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.
Modern research casts grave doubts upon the absolute validity of the assumed ratio.
More importantly, individual subjects disclosed coefficient variations ranging from .95 percent to 12.56 percent. 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. The short answer is that a breath test is cheap, easily administered and the public generally has been convinced that they are reliable.