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Chris' Blog

The blog of Chris Perri Law, written by Chris Perri and Shannon Perri. Read the latest in exciting cases where justice is served.

Filtering by Tag: blood alcohol concentration

Breath Test Machines: Less Reliable Than You Think

Chris Perri

Chris Perri, next to his very own breathalyzer.

Chris Perri, next to his very own breathalyzer.

In DWI investigations, breath test results are very common evidence. Although I advise clients to refuse to provide a sample of their breath, the case isn’t over just because the machine says that the sample is over .08 BAC.

At the outset, it’s important to realize that BAC means blood alcohol concentration, not breath alcohol concentration. Breath test science relies on the assumption that it can obtain a reliable breath alcohol concentration that mirrors a particular person’s blood alcohol concentration. This assumption is fraught with difficulties, as I’ll explain below.

Let’s start with a very simplified description of the mechanics of the breath-test machine (also known as an intoxilyzer or breathalyzer). An arrested person blows into the machine, which takes this breath sample and shoots it through a tube that’s then injected with infrared light. Because alcohol particles block infrared light, the machine detects the amount of alcohol in a subject’s breath by determining how much of the infrared light has been blocked. It then takes this number, makes some calculations, and reports a breath alcohol concentration.

The calculation of this breath alcohol concentration is problematic because there’s a lot less alcohol in the breath than in the blood. For example, in an average person, the number of grams of alcohol in 1 part of the blood is equivalent to the number of grams of alcohol in 2100 parts of breath. This 1:2100 ratio is known as a “partition rate.” So, while a person’s blood alcohol concentration is defined as the number of grams of alcohol in 100 milliliters of blood, that same person’s breath alcohol concentration is defined as the number of grams of alcohol in 210 liters of breath. (Note: 100 milliliters x 2100 = 210 liters). In other words, if you have .08 grams of alcohol in 100 milliliters of your blood, then it’s assumed that you have .08 grams of alcohol in 210 liters of your breath.

While it’s not hard for police to obtain 100 milliliters of your blood, it’s impossible for them to obtain 210 liters of your breath (think about a 1 liter bottle and imagine filling up 210 of those bottles with your breath). For this reason, the breath test machine must multiply any amount of alcohol that it detects by a very large number. As an example, if you provide the machine with one liter of your breath, then the machine multiplies the amount of alcohol it detects by 210 in order to determine the number of grams of alcohol per 210 liters of your breath. This calculation is then reported as your BAC.

Consequently, any error by the machine in determining the amount of alcohol in a given sample would be exacerbated when it multiplies that incorrect number by two-hundred-fold. Such errors can occur when the machine interprets non-alcoholic particles in the breath as alcohol. For example, the machine cannot distinguish acetone (a common substance in the breath of diabetics) from alcohol. Or, imagine the complication of a stray particle of liquid alcohol entering the machine in the form of spit. Any error in the initial measurement of alcohol will render the entire breath test unreliable.

Photograph courtesy of Oregon Dept. of Transportation

Photograph courtesy of Oregon Dept. of Transportation

For my next point, let’s give the machine the benefit of the doubt and assume that it can accurately measure the quantity of alcohol in a person’s breath. Even then, the machine makes a critical and troublesome assumption: that the subject’s partition rate is 1:2100 (recall from above that this means that the amount of alcohol in one part of blood is equal to the amount of alcohol in 2100 parts of breath). This assumption is not true across the population, as studies show that partition rates of normal people vary from 1:1100 to 1:3000.

As an illustration, let’s take a hypothetical subject arrested for DWI and call her Sue. She has a partition rate of 1:1100, which means that the number of grams of alcohol in 1 part of Sue’s blood is the equivalent of the number of grams of alcohol in 1100 parts of her breath. On this particular evening, Sue has consumed enough alcohol that her BAC is .06 grams of alcohol per 100 milliliters of blood. Since her partition rate is 1:1100, there would also be .06 grams of alcohol in 110 liters of her breath (note: 100 milliliters multiplied by 1100 equals 110 liters).However, the breath machine is going to overestimate the BAC by nearly a factor of two. Here’s why:

When Sue takes a breath test, the machine is programmed to incorrectly assume that her partition rate is 1:2100. As a result, it will determine the number of grams of alcohol in 210 liters of her breath. Since Sue’s correct BAC is .06 grams of alcohol per 110 liters of breath (due to her partition rate of 1:1100), she has .11 grams of alcohol per 210 liters of her breath. Due to the machine’s inaccurate assumption that Sue’s partition rate is 1:2100 instead of 1:1100, the breath-test machine will report a BAC of .11, which is nearly twice as high as her actual BAC of .06. This inaccuracy results because the machine is improperly measuring the number of grams of alcohol per 210 liters of Sue’s breath, as opposed to the number of grams of alcohol per 110 liters of her breath. Thus, the machine’s assumption that everyone has a partition rate of 1:2100 creates a critical error by reporting that Sue is intoxicated even though she’s actually below the legal limit of .08 BAC.

