What are the causes of false positives on a breathalyser?

Written by nelly morrison
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What are the causes of false positives on a breathalyser?
Not all breathalyser tests results are accurate. (ronstik/iStock/Getty Images)

About half of all fatal traffic accidents involve drivers with a blood alcohol level higher than the legal limit. A large proportion of drunk drivers are repeat offenders, and breathalyser machines are an important tool in keeping them off the roads. Unfortunately, there are many factors that show not all breathalyser test results are accurate.

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From as early as 1874, scientists were trying to find a breath test that could measure alcohol consumption. Drunk driving laws were first introduced in New York in 1910. Other countries followed suit, but enforcement relied on the subjective opinion of police officers using field sobriety tests. In 1927 Emil Bogen discovered the relationship between alcohol in expelled air and alcohol in urine. In 1938, this science was used to create the "Drunkometer," followed in 1941 by the "Intoxometer." In 1954, Professor Brokenstein, a forensic scientist, invented the first "Breathalyzer," the forerunner of machines used today.

Technical reliability

A breathalyser does not measure, but estimates, blood alcohol levels. Different machines can give estimates as much as 15 per cent higher than actual blood alcohol levels so not all results are reliable. Other false positives can be caused by imprecise calibration, and many DUI (Driving Under the Influence) cases have been dismissed because estimated readings are deemed inaccurate. Breathalyser readings can be challenged because assumptions of the blood alcohol to lung air alcohol conversion vary from person to person.

False readings

Ethyl alcohol molecules, which are found in alcoholic beverages, are very similar to methyl molecules, which appear in 70 or 80 per cent of compounds found in human breath. Acetone is a methyl compound and is commonly found in the breath of diabetics and people practicing low-calorie diets. Those following high-protein diets are especially at risk of having false positive breathalyser results as the body is producing extra ketones, which are a form of acetone.

Other influences

False positive breathalyzer readings can also be a result of alcohol, blood or vomit in the mouth at the time the reading is taken. Both ambient and human temperature can raise results as can interference from mobile phones or police radios, dirt, moisture and tobacco smoke. In Frankston, Australia, it was shown that two bites of ice cream caused a breathalyser to register a positive result. Conversely, hyperventilation or intense physical activity lowers blood alcohol readings.

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