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It can be challenging to prove whether an erratic driver is “under the influence” of a chemical, drug, or some other impairing substance. Natural hormonal fluctuations, chemicals, pain, and other bodily processes can significantly impair a driver’s judgment. One study found that having to pee can cause cognitive impairment similar to a 0.05 percent blood alcohol content (BAC)! Other drivers are just reckless or inexperienced.

Separating the impaired from simply bad drivers requires a little science. So-called ‘preliminary alcohol screening (PAS)’ tests like breathalyzers can help law enforcement officers in the field quickly take the first step when determining whether a driver has consumed alcohol.

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Per Se Laws And DUI Convictions

While drivers can be impaired by a wide array of pharmaceuticals and substances, drunk driving remains widespread and deadly. The National Highway Traffic Safety Administration (NHTSA) reports that in 2020, drunk driving killed one American every 45 minutes. These sobering statistics persist despite a decades-long campaign against driving under the influence.

Alcohol impairs motor functions, decision-making, reaction times, and other cognitive abilities. Medical studies have demonstrated that nearly everyone’s driving abilities are impaired at a BAC of .08. Because of this, every state now prohibits operating a motor vehicle with a BAC of .08 grams of alcohol per deciliter (g/dL) of blood or more. (Utah’s legal alcohol limit for all drivers is .05 percent.)

These are called ‘per se’ laws. If field breath analysis or blood test evidence shows that a driver has a BAC above the legal limit, they are guilty of DUI; no additional evidence is necessary to prove impairment or intent.

What Is A Breathalyzer Test?

Since the 1940s, police have used breath analysis devices along with the field sobriety test battery to evaluate drivers for alcohol use. There are actually several different types of DUI test devices commonly used in field breath analysis testing. All depend on the same basic physiological principle: The amount of alcohol expelled in a person’s breath is a consistent, known percentage of their blood alcohol level.

If you can accurately measure the amount of alcohol in the breath, you can calculate the level in the blood. Each type of device requires a user to blow into a mouthpiece, collecting their breath in a chamber for analysis.

The Breathalyzer device was invented in 1954 by Dr. Robert Borkenstein of the Indiana State Police. It combines a breath sample with a mixture of sulfuric acid, potassium dichromate, silver nitrate, and water. The reaction of alcohol causes the mixture to change color along a predictable spectrum. A photocell measures the progression of the color change and correlates it to a quantifiable BAC.

Modern field analysis devices also employ other scientific methods of analyzing BAC. Some use infrared spectroscopy to differentiate molecules of alcohol based on their vibration patterns. Others, like the Alco-Sensor FST, use fuel-cell technology to measure the oxidation of alcohol molecules.

Courts have accepted evidence from professional-grade devices using all three types of technology. State laws set out requirements for BAC testing to be admissible in DUI proceedings; if a device meets those criteria, its evidence will generally be admitted. However, all breath analysis devices have some margin of error; intentional evasion techniques, medical conditions, and other factors can diminish the accuracy of the readings. You may be able to contest the test results based on one or more of these factors.

When Is A Blood Test Performed To Determine BAC?

A blood test is the most accurate tool to determine a person’s blood alcohol content. Unlike indirect analysis devices, a blood test directly measures the percentage of alcohol in a sample of blood. Because this procedure involves drawing blood, it must be performed by a medical professional; this is generally not feasible during a roadside traffic stop. To be accurate, the test must occur within 6 to 12 hours of a person’s last drink.

If a driver refuses to submit to a breathalyzer test, police may apply for a warrant for a blood draw test. They must show probable cause to suspect the driver of DUI before a court will approve the warrant. A warrant is unnecessary if a driver is unconscious based on the “exigent circumstances” exception.

The U.S Supreme Court determined that you can’t face criminal charges for refusing to undergo a warrantless blood test. However, you can be arrested for failing to submit to a field breath analysis test. Failure to submit to any type of sobriety testing will likely result in the suspension or revocation of your driving privileges, even if you are not convicted of DUI.

BAC Levels And Graduated Punishment

Some states now use measures of BAC as a tool to determine the penalties for driving under the influence. For example, Pennsylvania DUI law has a three-tiered scale, separating drivers into categories based on their BAC. As a driver’s blood alcohol concentration levels rise, so do the corresponding penalties. However, a driver with a lower BAC may still be subject to higher penalties if they:

  • Are a minor (under the drinking age of 21)
  • Are a commercial, school vehicle, or bus driver
  • Are involved in an accident that injures someone or causes significant property damage
  • Refuse to undergo breathalyzer or chemical testing

Although DUI laws differ from state to state, many consider these or similar characteristics as factors that enhance a convicted offender’s punishment.

Are There Any Other Defenses To DUI Charges?

In cases where per se BAC evidence is established, a driver may still be able to present an affirmative defense that negates their legal liability. For example, someone whose drink was spiked without their knowledge may be unaware they have consumed alcohol (‘involuntary intoxication’), and a person who is forced behind the wheel at gunpoint might claim they only drove drunk under duress. Laws regarding the admissibility of these kinds of defenses vary from state to state.

Facing DUI Charges? Consult With A Lawyer

If you or a loved one has been charged with DUI, an experienced attorney can help you evaluate your options. You may be able to challenge the results of a breathalyzer or present evidence to mitigate the charges.

Legal Disclaimer: This article contains general legal information but does not constitute professional legal advice for your particular situation and should not be interpreted as creating an attorney-client relationship. If you have legal questions, you should seek the advice of an attorney licensed in your jurisdiction.

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