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DUI Breath Testing Variables

Breath Testing Errors

Why The Breath Test Reading Is Wrong

Etoh breath testing gives the tester an idea of how much alcohol may be in the test subject's body. Notice that I said "idea." Breath testing does not actually measure how much alcohol is in a person's blood, and it certainly does not measure how much alcohol is in a person's brain.  Breath testing measures alcohol that has evaporated out of the blood and into a person's airways. In calculating a BAC reading, the machine is programed to convert the breath number to a number that reads the same as blood reading.  This way there is one standard instead of separate standards for breath and blood. The machine is programed to report a breath number by multiplying the measured amount of alcohol in a subjects breath by 2100.  The calculation, therefore assumes a a static relationship between the amount of alcohol in breath to blood.  This assumption is not true. There is a relationship, but it is not static. The relationship varies from person to person, and even from one moment to the next with the same person. The relationship is not simple, but complex and subject to many variables.

And this is the problem with breath machines. It is not that they don't work well as reading the amount of alcohol in a person's breath, it is that the breath reading may not accurately reflect what is in a person's body. Readings may give an officer a good idea what is in the person's body, and to this degree, the tool can be useful.  The problem is, however, is that prosecutors are enforcing "per se" DUI laws with machines that under some circumstances can overestimate a person's true BAC by 75%. The machines are not designed to say with certainty how much alcohol is in a person's body.

The Ten Percent Margin of Error

Compounding this problem is the prosecution's insistence in portraying these machines as the next best thing to sliced bread. Prosecutors and their criminalists (cops in white robes) without fail assert to juries that these devices are 90% or better accurate. Juries want to believe them, and often do. Juries often have a hard time believing defense attorneys and their experts when they tell them the machine actually have uncertainty in measurement closer to 35% and possibly up to 75%. Many jurors find it impossible to believe the legal system would permit the use of a device in criminal proceedings which can be so wildly inaccurate.

What Calibration Checks Really Mean

When the prosecution and their criminalists claim that the device is 90% accurate, they inevitably point to the calibration checks routinely done on the machines to assure they are working properly. A calibration check is done by taking a known alcohol solution and having the machine test it for its alcohol content. If it tests within 10% of the "correct" value the machine is said to be working properly.There are times when a machine just does not work properly. The machines break down like any other mechanical / electronic type of equipment. Testing with a known solution helps find those machines that are not electronically or mechanically working correctly. That being said, it is intellectual dishonesty to contend that because a machine can accurately test a known solution it can just as accurately test people.

Calibration Checks Control All The Variables

It is intellectual dishonesty because the calibration test procedure perfectly accounts for all variables. Because the process is tightly controlled, consistent and accurate results can be obtained. However, in the real world, variables exist, and the machines do not account for these variables. Instead, the machines are programmed with a standard set of numbers and assume everybody is the same. A "one size fits all" solution to the problem. The problem is, everybody is different. Because of this, statistically speaking, it is virtually impossible for a breath reading to be a true measurement of how much alcohol is in a person's body.

In this state, and in many others, people's lives hang in the balance in criminal DUI proceedings.  In Arizona, our lawmakers not only allow law enforcement and prosecutors to use these machines, but have also passed legislation limiting a defendant's ability to challenge them. Because of this, prosecutors routinely send people to jail and prison based upon these guesstimates.

What Are The Variables?


The temperature of the person's breath sample is incredibly important. Think about a pot of water on the stove. At medium heat, it will steam. However, if you turn the heat up to high, the water boils. The higher the temperature, the more water evaporates from the pot. This exact same principle applies to alcohol in the lungs. The higher the body temperature, the more alcohol diffuses out of the blood and into the lungs. At least one breath testing machine accounts for this variable by measuring breath temperature. In Arizona, law agencies use the Intoxilyzer 8000 device. These machines do not measure breath temperature. Instead, they assume a breath temperature of 34 C. Recent research examining the logs of machines that do take breath temperature readings has shown that the average temperature is not 34 C as the CMI Intoxilyzer presumes, but 35 C. If the temperature of a person's breath sample is 1 degree above 34 C, then the person's reading will be about 7% higher.  Scientists today also know that normal body temperature is actually a range. Variances in body temperature can inflate the reading on a breath machine by more then 20%.

Breathing Technique

A person's breathing technique (the way a person blows into a breath testing machine) will also effect the accuracy of the reading produced by the machine.  Dr. Michael Hlastala of the University of Washington Department of Medicine, a leading expert on breath alcohol testing, has confirmed that breathing technique can significantly impact a breath test reading as it relates to a person's true BAC.  If a person holds their breath, or is a shallow breather, that can cause the reading to be over 20% higher than the true BAC. Given that the breathing pattern is not controlled, it is impossible to determine whether or not a person's result on the breath test is artificially high and to what degree.

Breath Blood Partition Ratio

All breath testing equipment currently used in the United States fixes the relationship between breath and blood at 2100 to 1. In other words, for every part alcohol in breath, there is 2100 times greater in blood.  According to the theory, if one took 2100 cubic centimeters of our deep lung air and analyzed the amount of alcohol in that sample, the amount of alcohol in that sample would be equal to the amount of alcohol in one milliliter of our blood.  In fact, recent research shows this ratio to vary from 990 to 1 to 3005 to 1. If a person has a ratio lower than 2100 to 1 then the corresponding breath test result will be artificially high. On the other hand, if a person's ratio is higher than 2100 to 1, then a person's breath test sample will be artificially low. A person's breath to blood partition ratio varies from person to person and even varies within the person over time.

In the absorptive phase, the average mean for a healthy white male is 1776 to 1. Scientists estimate the vast majority people will fully absorb a drink within one hour, but studies have also shown that it may take some people up to 5 hours after a person's last drink on a full stomach and 2 on an empty stomach. Current breath testing equipment assumes that a person is in the post-absorptive phase. Even the leading proponents of breath testing concede that it is virtually impossible to determine at which stage a person is in at the time of the test and therefore impossible to determine whether or not the person is overestimated or underestimated.

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