how do breathalysers work to detect alcohol in breath and how accurate are they?

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Surely a blood test would indicate exactly how much alcohol is in ones blood, but breathalysers can be used as evidence against a driver. So my question is how a breathalyser works to detect alcohol in breath and how accurate are they?

In: Technology

Alcohol is not digested by your body and directly enters your blood stream. Our blood also passes through our lungs where some of the alcohol passes through membranes and evaporates in the lungs because of its volatile nature. Now when you blow into a breathalyzer, if there is alcohol in your blood a chemical reaction occurs in the machine which causes a colour change. This shows the amount of alcohol in your bloodstream.

Alchohol circulates in your blood, and your blood reaches your lungs. Alcohol also diffuses out of your blood, so it mixes with the vapors in your lungs. This means it really is “on your breath”.

Breathalyzers are fairly accurate. They measure the amount of alcohol in your breath and multiply it by 2100 (breath-to-blood alcohol ratio is 2100:1) to get a decent estimate of your BAC. This is still just an estimate, though.

There are a few things that can affect your readout, though, including; higher body temperature, differences in blood composition, how recently you drank, electrical interference, presence of methyl compounds (from things like paint remover, vinegar, cleaning fluids, diabetes compounds, etc), acid reflux, blood or vomit in the mouth, etc, etc. Heck, even having recently smoked a cigarette can affect your readout.

All of these things can add up to about a 15% difference between your breathalyzer readout and your actual BAC. If you’re ever arrested for a DUI, police will probably run a blood test when they get you back to the station. There are even lawyers who make their living challenging the accuracy/reliability of breathalyzer tests.