The Blood-Breath Partition Ratio
Q. Is measuring breath as a means of determining blood accurate?
A. Probably not. This is the controversy which surrounds the Blood Breath Partition Ratio. The Blood Breath Partition Ratio assumes that 2100mL of breath contains the same amount of alcohol as 1 mL of blood. It was reached over forty years ago by the National Safety Council's Committee for Tests on Intoxication. Even with the primitive technology available at that time, it was only an average figure. Of some interest is the fact that one of the signers of the document which approved the Blood Breath Partition Ratio was R.N. Harger. His device, the Harger Drunkometer, was one of the machines that was found acceptable by the Committee.
Q. Have experts found difficulties with the Blood Breath Partition Ratio?
A. Yes. Modern research casts grave doubts upon the absolute validity of the assumed ratio. In Intoxication Test Evidence 2d (Clark, Boardman, Callaghan, 1995), Fitzgerald noted that Kurt M. Dubowski and others have found the normal range in test populations to vary from 1100:1 to 3000:1. These variances can be profound; as Fitzgerald notes: "All breath test devices, for example, will report a 0.10% for a subject who has a true BAC of 0.07% if he or she has a partition ratio of 1500:1 (instead of 2100:1), and conversely, a 0.10% for a subject who has a true 0.14%, if he or she has a partition ratio of 3000:1."
In a paper presented in part at the Seventh International Conference on Alcohol and Road Safety, noted alcohol researcher A.W. Jones observed that the blood/breath partition ratio varies not only between individuals but within specific individuals across a one hour period. In a study conducted upon 21 males aged 30-55, Jones observed blood/breath partition coefficients ranging between 1837 to 2666. More importantly, individual subjects disclosed coefficient variations ranging from .95 percent to 12.56 percent.
Q. Are there other sources of error?
A. There are. Hematocrit levels, that is, the ratio of the solid component of blood to the liquid, has been reported by Fitzgerald and others as affecting the validity of the ratio. Likewise, variations in the temperature of the breath sample can affect the results. As I set out in my books, an increase in breath temperature of 1°C can increase reported alcohol content nearly 23 percent.
Q. Well, if breath analysis is that prone to error, why is it used?
A. The short answer is that a breath test is cheap, easily administered and the public generally has been convinced that they are reliable.