Tuesday, December 11, 2007

The Alcohol Breath Test, a review


In most states, police enforcement agencies utilize the Intoxilyzer 8000 or a similar machine to estimate blood alcohol content based on the amount of ethyl alcohol molecules expired in a single breath. Evidence from these machines is presented during trials as being reliable and accurate. If one suspect blows .079, he will likely be acquitted or charges won't even be filed; but if the same subject blows .080, prosecution will ensue. We are talking about a difference one one-thousandths of a percentage point that separate that guilty from the innocent when these machines are used. With so much on the line, lets discuss some of the science behind the machine:

1) The affect of body temperature: The machine assumes that all DUI suspects have a body temperature of 37 degrees Celsius and exhale their breath at 34 degrees Celsius. This assumption is based on research from 1950 involving 6 test subjects. These assumption have been studied by pulmonoligists and biologists since 1950. Several treatises in peer-reviewed articles conclusively show that the average person actually exhales at 35 degrees. This conclusion was reached in 1996 after observing 700 test subjects. So why does it matter? Research that compares BAC (measured by blood draws) and BrAC (breath alcohol content measured by a similar device) shows that for every degree of body temperature above the assumed 37 degrees results in an overestimation of BAC by 8.6%, with a maximum of a 23% overestimation (Fox & Hayward, both PhD's; Schoknect). By example: let's assume that I have a .079% BAC in my system at the time that I blow into the machine. Because I am very nervous, my heart rate increases as well as my body temperature, though only slightly by 1 degree. When I blow into the machine, the BrAC will register at .085%. Even though my BAC is in fact below the limit, it is likely that I will be convicted at trial merely because my body temperature is slightly elevated when I blew into the machine.
Now consider the role this plays when somebody during an Arizona summer leaves a barbeque at the lake after being in the hot sun all day.

2) Partition Ratio: Partition ratio derives from Henry's Law, which, in a nutshell, stands for the proposition that, in a closed container, a portion of the gas dissolved in a liquid will leave that liquid into the air above and reach equilibrium at a particular proportion. Let's think about it this way: ethyl alcohol is a gas when present in the blood stream. When the blood reaches the lungs, which operate enough like a closed container, the EtOH gas tranfers into the air above the liquid blood until a certain proportion is reached. The Intox. 8000 assumes that all people have a 2100:1 ratio, which means that for every 2100 EtOH particles in the blood, 1 such particle will be in the air above the blood in the lungs. It measures, by means of an electromagnetic wave, how many particles of EtOH are in a measured amount of air breathed into the machine, multiply it by a factor of 2100, to reach an estimation of BAC.

Here's the problem: depending on individual physiological differences from suspect to suspect (i.e. male/female, genetics, dehydration, health of the lungs, etc.) a person's partition ratio can vary 1600:1 to 2900:1. So why is this a problem? A DUI suspect with a 1600:1 partition ratio will have a breath test result 20% higher than his actual BAC--so if he is at a .07 actual BAC, his breath result will be well above the limit, and he will be unjustly convicted. A DUI suspect that has a 2600:1 ratio will have a result that is 20% lower than his actual BAC, resulting in a strong likelihood of sliding past prosecution with an actual BAC of .09%. Most people have a partition ratio between 1800:1 and 2400:1, but even the smaller variation has serious ramification on who is unjustly convicted and who get away with one.

3) The rest: Breathing patterns, lung capacity, whether the suspect is in the aborption/elimination phase of alcohol consumption, dispersion rate, hematocrit, and other factors play into the accuracy/reliability of the machine. These effects are usually not as significant, but play a role. I won't bore you with any more science.

I realize that many people are of the opinion that a person should be convicted of DUI after even one drink. Believe me--I completely understand those feelings; I have seen the effect of drunken driving. But that's not the point--the law is .08%, and that law needs to be applied uniformly to all people in such a way that the guilty are convicted and the innocent are exonerated. Yet, the states rely on a device clearly susceptible to variation, up to + or - 40%. This creates a strong public policy concern: How can law be applied uniformly where the primary investigative technique is inherently flawed? The solution is simple: draw blood, then these concerns fall.
Here is an article by a Professor of Pharmocology with a review of these issues:

1 comment:

peter said...

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