It is a common scenario: you are pulled over after a party. You’ve had very little to drink because you were the designated driver. Now you’re on the side of the interstate with flashing lights in your rear view mirror.

The officer brings out a breathalyzer. This little hand held tool is supposed to save society from your public drunkenness. After a perfect walk, and a completely clear conversation with the officer, you are convinced you are fine. However, you blow a .09 % BAC into the breathalyzer. The test has to be flawed, but how possible is that?

Inaccurate breathalyzer results are very possible because breathalyzers are known not to be accurate. Court cases across the country have recognized something called an ‘inherent error’ in all breathalyzers. This error states that the normal breathalyzer result may be off by .01%. Other studies say that breathalyzers range in accuracy from .0005 to .04% error. That means your .09% may actually have been anywhere from a .05% to a .13%, which can be a HUGE margin of error!

The problem with accuracy is all in the design. Breathalyzers measure blood alcohol level via the breath. They do not actually test the blood. They function by checking for the presence of alcohol in the breath and then estimate how much might be in the blood. False results can be triggered by the presence of other alcohol like chemicals such as acetone, paint fumes, plastics and adhesives.

Even though all breathalyzers work the same way, the method of alcohol detection can vary. There are basically three different types of breathalyzers that are grouped by the type of sensor technology they utilize. These different sensors range in both cost and accuracy.

Police departments normally use a larger machine that is generally considered quite accurate. These stationary breathalyzers include either a spectrophotometer or a fuel cell. The spectrophotometer sensor takes advantage of infrared technology to identify alcohol on the breath. The fuel cell sensor uses an electrochemical reaction to verify the presence of alcohol. These types of breathalyzers are expensive and not very portable.

Traffic officers carry a different type of breathalyzer. Although sometimes utilizing fuel cell technology, most DUI breathalyzers are based on semiconductor technology. Semiconductor sensors use materials, such as silicon oxide, that will react with alcohol and cause resistance. Because this type of breathalyzer works with resistance, rather than chemical reaction or infrared lasers, it needs to be regularly calibrated. Calibration is often quite expensive, and can easily be overlooked.

In addition to variations in the type of sensors a breathalyzer can have, there are also different types of software. The software’s responsibility is to interpret the results of the sensors. The problem is: just like on a computer, software is capable of error. Accuracy can also be diminished based on the user not knowing how to use the software.

Besides the mechanical issues, breathalyzers can also fail because they are based on the average person. Metabolism, sex, height and weight can affect how much alcohol is present in the breath. The amount in the breath, however, may have nothing to do with what is actually in the blood. Tests do their best to take human variation into consideration, but there is always someone who doesn’t fit the norm.

Breathalyzers are flawed in multiple ways. First, they attempt to measure blood alcohol levels through the breath. Second, they are machines that can suffer from mechanical defects and user error. Third, they are based on a standard that may not be appropriate for all users. Based on this, the opportunity for a false reading is quite present.

Were you actually over the legal limit? Well, it may be safe to say that is debatable.