How breathalyzers work

Alcohol leaves the blood through the lungs

Alcohol in your blood passes into the air sacs of your lungs, where blood and air meet across a membrane a fraction of a micrometre thick. A small, nearly fixed share of it ends up in the air you breathe out. That is what a breath tester picks up: not alcohol in your mouth, which it tries to avoid, but alcohol in deep-lung air that has been in contact with blood.

Because the share is roughly constant, breath alcohol tracks blood alcohol. The ratio used in most of Europe is about 2,000 to 2,100: a litre of blood holds as much alcohol as two thousand litres of deep-lung air. That is why a blood limit of 0.5 ‰ usually comes with a breath limit near 0.25 mg per litre.

Each country fixes its own pairing

Here is the detail people miss: the breath limit is not a conversion that any device or website should do for you. It is a separate figure written into each country's law, and the pairings are not identical. Germany and France pair 0.5 ‰ in blood with 0.25 mg/l in breath. Finland pairs 0.5 ‰ with 0.22 mg/l. The Netherlands pairs 0.5 ‰ with 220 µg/l. Japan's order names 0.3 mg per ml of blood and 0.15 mg per litre of breath side by side. The United Kingdom pairs 80 mg per 100 ml of blood with 35 µg per 100 ml of breath.

Those differences are small, but at a limit they decide cases. That is also why this site's calculator works only in blood terms and never prints a computed breath value: the legal limits page shows each country's breath figure exactly as its statute words it.

Two kinds of instrument

Fuel-cell testers

Most roadside screening devices and good personal testers use a fuel cell. Alcohol in the breath sample is oxidised on a platinum electrode, which produces a tiny electric current; the more alcohol, the more current. Fuel cells are specific to alcohol and accurate at low levels, but they drift with age and need recalibration, and a cheap one left in a hot car will not stay accurate for long.

Infrared evidential instruments

The machine at the police station that produces the reading used in court is usually an infrared instrument. It shines infrared light through a chamber of breath and measures how much is absorbed at the wavelengths where ethanol absorbs. These instruments are large, calibrated against reference gases, and run a strict procedure: a waiting period so there is no alcohol left in the mouth, two samples that must agree, and an automatic check for mouth alcohol.

Semiconductor testers

The cheapest personal testers use a semiconductor sensor whose resistance changes in the presence of alcohol — and of many other things, including acetone, cigarette smoke and some cleaning products. They are useful for showing that alcohol is present. They are not reliable for the decimal places that separate legal from illegal.

Why your own tester and the police can disagree

• Mouth alcohol. A drink, a mouthwash or even a burp in the last fifteen minutes leaves alcohol in your mouth that reads far higher than your blood. Evidential procedures wait for it to clear; a quick personal test does not.
• Rising levels. If you drank in the last hour, your blood alcohol may still be climbing. A reading at the bar is not your reading thirty minutes later in the car.
• Calibration. A personal tester that has not been calibrated for a year can be off by more than the margin between two legal limits.
• Breathing. Holding your breath, breathing hard or blowing a short sample all change the result.

What a personal tester is good for

A personal tester can show you something useful: that there is still alcohol in your body when you thought there was none. It is a good tool for stopping you from driving. It is a bad tool for permitting you to drive. A reading just under the limit on a home device is not a reading under the limit on the police instrument, and in countries with a zero limit for new drivers, any reading at all is too much.

The same goes for this site's calculator. It turns your drinks and your weight into an estimate from a forensic model. That is good for understanding the scale of what you drank and how long it takes to clear. It is no substitute for a measurement, and a measurement is no substitute for not drinking before you drive.

Use any tester, and any calculator, only to decide not to drive. The only safe reading is 0.00.

Why a reading can climb after you stop

Alcohol is absorbed mainly from the small intestine, and how fast it gets there depends on what else is in your stomach. On an empty stomach most of a drink reaches the blood within thirty to sixty minutes. After a full meal the stomach empties more slowly and absorption can stretch over two hours or more. During that time the level in your blood — and therefore in your breath — keeps rising even though you have stopped drinking. A reading taken in the car park straight after the last round can be lower than the one taken at a roadside check twenty minutes later. That is one more reason a personal test is useless as a permission slip: it measures a moment on a curve that may still be going up.

Breath, blood and the law

In most of Europe a breath result from an evidential instrument is enough for an administrative offence. For a criminal case, many countries take a blood sample as well, and blood is the final word if the two disagree. The blood figure is what the BAC calculator estimates. Both follow the same curve over time — rising for the first half-hour to two hours after drinking, then falling at roughly 0.15 ‰ an hour.

In short

A breathalyzer measures alcohol in deep-lung air and the law of each country decides what breath figure matches its blood limit. Evidential instruments are careful and calibrated; personal testers vary widely and are confused by mouth alcohol. Use them, and estimates like ours, only as a reason not to drive.