Know Your Rights: How Breathalyzers Work


The first retail breathalyzer was used by Depression-era women to determine if their husbands had spent the night boozing. Based on the “evidence,” she could then decide whether or not to let him into the house.

Much more advanced retail devices are now used by both party-goers and law enforcement. How do these breathalyzers work, and are there any problems associated with their use?

What Exactly Do Breathalyzers Measure?

Common breathalyzers use an electrochemical fuel cell to indirectly measure the concentration of ethanol molecules transferred in their gas phase.

An iPhone attachment version, called the iBreath, measures blood alcohol content in a slightly different manner. The iBreath measures  electrical resistance that results from a reaction between a layer of tin dioxide and ethanol.

Another larger scale, more expensive breathalyzer, often called an intoxilyzer, measures the absorption of infrared light in the presence of a breath sample. The intoxilyzer looks for vibrational characteristics common with an alcohol functional group in a molecule of ethanol as the molecules are stimulated in the presence of infrared light.

Problems with Testing via a Breathalyzer

The use of a breath sample to determine blood alcohol content, the truly important measurement, brings up a significant problem. The amount of blood in your breath is NOT necessarily the amount of alcohol in your blood. In the United States and Canada, blood alcohol content is the criteria for arrest, but one can show an abnormal blood alcohol content via a breathalyzer examination in a number of situations.

If you are arrested from data obtained via a breathalyzer, a blood test will often be administered to determine the amount of alcohol actually in your blood. The alcohol is being metabolized, so the technicians will use established techniques to determine the amount of alcohol actually in your blood at the time of arrest.

These techniques include the Widmark Formula, which takes into account the weight of the suspect and their gender. The constant applied in the formula for males and females has come under fire in recent years, with the roughly 17 percent difference in the constant used in many DUI cases to aid the defendant.

The Unusual Effect of Running on Breathalyzer Tests

If you increase the number of breaths you take at in a given span of time, you decrease the number of alcohol molecules in each breath. A person with a BAC of 0.10 who just ran a mile (granted, without hitting a wall, passing out, or vomiting) could potentially come under the legal limit for BAC (0.08) if the test is given right after he or she finishes running.

Alcohol not yet absorbed by the body or alcohol brought up after vomiting will increase the concentration of alcohol in your breath. Before administering a breathalyzer test, law enforcement officials are supposed to wait 15 minutes after first contact or vomiting in order to allow breath alcohol to dissipate.

Invasive Installation?

As of November 2012, breathalyzers will be mandatory in all cars in France, with those failing to carry a breathalyzer in their car subject to a small fine. Disposable or multi-use breathalyzers like the ones described in this article are the easiest to acquire, but cars with built-in breathalyzers, called “alcolocks,” are rising in popularity in France due to the new law.

Could we see an ethanol-based ignition check point placed in North American cars in the next decade? If France’s effort is successful in curbing fatal accidents linked to drunk driving, it’s quite possible.


Image Sources: Wikimedia Commons

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