Finally, it’s important to remember that it’s only illegal to be intoxicated while driving. It’s not a crime to be intoxicated 10 minutes, 30 minutes, or an hour after operating a motor vehicle. But these chemical tests usually occur over an hour after driving, and the prosecution has the burden of proving how that BAC measurement relates to the time of driving. Unless law enforcement knows a lot of information about a particular person (e.g., number of drinks consumed, when the drinks were consumed, the type of alcohol consumed, amount of food consumed, when food was consumed, etc.), it’s impossible to conduct a reliable retrograde extrapolation, which is the science of determining a past BAC level based on a known BAC level. In other words, if the breath test machine reports that a person has a .11 BAC over an hour after driving, we don’t know whether the person’s BAC at the time of driving was below, above, or the same as the level reported by the machine.

While breath tests are an important tool for law enforcement in that they give a ballpark figure about an arrested person’s intoxication level, it’s a common misconception that a breath test machine provides an exact measurement of a person’s BAC at the time of driving. If you or a loved one have been arrested for DWI and submitted to a breath test that reported a BAC over .08, don’t despair. Call an experienced criminal defense attorney to fight the machine’s potentially inaccurate result.

Lowering the Legal Alcohol Limit for Drivers Would Backfire

Chris Perri

Recently, The National Transportation Safety Board released an official recommendation for all states to lower the legal alcohol driving limit to a .05 blood alcohol concentration (BAC). Currently, the blood alcohol legal level is .08. See the chart below to see how these limits actually translate to individuals’ alcohol consumption.

bac-chart.jpg

At Chris Perri Law, we believe that lowering the legal limit would be a mistake and lead to injustice. “I think it dilutes the standard for intoxication,” Chris Perri says. “There is about a .02 margin of error on these breath tests. People that aren’t drunk and even had only one beer could register over the legal limit.” According to Chris, this would cause even more people to refuse to cooperate with police officers, as it puts those who have just had one drink at risk of severe legal consequences. In fact, Chris believes that raising the legal limit would actually lead to safer roads, as then the crime would be more stigmatized by our community. At present, Chris feels it is too easy for anyone to get a DWI, and if the limit was lower, it would seen as even less of a big deal to have been convicted of a DWI. To Chris, it is just not okay for someone who registered at a .08 BAC to be facing the same charge as some registered at, say, a .14.  Currently those arrested with a very high BAC actually benefit from the fact that they are lumped together with those just barely over the limit—the community sees all these crimes as one.

Another concern is that the police are under pressure to arrest anyone who has possibly had one drink for their own liability reasons. If they let someone go and that person has an accident, the city could be sued. However, this leads to innocent, law-abiding citizens spending nights in jail, carrying criminal records, and causing additional tax money to be spent on the criminal justice system. Further, if the BAC limit was say, .12, then when someone is arrested at this BAC, there would be no question that the person was drunk and needs a steep punishment. Currently, having a DWI is not a major stigma because the population understands that even those who aren’t drunk can end up with an arrest.

The National Transportation Safety Board states that more than 100 countries around the world have adopted a .05 BAC legal limit, and that this had led to fewer alcohol-related accidents. However, what is unfair about this comparison is that in these other countries, readily-accessible alternative transportation options exist. Chris Perri believes that in cities in like San Francisco, Chicago, and New York City there is absolutely no reason to ever drive after drinking alcohol. Yet, in Austin, we lack a viable public transportation system. A much better way to spend our tax dollars would be on developing better transportation options – not prosecuting individuals with a .05 BAC.

A forgotten element in this debate is how lowering the legal BAC would adversely affect the indigent population. Those with a lower socio-economic status are less likely to have funds for a taxi service. Also, if arrested this population often does not have ability to pay for a private attorney, so they must rely on a court-appointed attorney, which can be a bit of a crapshoot. Let’s not forget, court-appointed services are also paid with our taxes. Furthermore, giving more people criminal records, especially those already facing hardship, does not help anyone, but instead harms our entire community. Having a criminal record makes getting a job harder and also increases one’s likelihood of repeating the crime, as one’s sense of identity begins to shift due to the community’s label of that person as a “criminal.”

At Chris Perri Law, we believe strongly in finding ways to reduce accidents related to drunk-driving. However, Chris feels that by making a DWI more stigmatized and also offering improved public transportation options are much better ways to focus our efforts than punishing those for driving after just one drink.

We’d love to hear what you think, too. Let us know in the comment section.