . . the key point is that for every 1 degree Centigrade change in alveolar air temperature, the conversion ratio, and consequently the % BAC changes by 6.5%.” Labianca, supra. at 2.

Like the 2100:1 partition ratio, the assumed fixed temperature of 34° C guarantees that some defendants suffer overestimated BACs. In the span of 1941 to 1995, multiple published studies examined expired air temperatures in humans.  See, Dale A. Capenter and James M. Buttram, Breath Temperature: An Alabama Perspective, Int. Assoc. for Chem. Testing Newsletter Vol. 9, No. 2  (July 1998).  For example, Carpenter and Buttram’s study found that as many as 81% to 93% of test subjects exhibited breath temperatures above 34°.  Id. at   16.  For defendants with breath temperatures above 34 ° C, the breath testing device yields a falsely elevated result.

Utilizing 34° C is a mean or average breath temperature, to calibrate breath testing devices, and also calculate blood to breath alcohol ratios, provides yet another scary breath testing assumption.  The temperature assumption sullies breath testing as a forensic discipline and causes it to run afoul of the National Research Council’s mandate for reliable scientific methodologies.