How Does a Breathalyzer Work?

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How Does a Breathalyzer Work?
(Elza Fiúza/ABr (CC-By 2.5 Brazil))

The term "breathalyzer" is actually the brand name of the breath-analysing products manufactured by Smith and Wesson in the late 1950s, an improvement of the much more cumbersome "drunkometer" developed in 1938. The term, nevertheless, has come to be a generic term for any make or model instrument designed to determine the amount of alcohol in the blood of a subject by analysing the breath. Literally dozens of breathalyzer models exist, most hand-held. Because of their frequent use as evidence in criminal courts, their design and performance is subject to highly prescribed regulations.

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What is a Breathalyzer?

The term "breathalyzer" is actually the brand name of the breath-analysing products manufactured by Smith and Wesson in the late 1950s, an improvement of the much more cumbersome "drunkometer" developed in 1938. The term, nevertheless, has come to be a generic term for any make or model instrument designed to determine the amount of alcohol in the blood of a subject by analysing the breath. Literally dozens of breathalyzer models exist, most hand-held. Because of their frequent use as evidence in criminal courts, their design and performance is subject to highly prescribed regulations.

How Does it Work?

Alcohol is not digested in the body; it is simply absorbed into the tissues of the mouth, throat, stomach and intestines and eventually excreted. In part, alcohol leaves the body via the breath; because the per cent of the total blood alcohol that exists this way is predictable, the alcohol content of the breath can be used to calculate the total blood alcohol content, or BAC. The hand-held breathalyzer uses electrochemical fuel cell technology to identify the elements found in the sample, of course, searching specifically for alcohol. Some devices use an infrared spectrophotometer to scan the sample, identify alcohol and calculate its percentage content. The BAC is then figured based on a partition ratio, the ratio of breath alcohol to blood alcohol, of anywhere between 1700 and 2400. The output of the breathalyzer is a blood alcohol reading expressed as a percentage of the blood that is alcohol.

Factors Affecting Breathalyzer Accuracy

While the principles behind breathalyzers are sound, they can only more or less estimate BAC since they do not measure it directly. An artificially high or low reading can result due to several different factors, all of which are frequently exploited by defence lawyers. For starters, body temperature and blood composition can alter the partition ratio -- individuals with reduced blood content will show an artificially high BAC reading. Vomit or blood in the mouth, or acid reflux can also skew readings to the high side. Most breathalyzers also cannot differentiate between ethyl alcohol and chemically similarly compounds such as ketones, acetone and gasoline, so the presence of these substances in the testing environment or in the subject’s metabolic system will also produce a falsely high reading. A recent drink can also produce an aberrant reading since the presence of alcohol in the bloodstream lags consumption by several minutes. Finally, electrical interference from cell phones and police radios or inaccurate calibration can produce unreliable results.

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