One of the main scientific underpinnings of breath testing is this thing called “partition ration.” This refers to the difference in breath alcohol verses blood alcohol. this is an important subject because testing the breath can only tell you how much alcohol is in the breath. And, there is always less alcohol in the breath than there is in the blood.
But breath alcohol does not impair or intoxicate. For that to happen the alcohol must get into the brain, and this of course requires that the alcohol be in the blood.
Because of this, in order for breath alcohol to have any relevance it must be converted to blood alcohol. To do this a ratio of breath and blood alcohol must be used. This is where partition ratio comes in, and this is one of the dirty little secrets of breath testing.
All breath testing machines everywhere in the United States “assume” that your breath blood partition ration is 2100/1. This means every molecule of alcohol in your breath is multiplied by a factor of 2100. But, not everyone has a partition ratio of 2100/1. Some people have a partition ration as low as 1784/1 to as high as about 3400/1, and anyone who has a partition ration that is different has a breath test score that is either too high or too low.
Because of this many scientists have recommended that a “safety factor” be applied to breath test scores. The idea is that this will cause there to be fewer false convictions. For example, Kurt Dubowski suggested this “safety factor” because data from his own research showed that at least 14% of the subjects had blood breath partition ratios less than 2100:1.[i] Dr. Dubowski also stated that for states using the two decimal system, (such as Michigan), a 0.025 deduction would round off to 0.03. This would make a .10 breath test score actually a .07 – below the legal limit!
Another top expert, A.W. Jones recommends a 0.015 g/210 liter reduction from the mean of two subsequent breath tests. This is less than Dubowski, but can still be significant. The highest recommendation is from Dominick Labianca who suggested a 0.055% reduction from the raw result. This would make a .13 at .075, again, below the legal limit!