Breathalyzer test basics at home and at the roadside

When you drink, alcohol passes from your stomach and small intestine into your bloodstream. As blood flows through the lungs, a small share of that alcohol moves into the air sacs, so the air you breathe out carries alcohol in proportion to the alcohol in your blood.

A breath tester measures the alcohol in a known volume of exhaled air and converts it into an estimate of blood alcohol concentration, usually shown as BAC. In the United States that conversion uses a fixed ratio between breath and blood, which is why devices display a number such as 0.05 rather than a reading in breath units.

The key word is estimate. The ratio between breath and blood differs a little from person to person and changes during the hours after drinking, so even a perfect sensor gives an approximation of what a blood test would show.

This is an estimate. It cannot tell you whether you are safe or legal to drive. If you have been drinking, do not drive.

What sits inside the device

Personal testers use one of two sensor types. A semiconductor sensor changes its electrical resistance when alcohol touches a heated metal oxide surface, which makes it cheap and compact but also sensitive to other substances. A fuel cell sensor oxidizes alcohol in an electrochemical cell and produces a small current, which is more specific to alcohol and more stable over time. Our fuel cell vs semiconductor breathalyzer comparison goes through the differences in detail.

Police and workplace evidential instruments often add infrared spectroscopy, which identifies alcohol by the way its molecules absorb light, and many combine infrared with a fuel cell for a second check.

Most makers give similar instructions. Check your own manual, because warm-up times and blow durations vary by model.

• Wait at least 15 minutes after your last drink, food, cigarette, vape or mouthwash. Residual alcohol in the mouth can push a reading far above your true level.
• Rinse your mouth with water if the manual allows it, then wait a few more minutes.
• Turn the device on and let it finish its warm-up or countdown before you blow.
• Fit a clean mouthpiece if your model uses one, so you get a consistent seal and avoid moisture building up in the sensor.
• Take a normal breath and exhale steadily into the device for the full time the manual specifies, usually several seconds, until it beeps or signals the end of the blow.
• Read the result and note the time.
• If you want to confirm, wait at least 15 minutes and test again. A falling or rising trend tells you more than a single number.

Look after the tester between tests

Store the device at room temperature, away from a hot dashboard or a freezing glove box. Keep it away from perfume, hand sanitizer and cleaning sprays, since alcohol vapor in the air can affect the sensor. Replace or charge batteries when the maker recommends, and follow the calibration schedule in the manual. Our guide to calibrating a breathalyzer explains why that schedule matters.

Most surprising readings come from the person, not the machine. These are the errors we see described most often in maker documentation and support pages.

• Testing too soon after a drink. Mouth alcohol makes the number jump, sometimes far above the real level.
• Blowing too short or too softly. The device needs deep lung air; a quick puff measures air from the mouth and throat and reads low.
• Blowing too hard or holding your breath first. Holding your breath can raise the reading, while hyperventilating can lower it.
• Testing right after mouthwash, breath spray or certain medicines that contain alcohol.
• Using a cold or overheated device, or testing while a semiconductor unit is still warming up.
• Ignoring calibration. An overdue sensor drifts, and the error grows quietly over months.
• Comparing two devices and expecting them to match to the third decimal. Our guide on how accurate are breathalyzers explains why they rarely do.

Choosing a tester for personal checks

If you want a number you can lean on for a personal decision such as calling a ride, a fuel cell handheld is the most practical choice for most people. A keychain semiconductor unit is fine for a rough idea and for learning how your body handles alcohol. Families and parents of teen drivers usually do well with a robust handheld. Start with our ranking of the best breathalyzer options by use case, or compare fuel cell breathalyzer models side by side.