How Does a DUI Breath Test Really Show How Much I Had to Drink?

A common question that is asked is about how a DUI breath test can really show how much a driver has consumed before driving. This is a very fair question. Common sense does not lead someone one to a conclusion that a puff of air from someone's mouth can reveal precisely how much alcohol is coursing through someone's body.

History of Breath Alcohol Testing

The human understanding of physics and chemistry has been amazing for a very long time. Exhaled breath has been analyzed to make estimations about amounts of alcohol as long ago as the 1870s. Of course, these measures were not related to DUIs, as automobiles would not be common for another 50 years.

As early as the 1920s there were at least theories that measurement of exhaled breath could reveal at least estimations of the amount of alcohol in someone's blood. The first widely accepted device for use in DUI investigation was produced in the 1950s, and was called the Breathalyzer.

Breath Test Physics

A basic premise that underlies DUI breath alcohol testing is the fact that alcohol is a volatile chemical. In this context, "volatile" means a chemical that is at least partially gaseous at normal atmospheric temperatures. Alcohol that is consumed as a beverage is the chemical "ethanol." Ethanol is a hydrocarbon, and like virtually all hydrocarbons, is at least partially gaseous at room temperature and body temperature. When a liquid mixture containing a volatile chemical is at equilibrium at a known temperature and air pressure, there is a measurable and consistent ratio between the concentration of that volatile in the liquid and the concentration of the volatile in the air around the liquid (in a hypothetical closed system at least). This fact of physics led to the idea that knowing the temperature and the ratio would allow a measurement of alcohol concentration in the air to reveal the concentration in the liquid.

Applying this science to the human lung physiology was the next step. The primary purpose of blood circulating through the body is to to deliver oxygen to the brain. A close second is the reciprocal purpose of delivering carbon dioxide from the body back to the lungs for expulsion from the body. The exchange of these gasses to and from the blood to the breath occurs in the alveolar sacks in the lungs. It was realized that a volatile like ethanol would be equalibriating in the alveoli, allowing a measurement of the air coming from these sacks would reveal the amount of ethanol in the liquid blood that flows to those sacks. Comparisons of breath alcohol test results to simultaneously conducted blood tests can be remarkably consistent, considering some of the dynamic variables that cannot be controlled.

The Problem With Breath Results as Forensic DUI Evidence

Although controlled laboratory experiments have shown that breath testing can be reasonably accurate, there are many variables that can render them inaccurate.

Measuring Exclusively Alveolar Air

Forensic breath theory assumes that the exhaled air being measured is air that represents the air in the alveolar sacks that has not been contaminated by any alcohol that is in fact all along the breathing tract and in the mouth. Recent alcohol consumption or even burping could compromise this assumption.

The Suspect Has the Assumed Body Temperature

Recall that the estimated ratio between liquid and air is only predictable at a constant temperature and pressure. Different people have different body temperatures. One person has different body temperatures at different times of one day. Any deviation in body temperature from the assumed temperature will introduce changes in the measurement of breath alcohol concentration.

Assuming the Machine is Properly Calibrated and Maintained

Police DUI breath machines are generally calibrated and maintained by police employees. The records that represent that the machines are working properly and have been properly maintained are largely generated by people who work for the police department. Most law enforcement agencies are not particularly open to independent toxicologists or chemists coming into their agencies and inspecting the breath devices or testing the calibration. There are regularly cases discovered in which maintenance records have been deleted or falsified by chemists employed by police agencies.

Lack of Training of Police Officers/ Bias

At the time breath tests are conducted, the machine is usually operated by a police officer. Often, it is the same officer who made the arrest for DUI. Police officers are not scientists. There are a number of factors that could compromise a breath analysis that a minimally trained police officer would not recognize or respect. Testing conducted by an arresting officer also compromises the integrity of the whole process, as the breath test result is the objective evidence that confirms the correctness of their arrest. An officer who has already made an arrest for DUI has generally already made their mind up that a suspect is DUI. Consulting a different officer to conduct the chemical test could improve the forensic integrity of the investigative process.

No Right to Re-Testing

When police obtain a blood result in a DUI investigation, an additional blood vial is generally collected. This second vial allows a suspect to have an independent test conducted on the collected blood. This "second opinion" so to speak allows any errors in analysis to be discovered through a separate analysis. Breath testing does not allow for an independent check of alcohol results. Technology exists to capture and preserve air analyzed in a DUI breath test, but virtually no state provides this protection to DUI suspects.