What do breath test devices really measure?

Law enforcement officers often use roadside breath tests to determine whether a driver is over the legal limit, which is 0.08 in Maryland and in many other states in the U.S..

After blowing a breath sample into the device, the machine gives a reading based on the amount of ethanol alcohol found in the exhaled sample. Yet, studies show that the devices measure more than just the amount of alcohol. In fact, at least one in four people who use the devices will show inflated BAC results, according to the State University of New York at Potsdam. 


Researchers compared the blood alcohol content level reading from a breath test device to the level obtained from an actual blood sample. The results showed a variance by more than 15%, which could lead to a wrongful DUI arrest and possible conviction. 


In addition to ethanol alcohol, breath test devices pick up other substances that have similar methyl structures. These include the following: 

  • Any residual food, drink, vomit and blood found in a person’s mouth 
  • Inhaled gasoline fumes, cleaner fumes and cigarette smoke 
  • Pollution in the air 
  • Residual temperature and humidity of the air 
  • Compounds in human saliva 
  • Electrical interference from police radios and cellphones


The accuracy of breath test device readings is also dependent on whether the machines have been calibrated properly and regularly. It is also imperative that the officer administering the test knows how to do so correctly in order to minimize or avoid inaccurate results. 



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