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Dear Dr. Vinny,

I am fortunate to live near wine country, and visit often for tasting. Since I always do the driving, and like to avoid mid-afternoon inebriation, I always spit out the wines. How does the alcohol on my breath affect my standing in the eyes of the law? Since the machine is called a "breathalyzer," and it works by blowing into it, does having had wine in your mouth somehow affect its calculation of your BAC?

—Chum L., Mendocino, Calif.

Dear Chum,

Great question. And yay, designated driving! Breathalyzers were invented to be a noninvasive way to get a sense of your blood alcohol content (BAC) using chemical oxidation, photometry or infrared spectroscopy. So breath alcohol tests don't directly measure your BAC—you need a blood sample for that—instead, they measure deep lung alcohol.

I checked with Sgt. Terry Gonsalves of the Napa Police Dept. to make sure I understood how breath alcohol tests work. The most accurate results occur if the breath sample came from alveolar air—air exhaled from deep within the lungs. But if you taste and spit wine and immediately breathe into the breathalyzer, there might still be alcohol in the mouth or throat that the breathalyzer will read, screwing with the results.

That's why, says Sgt. Gonsalves, the Napa police have a 15-minute waiting period before being tested, where a subject is observed to make sure they don't add anything to their mouth that might alter the results, and to make sure they don't burp, which can exaggerate the reading. "The accuracy of any breath alcohol test is dependent upon the relationship between the concentrations of alcohol in the blood and deep lung breath," says Gonsalves. "The amount of alcohol in a properly collected breath sample is governed by the amount of alcohol in the bloodstream circulating in the lungs, not the mouth. Bottom line, we test deep lung alcohol, not mouth alcohol."

Our conclusion is that if you truly have been spitting, not swallowing, then the breathalyzer's analysis of your deep lung alcohol should show you in the safety zone. But make sure to take that 15 minutes the officer gives you before taking the test, to ensure that any alcohol still in your mouth dissipates and doesn't elevate the reading.

—Dr. Vinny

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