Breath Testing

One of the ways that a police officer might confirm that someone is driving under the influence of alcohol is by administering a breath test, also known as a breathalyzer. On this page, we explore the science behind the breath test and help you understand some common issues with this type of test. Learn more here!


In Danbury, breath testing machines are used by law enforcement to test for the presence of alcohol after arresting a driver for a DUI. These machines, also called breathalyzer tests, use infrared light to identify the amount of alcohol in a person’s blood. The way that this works is that the subject will exhale into a mouthpiece, which releases their breath into the device, and the molecules in the breath pass through infrared light beams inside the detector, which then measures the absorption of ethanol. Breathalyzer results can constitute evidence against a person in a DUI case. You can learn more about these breath tests here.

Breathalyzer Science

The breathalyzer is based on science known as Henry’s Law, which refers to the “mass of gas that dissolves in a definite volume of liquid which is directly proportional to the pressure of the gas provided that the gas doesn’t react with the solvent.” This means that if a gas and a liquid are contained in a closed area, the concentration of gas in the air above the liquid will be equal to the gas dissolved in the liquid.

An example is if there is blood inside a container and the blood evaporates until the concentration in the air above it is equal to the concentration in the blood. This is how the breathalyzer works.

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There is also a term called Henry’s constant, which refers to the ratio of the blood in the human body compared to air, which is measured at around 2100:1. On average, the concentration of alcohol within the blood is around 2100 times more than in the air.

Issues With Breath Testing

However, because each person’s size is different, this average ratio is not necessarily accurate for each person. The accuracy of the breathalyzer is questionable because the constant on which the science is based may differ from person to person. Scientists estimate that there can be up to a 0.03% error or more with a breath machine due to the normal ratio being used as a constant with the machine.

In addition to issues of the ratio of the breathalyzer, the temperature of the individual giving the sample can impact the results. What this means is that a temperature increase of only two degrees will cause about a 10% increase in the blood alcohol concentration results. Other things like taking aspirin, being sick, having diabetes, or taking medications can alter the accuracy of the breathalyzer.

The breathalyzer itself can cause inaccurate results as well because even without any outside influence on its reliability, the device could be calibrated improperly or poorly maintained. This is a very common defense against the admissibility of breath test results. If a device is calibrated incorrectly or not properly maintained, there is no telling what the degree of inaccuracy was at the time of the breath test.

Administering officers must be trained and certified to administer breath tests. Things like burping, vomiting, eating, or smoking before taking the breath sample could lead to inaccurate results. Also, it is important to note that in order for breath test results to be admissible, there must be two clear results that do not deviate in measurement of greater than .02 from one another.

Connecticut law has something called implied consent, which means that any driver has implicitly consented to a breath test, though any person can still decline to take it and instead be penalized by the DMV with a license suspension. However, the consequences of consenting to taking the breath test could be much more disastrous. Any good DUI lawyer will be able to analyze the case and find problems that may exist, and determine what possible defenses there may be. If you were arrested for a DUI in Danbury or have more questions, call and speak to a lawyer today.

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