The booze on your breath, or how a breathalyzer works

Bar at Deanna's by Alan Turkus on Flickr. Used under Creative Commons license.

(Alan Turkus/Flickr)

I had to make a quick trip to the drug store one night last week. Climbing back into my car after my shopping was done, I happened to glance at the car parked next to me, and noticed that the driver was breathing into a breathalyzer ignition lock.

Naturally, the first thing that came to mind was, “You know, I really don’t know how a breathalyzer works.” Which led to, “How does alcohol get on your breath in the first place?” Which of course became, “Hey, I could blog about this!”

So here we go. I’ll start with how liquor gets on your breath and work our way back to the parking lot.

From lips to liver…

When you have a drink, most of the alcohol — that is, the ethanol — that gets into your bloodstream gets broken down in the liver (though the brain, pancreas, and even the esophagus also break some down). While the liver has a few ways of doing this, most of the ethanol gets metabolized in a two-step process.

Alcohol metabolism (NIAA)

First, an enzyme called alcohol dehydrogenase (ALD) knocks a hydrogen off the ethanol molecule, turning it into something called acetaldehyde. Acetaldehyde is a bad actor. It’s carcinogenic and toxic to cells. This is where the second step comes in: a second enzyme called aldehyde dehydrogenase (ALDH) knocks another hydrogen off the acetaldehyde, turning it into a molecule of acetic acid. The acetate then gets further broken down into water and CO2.

…to lungs…

But not all of the alcohol you drink goes through this process, because it takes time. While the ALD and ALDH in the liver are chugging along, the alcohol you’ve absorbed from your stomach is coursing through your bloodstream, getting into your muscles, your brain, your skin…and your lungs.

Chemically speaking, ethanol is pretty volatile…that is, it likes to evaporate. Especially when in a nice warm environment like your lungs, where there’s lots of air flowing in and out. So when ethanol reaches the alveoli (the air sacs) your lungs, it passes out of your blood and gets exhaled just like carbon dioxide.

…to the law

At this point we can start talking about the breathalyzer.

The idea that the amount of alcohol in your breath can tell the police how much alcohol is in your blood comes from a physical law called Henry’s law. Formulated in 1803 by British chemist William Henry, the law states:

At a constant temperature, the amount of a given gas that dissolves in a given type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid.

To see what this means, I dug up this video lesson on Khan Academy:

All of this is a fancy way of saying that if you know how much of a volatile molecule or gas is in the just above the surface of a liquid — say, the amount of vaporized alcohol  in the air at surface of the gas exchange membranes that separate air and blood in our lungs  — then you can calculate how much of that gas is in that liquid.

Based on Henry’s law, in humans the ratio of exhaled alcohol to blood alcohol is 2,100 to 1 — put another way, there’s the same amount of alcohol in 2,100 milliliters of exhaled air as there is in 1 milliliter of blood.

And it was that knowledge that allowed Robert Borkenstein, an working for the Indiana State Police, to invent the breathalyzer in 1954.

A breathalyzer

A breathalyzer. (US Marine Corps/Wikimedia Commons)

So how does the breathalyzer then take that knowledge and put it to use? The actual operating principle depends on the device, because while I’ve been writing breathalyzer with a lower case ‘b’, there are a few different breathalyzer devices that each measure breath alcohol (and, by extension, blood alcohol) in a different way.

For example the Breathalyzer (with a capital ‘b’) relies on a chemical reaction between alcohol, sulfuric acid, and reddish-orange potassium dichromate that produces greenish chromium sulfate. A suspect breathes into the device, their air is bubbled through a vial containing the reaction mixture, and a sensor the measures the amount of color change.

The Intoxalyzer, on the other hand, relies on infrared spectroscopy. Different molecules absorb infrared light differently based on their chemical structure, meaning that alcohol in exhaled air will absorb IR light in a certain, measurable way.

A third device, the Alcosensor, takes the principles of fuel cells and applies them to breath alcohol measurement. The device contains a pair of electrodes with an electrolyte between them. The suspect’s air goes in and the interactions between the electrolyte, the electrodes, and the alcohol causes an electric current that can be used to calculate the suspect’s blood alcohol. (HowStuffWorks has a good cartoon for how this works.)

The end result of all three technologies (and I’m sure there are others I’m missing) is the same: From the amount of alcohol exhaled into the device, it can calculate — based on Henry’s law and that 2,100:1 ratio I mentioned above — whether the person breathing into it is OK to drive, or should have taken a taxi home.

Police car emergency lighting fixtures switched on by Scott Davidson on Wikimedia Commons. Used under Creative Commons license.

(Scott Davidson/Wikimedia Commons)


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