Almost everyone in Utah knows that the “legal limit” for a DUI is .08, but what does that mean? What .08 refers to is your blood alcohol content (BAC) – the percentage of alcohol found in your blood or breath. The more alcohol in your blood, the more impaired you will become. And the state legislature has decided that at .08 you are too impaired to safely operate a motor vehicle. Thus, if you are above .08 BAC you are guilty of driving under the influence (DUI) due simply to the fact that your blood alcohol content is above the legal limit.
This kind of DUI violation – where you are guilty because your BAC is above the legal limit of .08 – is known as a “per se” violation. Because your BAC is .08 or greater, you are guilty “per se”.
Whether or not a particular individual is too impaired to safely operate a vehicle when their BAC is above .08 is a topic for another discussion. If you are dealing with a “per se” violation, all that is important is that the legislature has made it a crime to drive with a BAC above this arbitrary .08 BAC limit, and you and your attorney are now tasked with attacking either the the accuracy of the chemical test evidence given by the prosecutor, or (as always) the Constitutional basis of your arrest.
DUI charges based on this per se violation are by far the most common in Utah courts, and a large majority of these charges are based on breath test evidence, although in some cases blood tests are given instead of breath tests. Unfortunately for Utah drivers, breath test machines are inherently inaccurate because they are unable to account for circumstantial, environmental, and subject-based variations in data. What follows is a detailed scientific explanation of the problem with trying to obtain an accurate blood alcohol concentration from a breath sample with breath test machines (breathalyzers).
Henry’s Law and the “closed system assumption” in DUI cases
The problem with testing a BAC level is that taking the subject’s blood is time consuming, and intrusive. Breathalyzers were originally developed to provide law enforcement with a more convenient way of testing a subject’s BAC level. The basic idea is that if you can take a breath sample, and measure the amount of alcohol present in that breath sample, you can then calculate the amount of alcohol that will be found in the subject’s blood. This sounds simple, but as you will see, there are plenty of scientific problems with taking these calculations from theoretical to actual.
Henry’s Law is the basis of the modern day breathalyzer test. It is impossible to calculate a subject’s blood alcohol content based on the amount of alcohol found in their breath without a scientific law that shows the ratio between gases and liquids. Henry’s Law provides the basis for the calculation of a person’s BAC based on the amount of alcohol found in their breath. Henry’s Law states that if there is a “closed system” at a “constant temperature”, then a number of molecules in their liquid state will move into a gaseous state above the headspace of the liquid. Once this occurs, Henry’s Law states that the number of gaseous molecules will be directly proportionate to the number of liquid molecules within the closed system.
If it is true that the human body is a closed system like that in which Henry’s Law would traditionally apply – which I imagine would be something like a sealed test tube – then we can rely on a test which gives us a concentration of alcohol in the breath to convert that amount into the concentration of alcohol in the blood. This is called the “closed system assumption”.
The first problem we’ll discuss, however, is that it is unclear whether the human body constitutes a “closed system” such as one in which Henry’s Law would apply. It is easy to understand how Henry’s Law can be accurate where you have one chemical solution in a single sealed test-tube where the temperature and pressure do not change. But that is not really how a human body works is it? We are constantly breathing in and out, changing the pressure within our bodies, and skewing the calculations provided by Henry’s Law.
Nevertheless, the science behind breathalyzers must assume that the human body works as a closed system, whether or not this is actually true; otherwise, there is no way to accurately predict the amount of alcohol in someone’s blood based on the amount found in their breath. Consequently, breathalyzer developers use the constant blood to breath ratio of 2100:1 to calculate a subject’s BAC based on a sample of their breath.
Unfortunately, the “closed system” assumption is not the only problem with breathalyzers.
Alveolar exchange and DUI breath tests
DUI Breathalyzer Problem #1: you can’t get a sample of alveolar air during a breath test
Another problem with the accuracy of breathalyzers is that they function based on a ratio of “alveolar air” to “pulmonary blood”. This is not the same thing as a ratio between breath in general and blood in general. The accuracy of the 2100:1 ratio depends on comparing blood being exchanged in the lungs, and breath coming from deep in the lungs where, it is thought, the alveolar sacs exchange alcohol with the blood. You can imagine how alveolar air might contain a different amount of alcohol than breath higher up in the lungs or airways (where there are no alveolar sacs exchanging alcohol with the blood). You can also imagine how blood found coursing through other parts of the body would not be relevant for comparison to the amount of alcohol being exchanged between breath and blood in the lungs, since alcohol is not exchanged between breath and blood in other parts of the body.
So, if we could get a sample of purely alveolar air from a person, perhaps we could rely on the 2100:1 alveolar breath to pulmonary blood ratio, and we could then use the amount of alcohol found in that alveolar air sample to calculate the amount of alcohol that should be found in the subjects blood. The problem is that it is impossible to get a sample of purely alveolar air, because this air is located inside of the alveolar sacs within the lungs. What is being sampled when a person submits to a breath test for suspicion of DUI is air from the deepest part of the lungs, but it is not alveolar air! Thus, the breathalyzer is comparing apples to oranges by collecting deep lung air, but basing its calculations on pure alveolar air.
DUI Breathalyzer Problem #2: Alveolar Exchange of alcohol in the lungs is only a theory, and there is evidence that it may be incorrect
The basic assumption upon which breathalyzer technology is based, is the assumption that alcohol, in the form of ethanol, is exchanged between the blood and breath in the alveoli. Decades ago, when breathalyzers were first introduced, this assumption was generally accepted, and science had not progressed to the point where such an assumption could be challenged by conflicting data. However, that is no longer true. It is now possible to test the theory of alveolar exchange, and new studies have led some scientists to question whether the alveolar theory of alcohol exchange is correct.
For example, some studies have shown that oxygen and carbon dioxide do, indeed, exchange between breath and blood in the alveoli. However, other compounds like ethanol, exchange much higher up in the airways.
There are three phenomenon that occur in breath tests that indicate the alveolar exchange theory may be incorrect. First, if it was true that alcohol only exchanged between breath an blood in the alveoli, then the initial breath blown into a breathalyzer machine would contain no alcohol, because all of the alcohol found in the breath would be coming from far down deep in the lungs. However, this is not what we see in breath tests. When a DUI breathalyzer is taken, there is alcohol being registered in the initial breath blown into the machine, and there is a steady increase of the amount of alcohol found in the breath as the subject continues to blow. (An aside, this is also why the officer tells you to blow for a very long time when she administers the breath test!) This suggests that there is ethanol being exchanged between blood and breath much higher up in the respiratory tract.
The second phenomenon that suggests the alveolar exchange theory may be incorrect is that the longer the subject blows in a DUI breath test, the higher the concentration of alcohol found in the breath becomes. If the alveolar exchange theory were correct, then there would be a point during the breath sample where the air entering the sample chamber would be mostly alveolar air. The initial air coming from higher up in the respiratory tract would have already entered the chamber, and the amount of alcohol found in the breath at that point would flatten out. In other words, after the initial air was blown into the breathalyzer, the alveolar air would be expelled by the subject, and the alcohol content at that point would be the same as the content blown into the chamber later on in the breath.
But that is not what happens in reality. In reality, we find that the longer a subject blows into the breathalyzer, the higher the alcohol content of the sample, and the conventional alveolar exchange theory cannot account for why this happens.
The third phenomenon that suggests the alveolar exchange theory is incorrect is that it has been shown that the subject’s breathing pattern prior to taking the breath test affects the results of the test. Studies have shown that hyperventilation and hypoventilation immediately prior to taking the breath test can cause up to a 15 percent difference in the test results