How Does A Breathalyzer Work?

Breathalyzers are part of modern culture. Are they based on scientific principles? How is it possible for them to determine the level of a person's intoxication?

We've all heard about them. Breathalyzers are part of modern culture. We read about their use in newspaper articles and magazine accounts of drunk driving arrests; we see them being used in movies. They are mounted on the walls in bars. Single-use breathalyzers are even for sale to the general public. Police officers use them to determine whether the person they've pulled over is legally drunk and courts use the numbers that they produce for drunken driving convictions. But for all of the general public's familiarity with the term "breathalyzer," what do we really know about how they work. Are they based on scientific principles? Or are breathalyzers just pseudo-science, used to create difficulties for an unsuspecting public?

The fact is, breathalyzers are entirely scientific. They provide a useful, noninvasive technique by which police officers can identify drunken drivers, to remove them from the streets. Although roadside sobriety tests are a good first-line defense against such drivers, they are fallible and often subjective. It is relatively simple for someone who is actually drunk to pass one of those tests. Breathalyzers determine drunkenness in objective terms, based on the blood alcohol concentration (BAC) in the driver's blood.

What is Blood Alcohol Concentration (BAC)?

The legal limit for legal blood alcohol level is 0.08 in most states, down from the commonly accepted 0.10 level from some years ago. The American Medical Association (AMA) states that even that level is somewhat high. Statements that the AMA has release indicate that people can become impaired with a blood alcohol level of only 0.05. If a driver being tested has a BAC of 0.15, that means he or she has 0.15 grams of alcohol per 100 ml of blood.

Determining BAC using a Breathalyzer

How is it possible to determine the level of alcohol concentration in the blood by using a person's breath? Unlike milk or fruit juice, the body does not digest alcohol. Rather, it is absorbed through the membranes in a person's mouth, throat, stomach, and intestines. Once absorbed by the body, the alcohol passes immediately into the bloodstream, where it circulates until it is expelled through evaporation in the lungs. Evaporation occurs because alcohol is "volatile" in a solution, meaning that its molecules do not combine with the liquid that it mixes with. Due to this volatility, as the blood passes through the lungs, some of the alcohol passes over the alveoli (the lungs' air sacs), allowing it to be released by the person's breath. The expulsion of the evaporated alcohol through the breath permits the BAC to be accurately measured, since the percent alcohol being expelled contains the same level of alcohol that is contained in the blood. The amount of alcohol in 2,100 ml of expelled breath is exactly equivalent to the amount of alcohol in 1 ml of blood. With these equivalent measures in mind, it is possible to attain an accurate measure of the driver's intoxication, based on the figures set for legal limits. But how is the concentration of evaporated alcohol in the breath measured?

The breathalyzer operates on a similar principle as diabetic testing strips or pool testing kits: a certain level of alcohol produces a chemical reaction that causes a color change on a piece of paper or in a solution. The color changes produced by these testing kits are usually interpreted by comparison to a color chart. This comparison is subjective, based on the observation of the individual performing the test. In the case of the breathalyzer, however, the expelled breath travels through a chemical solution, creating the color change that is interpreted objectively using a device that police officers are trained to use.

A breathalyzer consists of a collection device, a "straw" attached to a cylinder. The cylinder contains two vials, which contain a solution of sulfuric acid, potassium dichromate, silver nitrate and water. The individual being tested blows into the straw for approximately two to four seconds. The object is to test the air from deep in the lungs, which will produce the most accurate reading. The expelled air travels into the vials, where the silver nitrate acts as a catalyst to initiate and speed up the analysis. The first thing that happens is that the sulfuric acid removes the alcohol from the air. The sulfuric acid potentially produces the acidic condition necessary for the remainder of the process to take place.

Once removed from the air, the alcohol is then absorbed into the liquid solution, where the potassium dichromate proceeds to break it down. The alcohol is broken down into chromium sulfate, potassium sulfate, acetic acid, and water. This portion of the process causes the color change. The dichromate ion is reddish-orange, while the chromium ion that the dichromate is broken down into is green. The observed degree of color change is based on the amount of evaporated alcohol expelled in the breath.

The chemical reaction takes place in only one vial. The other vial remains in its original state, as part of the photocell system. This system produces an electrical current, causing the needle in the gauge to move. The breathalyzer operator waits for the needle to stop moving after the reaction has taken place, and then rotates a knob that returns the needle to its point of origin. The more the knob must be turned, the higher the percentage of alcohol in the solution.

Although the breathalyzer provides an accurate and objective method of determining the level of an individual's intoxication, it still needs to be maintained to retain its accuracy. Maintaining a correct calibration at all times is necessary to obtain accurate results based on the process. Prosecutors tend to rely on the results of full-sized breathalyzers over the smaller, portable systems for court proceedings, because they may be more reliable. In any case, it is important that people in charge of operating breathalyzers, such as law enforcement and court officials, are adequately trained in how to use and calibrate them.

Other Methods to Determine BAC

The breathalyzer is not the only method for determining an individual's level of intoxication. The Intoxilyzer uses infrared spectroscopy to measure the movement alcohol molecules based on the way they absorb infrared light. The Alcosenser also uses a chemical reaction, like a breathalyzer does. Unlike the breathalyzer, the Alcosenser but determines the quantity of alcohol present by the amount of an electrical charged produced by ionized alcohol molecules.

How Accurate are Consumer Models?

Some consumer models of breathalyzer are very accurate and may even be used as an initial screening device by some law enforcement officers. Although these devices might provide a good idea of a person's level of intoxication, they should not be relied upon for determining impairment before driving. Using a consumer model of breathalyzer will not absolve an individual of responsibility should he or she be pulled over for drunken driving.

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