How Does a Breathalyzer Work and What Does It Measure? | Breath, Not Blood

A breathalyzer estimates blood alcohol concentration by measuring alcohol in deep-lung breath — a value called BrAC — and never takes a blood sample.

One firm breath into a handheld device can gate a car’s ignition, but the number it prints is an estimate, not a verdict on sobriety. The device captures deep-lung air, senses the alcohol vapor in it, and converts that reading into breath alcohol concentration (BrAC) and a blood-alcohol equivalent. It measures what you exhale; regulators and courts treat that number as a proxy for what is in your bloodstream.

What Does a Breathalyzer Actually Measure?

A breathalyzer measures alcohol in exhaled breath, reported as BrAC; the displayed number is an estimate of BAC derived from that sample. It does not measure impairment, reaction time, or driving ability. Because alcohol moves between blood and air in the lungs, deep-lung air tracks circulating blood alcohol, and the device applies a conversion factor. The CDC frames BAC as the measure of alcohol in the bloodstream, and law enforcement treats the breath reading as its practical stand-in.

• BrAC — alcohol concentration in breath, what the sensor literally reads.
• BAC — alcohol concentration in blood, stated in the US as grams per deciliter (g/dL).
• 0.08 g/dL — the per se impaired-driving threshold for ordinary adult drivers under US law, with state variations and lower limits for some drivers.

How the Sensor Turns Breath Into a Number

Fuel cells and semiconductors both react to alcohol and produce an electrical signal converted into a BrAC value, but the chemistry differs. A fuel-cell sensor oxidizes alcohol at an electrode, generating current proportional to alcohol present; a semiconductor sensor uses a solid-state material that changes electrical properties in the presence of alcohol vapor. NHTSA identifies both as technologies used in ignition interlocks. Evidential instruments used by law enforcement carry more sophisticated measurement and verification than consumer screening devices — one reason a pocket unit’s reading is never legal authority. For consumer units, ratings, sensors, and accuracy claims vary wildly, which is why a roundup of the best at home breathalyzer options beats guessing from spec sheets.

How an Ignition Interlock Runs a Test

An ignition interlock combines a handheld breath unit with a control relay wired into the ignition, tamper detection, and data logging, permitting engine start only when the sample falls below its programmed threshold. The sequence:

1. Turn the ignition on or follow the start prompt, then wait for the ready signal.
2. Fit a clean mouthpiece, take a deep breath, and blow firmly and steadily.
3. The sensor analyzes the sample against the state or court threshold.
4. Below threshold, the engine starts; at or above it, the vehicle stays locked out.
5. After pulling away, provide prompted rolling retests — NHTSA describes these commonly every 20–30 minutes, and a Draeger interlock document spells out the same handheld-plus-control-unit design.
6. Pull over safely before any retest. An interlock is not built to shut off a running engine; a missed or failed test triggers warnings such as an alarm or flashing lights.

Interlock thresholds sit low on purpose. NHTSA describes a common starting point around 0.02 g/dL, with model-program thresholds of 0.025 g/dL or higher and typical set points of 0.020 or 0.025 depending on the program. That is a vehicle-starting condition and a cushion for measurement uncertainty — not a statement that 0.02 is the general legal driving limit.

Why a Reading Can Be Wrong

Residual mouth alcohol and a shallow breath sample are the two most common reasons a reading is unreliable — both distort the sample before the sensor does its job. Testing right after drinking, eating, vomiting, or using alcohol-containing mouthwash can push the number artificially high; blowing too weakly or failing to reach deep-lung air can produce an invalid test. Tampering with, blocking, or bypassing an interlock can be logged as a violation, since the unit records data. The CDC’s impaired-driving guidance notes that alcohol affects people differently — body composition, food, medications, and elapsed time all shift the relationship. A personal breathalyzer is a screening tool, not a green light to drive; a low reading proves neither sobriety nor legal permission.