The most common way of testing a driver's blood alcohol content is with the use of a breath test. The science behind all DUI breath tests is based on several assumptions, one of which is that people release alcohol from their blood into their exhaled breath at the same rate. However, more and more scientific studies are revealing that different people release different amounts of alcohol from their blood into their exhaled breath. In fact, there are dramatic variances from person to person, and even variances within the same person over different periods of time.
For the prosecution in a DUI case, the blood alcohol content of the defendant is one of the principal pieces of evidence. Most jurors in DUI cases do not understand just how unreliable a breath test can be. People generally have a tendency to assume that science must be accurate, otherwise it wouldn't be in use. Because they don't understand the complicated science behind the test itself, it is easier for them to assume that what seems to be objective scientific data is reliable enough for them to make their decision easy. A skilled DUI attorney will undermine these assumptions, and show the jury why there is reasonable doubt as to the accuracy of many breath tests.
How the Breath Test Works
All breath testing devices make an assumption that the amount of alcohol in a person's breath is proportional to the amount of alcohol in their blood. These tests assume that 2100 ml of breath contains the same amount of alcohol as 1 ml of blood. Using technical jargon, this means that each person is assumed to have a blood partition ratio of 2100 to one. Because the breath machines can detect the amount of alcohol in an air sample - the amount of alcohol in the breath is then used to estimate a person's blood alcohol content. However, there is growing evidence that the assumption each person has a blood partition ratio of 2100 to one is false.
Each person's lungs have tiny sacs called alveoli, which is where the air that you breath makes contact with your blood. It is in the alveoli that oxygen is absorbed into the blood and carbon dioxide is released into your exhaled breath. However, not all people breath as deeply as others, and not every person's alveoli work the same way. In addition, other factors will cause unreliability in the results - including a person's body temperature, the composition of a person's blood, and so on.
It was revealed in 1995 that different members of the population can have dramatically different blood partition ratios from individual to individual. A person's blood partition ratio can be as low as 1100, or as high as 3000. If a person's blood partition ratio is under 2100, they're less likely to be convicted of DUI. This is because a low blood partition ratio will cause the breath machine to report an erroneously low blood alcohol content. Conversely, if person's blood partition ratio is very high, their breath test result will over-exaggerate their blood alcohol content. The differences can be dramatic. For example, let's imagine a hypothetical person who should receive a 0.10 breath test result based on the assumption that their blood partition ratio is 2100 to one. If that same person has a true blood partition ratio of 1500 to one - the tests will report a 0.07 BAC. If the same person has a blood partition ratio of 3000 to one, the BAC will be reported as a 0.14.
As you can see, two different people could have the same blood alcohol content, yet register dramatically different breath test results. These testing irregularities become especially important in cases where a driver's BAC is close to the legal limit. In Colorado the legal limit is 0.08. If a person is charged with DUI when their blood alcohol content is 0.09, for example, it is critical that the jury becomes educated about just how far off the breath test results may actually be. Many factors will cause the test result to be false, including:
Every person has unique physiology, and the transfer of alcohol across the alveolar membrane is not consistent from person to person. This is a result of factors such as the permeability of the membrane itself, the cellular density of the blood - or the simple fact that an individual may breath more or less deeply than the average person.
If the person taking a BAC test has a higher than average body temperature, the breath test result will over-exaggerate the blood alcohol content. Each additional degree of body temperature results in a 7% higher value in the blood alcohol result. If a person is running a fever when they are tested, they have a higher risk of being falsely convicted of driving under the influence.
Exercise and Elevated Heart Rate
Exercise will decrease your breath test results. If you were to run up several flights of stairs immediately before you were given a breath test, this could decrease your breath test result by up to 25%.
The Temperature of the Testing Room
Because the water vapor in a person's breath will condense into liquid as it cools - and absorb alcohol with it — a colder testing room will result in a lower BAC result, and a warmer testing room, a higher BAC result.
When Denver area police officers take a breath sample from a person suspected of DUI, they use a machine called the Intoxilyzer 5000. Like all breath machines, the Intoxilyzer relies on the assumption that all people have the same blood partition ratio. If you took a breath test in your drunk driving case, and especially if you were just slightly over the limit, you should not plead guilty before having a skilled lawyer take a close look at the data.