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DUI Breath Testing and Criminal Prosecutions

A driver with a blood-alcohol concentration (BAC) of .08 or higher is presumed to be intoxicated. Blood tests are more trustworthy, but breath tests are commonly used.

The breath test is simpler to perform and administered by non-medical personnel with quicker results. Many situations exist that impair breath tests as a precise calculation of BAC.

Results Used for License Suspension and Criminal Prosecution

All states have implied-consent laws. These laws provide that every driver within the state is deemed to have given consent to chemical testing to measure BAC if law enforcement has reasonable suspicion of intoxication. A driver suspected of driving under the influence[1] with breath tests result greater than .08 or refusing to submit to testing is generally subject to driver license suspension or revocation. See Illinois DUI Law: A Criminal and a Civil Case.

The test results are routine evidence in criminal prosecutions, where the penalties can include jail or prison. Almost everyone has an acquaintance that has been arrested for a DUI. Knowledge of the evidence in these cases is important for the public.

From Drunkometer to Intoxilyzer

Rolla Harger invented the “Drunkometer” in 1937. His invention launched an industry that has seen much scientific evolution.  The Breathalyzer® was invented in 1954 to collect evidence of intoxication by measuring BAC through breath analysis. In addition to Breathalyzer, common brands include Intoxilyzer® and Intoximeter®.

Despite scientific efforts, all breath-testing machines are subject to many errors. All breath-testing devices base test results upon certain population averages and no individual is average. Human error, maintenance problems, interference from outside sources, and internal malfunctions can affect every breath-testing result.

The Science of Breath Testing

The breath-test measures the amount of alcohol in a deep-lung breath sample. The breath alcohol is then converted to blood alcohol concentration. Chemical analysis, infrared spectrophotometry, gas chromatography and fuel-cell detection are among the scientific methods relied upon.

With an infrared system, the person tested blows a breath sample into a chamber. An infrared beam shoots through the chamber. The beam absorbs alcohol molecules. Sensors in the chamber measure the beam’s diminution. The more light absorbed, the higher the alcohol reading on the machine. This is the theory, though other molecules in the same spectrum as alcohol may also absorb the light. Computer software translates the results to BAC for the average human.

The Blood:Breath Ratio

This ratio is the relationship between breath alcohol content and blood alcohol content. It defines the breath quantity that contains the same amount of alcohol as a given blood quantity. The law accepts a value of 2100:1 as the population average. This means 2100 parts of breath contain the same quantity of alcohol as 1 part of blood. The actual range in the population varies from 1700 to 3000:1. Beyond the variations between individuals, the blood to breath ratio changes over time for a specific person.

Individuals whose personal ratios at the moment of testing are greater than 2100:1 have actual blood alcohol concentrations that are greater than the test result. Persons tested that have ratios less than 2100:1 show test results that are greater than their actual BAC. The machines make scientific assumptions based upon population averages that may not be accurate for a particular person. See Forcon Forensic Testing.

Individual Variations from Personal Physical Conditions

The personal physical condition of the test subject also affects test results. Personal conditions that affect test results include: age, lung function, overall strength and size, a disease or condition such as asthma, diabetes, eardrum rupture, ketosis, emphysema, bronchitis, dental issues, fever or harelip, shock or trauma, certain types of special diets, or hiccoughing, burping, vomiting or hyperventilating. Heartburn can make breath results unreliable.

Other Technical Issues with Breath Testing

There are technical issues in the conduct of the test itself. These include:

  • The  machine must be warmed to the correct operating temperature.
  • An adequately deep lung sample must be ensured
  • The  equipment must be maintained properly, calibrated correctly and cleaned adequately.
  • Other  items such as mouthwash or adhesive, or lip ointment can affect test results.
  • Police radios may cause radio frequency interference (RFI) with the testing equipment.
  • The subject may have had exposure to a gas or vapor such as paint, floor sanding, varnishing or other chemicals.
  • Outside environmental causes in the surrounding air may cause inaccurate test results.

Evidence Issues in Criminal Prosecutions

Driving under the influence is a criminal matter.  A DUI defendant has the same constitutional rights as a defendant in any other criminal prosecution.  The state must prove the charge beyond a reasonable doubt, and follow proper criminal procedure providing access to evidence and the confrontation of witnesses.

Although widely accepted by the courts and public, the breath testing process has significant scientific issues and confrontation issues.  Using population averages to determine the guilt of a specific individual is questionable from a “beyond a reasonable doubt” perspective.  Makers of testing machines consider the computer programs behind the testing to be trade secrets.

These manufacturers have resisted providing information to defense attorneys about the operation of the machines.  The machines are the source of evidence in as many as 95% of DUI prosecutions, and the makers have been refusing to provide critical information to persons whose liberty is at stake based upon machine test results.

In many counties of Florida, just such issues have been working through the courts for several years.  The Florida Supreme Court now has several cases regarding access by defendants to test computer software for reliability.  These cases will have impact across Florida and potentially across the country.

Protecting the Public and Protecting Our Rights

Society always has a fundamental conflict.  There is an eternal balancing act in protecting public safety and not infringing upon the rights of a given individual.  The dangers that drunk drivers present are evident; the societal costs of convictions based upon evidence of questionable reliability and witnesses resistant to providing evidence in criminal prosecutions are not quite as obvious.  Laws enforced ignoring valued principles breed disrespect for the law generally.

In an effort to provide public protection from that danger, lawmakers have relied upon science as an answer. Breath testing is relatively quick and does not require the training and skill that blood testing involves. The financial costs are significantly less.  The science is also significantly flawed.

While inconvenient and expensive, blood testing is highly accurate.  It should be the norm rather than the exception.  Additionally blood evidence can (unlike breath evidence) be preserved.

Drunk driving is not to be condoned; neither is imposing criminal punishment based upon unreliable evidence and witnesses refusing to testify.


[1] All United States jurisdictions have criminal laws prohibiting the operation of motor vehicles on public highways under the influence of alcohol.  The name for this action varies among the states.  Among the names for these crimes: Driving Under the Influence (DUI), Operating Under the Influence (OUI), Driving While Intoxicated (DWI).

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