How Do Breathalysers Work?

We all know that driving drunk is dangerous (and not to mention, illegal), but how much alcohol is too much? In Australia, the legal BAC limit for driving is 0.05%. It’s almost impossible to guess your own BAC levels because the number is affected by so many different factors, including your weight, gender, and how fast you drink.

That’s where a breathalyser test comes in. These devices can check a user’s breath for alcohol to determine their BAC before they get behind the wheel. How exactly does a breathalyser work?

What is a Breathalyser?

A breathalyser is a device that can estimate a person’s blood alcohol content (BAC) from a sample of their breath. “Breathalyser” is actually a trademarked brand name for the device, but most drivers have come to use the brand as a generic term.

You probably associate breathalysers with police, but anyone can own one of these devices. Some businesses in the hospitality industry have public breathalysers that anyone can use to test themselves before they hit the road.

How Modern Breathalysers Work

Breathalysers do not actually measure a person’s BAC directly. To do this, a blood sample must be analyzed. Instead, a breathalyser indirectly measures BAC based on the amount of alcohol in your breath.

There are three main types of breathalyser:

  • Semiconductor oxide-based testers use a tin-oxide material to test the user’s breath for alcohol. The reaction between the user’s breath and the tin-oxide substance generates a current that is measured by the device to provide a reading. Semiconductor testers are affordable and portable, making them ideal for personal use.
  • Fuel cell testers use an electrochemical process to oxidize the user’s breath and create a current. While this works similarly to a semiconductor, a fuel cell tester is more accurate. These are used by law enforcement to test drivers during traffic stops. While these tests can’t be used as evidence in court, they are reliable enough to arrest someone based on probable cause.  
  • Infrared Spectrometry is a type of technology used in the large, stationary breathalyser units found in police stations. Instead of creating a current, these units identify alcohol molecules based on how they absorb a special infrared light. Spectrometers are the most accurate type of breath testing.

Common Causes of False-Positive Results

Breathalyser technology has come a long way, but false-positive results can still occur. The best way to avoid this is to know why these results happen – some of the most common causes are:

  • Incorrect calibration. The sensors in the devices can be interfered with by substances other than the alcohol in the breath. That’s why they need to be calibrated regularly to ensure they are testing for the correct levels of alcohol.  
  • Non-Specific Analysis. Older devices identify the ethanol in alcoholic drinks as well as other substances with similar reactivity and molecular structure. These devices oxidize the alcohol into acetic acid which is then tested but other substances can be oxidized producing the same levels of acetic acid leading to false results.  
  • Interfering Compounds. Substances like acetone and petrol additives that can accidentally be inhaled in the workplace can trigger false positives. Similarly, dieters and diabetics can have acetone levels that are higher than normal, leading to failed tests.
  • Mouth Alcohol. This is the most common cause of inaccurate results. Breathalysers test alveolar air (the air in the lungs) for specific ratios of alcohol to breath.  Alcohol in the stomach or intestines that hasn’t been absorbed into the blood can be burped up into the soft tissue of the oesophagus (the tube that connects the throat to the stomach). If a subject has taken a drink right before a test, the ratio will be thrown off. This is why officers should observe suspects for 15 minutes or so before taking a breath sample.


Having the ability to check your BAC before you get behind the wheel of a car can help you keep your license and even save lives. As more people realize the benefits of these devices and start to own personal breathalysers and take advantage of public ones, the roads will become a lot safer for us all.

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