Breathalyzer Accuracy

About half of all alcohol-related traffic accident fatalities involve drivers with a blood alcohol concentration (BAC) of about .16 or higher. A significant proportion of high-BAC drivers are repeat offenders, and breath analyzers (Breathalyzer, Intoxilyzer, Alcosensor and Alcoscan are proprietary names for various different devices) are an important tool in keeping them off the streets. However, numerous factors can affect the accuracy of these devices, prompting some states to not rely on breathalyzer accuracy in DUI prosecutions.

One major problem with breath analyzers is that they don't actually test BAC, but estimate it indirectly. Different devices use different techniques and larger machines generally yield better estimates than the hand-held models that police carry. Readings from hand-held breath analyzers can vary 15% from actual BAC.

Because the measurements are indirect estimations, the analyzers must always be calibrated for precision. Improper calibration is a frequent reason for discounting the analyzer results at trial.

Technical issues with the devices include sensitivity to ambient temperature as well as subject temperature; failure to properly account for variations in the human hematocrit (cell volume of blood) range; and false assumptions of the conversion factor used in converting lung air alcohol concentration to blood alcohol concentration.

Many alcohol breath analyzers assume a 2,100-to-1 conversion factor, but the ratio varies from person to person and within a person over time. All these factors can increase a BAC reading.

A problem with some alcohol testing machines is that they not only identify ethyl alcohol (found in alcoholic drinks) which is made of ethyl molecules, but they also identify any compound with a methyl molecular structure. Over one hundred compounds can be found in human breath at any one time and 70 to 80 percent of them contain methyl molecules.

One such compound is acetone, which can be found on the breath if you are using it, diabetic, or if you are dieting and producing a lot of ketones. This points to another dilemma with breath analyzers, which is a sometimes inflated reading due to alcohol in the mouth, not in blood.

Other things that can cause falsely elevated BAC readings are blood or vomit in the test subject's mouth, electrical interference from cell phones and police radios, tobacco, smoke, dirt and moisture.

On the other hand, physical activity and hyperventilation can lower apparent BAC levels. Studies have found that BAC readings decrease with vigorous exercise or hyperventilation.

Learn more about breathalyzers.