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Understanding DUI Scientific Evidence

This product provides an insider's perspective on the evolving technologies and procedures associated with the evidence associated with driving under the influence (DUI) charges. Leading defense attorneys guide the reader through the key stages and steps involved in successfully defending a client accused of driving while under the influence.

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Reliability of Breath-Test Results in a Drunk-Driving Case

In every state in the U.S., a driver with a blood alcohol concentration (BAC) of 0.08 or higher is presumed to be legally intoxicated for drunk-driving purposes. Each state has also enacted an implied-consent law. Implied-consent laws provide that every licensed driver within the state is considered to have given his or her consent to chemical testing to determine his or her BAC whenever a law enforcement officer has reasonable suspicion of intoxication. In most states, refusal to submit to such a test results in license suspension or revocation.

While a blood test is arguably more reliable, the breath test for BAC is more commonly used in the field because the testing equipment is convenient to transport and because the test is easier to administer, can be used by nonmedical personnel, is less physically intrusive to the person taking the test and produces results faster. However, under certain circumstances, breath tests can be less than reliable as an accurate measure of BAC.

If you are facing drunk-driving charges, contact an attorney who is thoroughly familiar with possible breath-test weaknesses.

History and science of the breath test

The Breathalyzer® was invented in 1954 to collect evidence of intoxication by measuring BAC through breath analysis. Since that time, several manufacturers have entered into the breath-analyzer market and the science behind the machine has evolved. The word "breathalyzer" is now widely used to refer to these apparatus regardless of manufacturer. Other terms used to refer to these devices include evidential breath tester (EBT), breath analyzer, breath-analysis product, breath-alcohol analyzer and breath-testing device or machine. In addition to Breathalyzer, common brands include Intoxilyzer® and Intoximeter®.

In very basic terms, the breath-test machine is designed to measure the amount of alcohol in a deep-lung breath sample and to use that amount to determine BAC. The four main scientific methods used in the various machines are chemical analysis, infrared spectrophotometry, gas chromatography and fuel-cell detection.

Breath-test vulnerabilities

A drunk-driving defendant, usually through an experienced attorney and often with the help of a scientific expert, may be able to challenge the breath-test results in his or her case. Several different factors may call into question the reliability of the results. Depending on the law of the state in which the drunk-driving charges were filed, evidence of a problem with the test result could make the BAC reading inadmissible in court or, if the BAC is still admissible, could cast serious doubt on whether the reading is reliable.

Arguments a drunk-driving defense attorney may make to undermine the breath-test results:

  • The breath tests' assumption of a 2100-to-1 blood-to-breath ratio may not be scientifically reliable.
  • The test was not administered correctly; for example, the administrator did not warm up the machine to the correct operating temperature or ensure an adequately deep lung sample.
  • The test administrator was not properly trained or qualified.
  • The equipment was not maintained properly, calibrated correctly or cleaned adequately.
  • The result was affected by some characteristic of the driver, such as 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. Even severe heartburn could have made the breath results unreliable.
  • The test administrator did not continuously observe the driver for an adequate period before the test to prevent him or her from putting anything into his or her mouth that could affect the result. For example, a product used in the driver's mouth, such as mouthwash or adhesive, or lip ointment could have affected the test result.
  • Police radio operation generated electromagnetic waves, causing radio frequency interference (RFI) with the testing equipment.
  • The driver was exposed to a gas or vapor before the test that made it unreliable, such as during painting, floor sanding, varnishing or other activity with chemical exposure. Similarly, an outside environmental cause in the surrounding air could have caused a high breath test result.

This long list of potential problems with breath test results is not close to complete. If a person charged with drunk driving is able to show that something affected the reliability of his or her test results, the state will need to rely more on other types of evidence to prove intoxication. Other types of evidence on which the state may need to rely include witness, officer and driver statements; witness and police observations; or field-sobriety test results.

Speak to a drunk-driving defense lawyer

If your situation involves a breath test to measure BAC, an experienced attorney who is thoroughly familiar with proper testing procedure and the science behind the test may be able to provide valuable assistance.

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