Want to trick a breath machine into a low test result? Or make sure that you're not causing a false high reading? Not that difficult, says a law firm of San Diego DUI attorneys and lawyers: just control your breathing.

The simple fact is that these breath machines which determine guilt or innocence in DUI cases are not exactly the reliable devices that law enforcement would have us believe. There are dozens of factors that can cause false test results. An example of that unreliability is the fact that the results will vary depending upon the breathing pattern of the person being tested. This has been confirmed in a number of scientific studies.

In one, for example, a group of men drank moderate doses of alcohol and their blood-alcohol levels were then measured by gas chromatographic analysis of their breath. The breathing techniques were then varied. The results indicated that holding your breath for 30 seconds before exhaling increased the blood-alcohol level by 15.7%. Hyperventilating for 20 seconds immediately before the analyses of breath, on the other hand, decreased the level by 10.6%. Keeping the mouth closed for five minutes and using shallow nasal breathing resulted in increasing the blood-alcohol level by 7.3%, and testing after a slow, 20-second exhalation increased levels by 2%. ("How Breathing Techniques Can Influence the Results of Breath-Alcohol Analyses", 22(4) Medical Science and the Law 275.)

Dr. Michael Hlastala, Professor of Physiology, Biophysics and Medicine at the University of Washington has gone farther and concluded:

"By far, the most overlooked error in breath testing for alcohol is the pattern of breathing... The concentration of alcohol changes considerably during the breath... The first part of the breath, after discarding the dead space, has an alcohol concentration much lower than the equivalent BAC (blood-alcohol concentration). Whereas, the last part of the breath has an alcohol concentration that is much higher than the equivalent BAC. The last part of the breath can be over 50% above the alcohol level... Thus, a breath tester reading of 0.14% taken from the last part of the breath may indicate that the blood level is only 0.09%." 9(6) The Champion 16 (1985).

Many police officers know this. They also know that if the machine contradicts their judgement that the person they arrested is intoxicated, they won't look good. So when they tell the arrestee to blow into the machine's mouthpiece, they'll yell at him, "Keep breathing! Breathe harder! Harder!" As Professor Hlastala has found, this ensures that the breath captured by the machine will be from the bottom of the lungs, near the alveolar sacs, which will be richest in alcohol. With the higher alcohol concentration, the machine will give a higher -- but inaccurate -- reading.