Investigating the reliability of breath test devices

Roadside breath test devices are commonly used by law enforcement departments in Maryland and throughout the United States. Law enforcement uses these devices as a way to determine whether motorists are driving with a blood alcohol content level over the legal limit of 0.08.

Many driver rely on breath test devices to provide accurate and reliable results, but studies show that the readings may become skewed and could result in a wrongful DUI conviction


According to research performed at the State University of New York at Potsdam, there is a significant variance when comparing actual blood test results and breath test device readings. The two may differ by more than 15% in some cases. Furthermore, at least 23% of people who take a breath test show inflated results. 


Factors that affect breath test readings 

Breath test devices can pick up more than the amount of ethanol alcohol found in an exhaled breath sample. Other substances in the body and the environment can affect the results, which could lead to wrongful charges. Factors that can alter breath test readings include the following: 

  • Residual drink, food, vomit or blood in the mouth 
  • Inhalation of gasoline and cleaner fumes 
  • Temperature and relative humidity of the air 
  • Electrical interference from cell phones and officer radios 
  • Pollution, such as cigarette smoke and dirt in the air 

Improper calibration of the machine can also lead to erroneous results. Something else to bear in mind is that officers may inadvertently use the breath test device incorrectly, resulting in bad results. 


How do breath test devices work? 

When drivers exhale a breath sample into the device, it measures the amount of ethanol alcohol. The device then converts the measurement into a blood alcohol content level. Instead of just picking up ethanol alcohol structures, however, the device also picks up substances that have similar chemical structures. This adds to the possibility of wrongful readings. 



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