Breathalyzer Test: What to Expect and How They Work

Alex Harmon · Updated June 2026 · Reviewed for accuracy

If you're ever pulled over on suspicion of drunk driving, a breathalyzer test is likely the first tool law enforcement will use. Understanding how it works — and where it can fail — is essential knowledge whether you're a driver, a legal professional, or simply curious about the science.

How a Breathalyzer Measures BAC

Breath alcohol testing is grounded in a principle of physical chemistry called Henry's Law: at a given temperature, the ratio of a volatile substance in breath to its concentration in blood is predictable and consistent. For alcohol, that relationship is the basis of every breathalyzer on the road.

The key number is the partition ratio — the assumption that 2,100 mL of alveolar (deep lung) air contains the same amount of alcohol as 1 mL of blood. This 2100:1 ratio is built into every breathalyzer's calculation. When the device measures alcohol in your breath, it multiplies that figure to estimate blood alcohol concentration.

There are two main technologies used in breathalyzers:

• Fuel cell breathalyzers (law enforcement grade): use electrochemical oxidation of ethanol. Alcohol reacts at a platinum electrode to produce an electric current proportional to the alcohol concentration — highly accurate and specific to ethanol.
• Semiconductor breathalyzers (consumer/roadside screening): cheaper and more portable, but less chemically specific. Can be triggered by acetone, paint fumes, certain foods, and other volatile compounds, making them less reliable for evidential purposes.

Results are expressed as grams of alcohol per 210 liters of breath — mathematically equivalent to grams per deciliter of blood for legal BAC purposes.

The Two-Stage Testing Process

A DUI stop typically involves two separate breathalyzer tests, each with different legal standing.

• Preliminary Alcohol Screening (PAS): the handheld roadside test performed at the scene. In most US states, PAS results alone are not admissible as evidence in court — they are used to establish probable cause for arrest and to justify transporting you for an evidential test.
• Evidential Breathalyzer (e.g., Intoxilyzer 9000, Datamaster): performed at the police station or in a dedicated testing vehicle. Two independent breath samples must agree within 0.020% of each other. This result is admissible in court and forms the core of most DUI prosecutions.

Before the evidential test, the officer is required to observe you for a deprivation period of 15–20 minutes. The purpose is to ensure no mouth alcohol — from a recent sip of alcohol, burping, vomiting, or even mouthwash — contaminates the sample. Failure to observe this period is one of the most common grounds for challenging results.

Sources of Error and False Positives

Breathalyzers are reliable tools, but they are not infallible. Several factors can produce readings that don't accurately reflect true BAC.

• Mouth alcohol: residual alcohol in the mouth from a recent drink, mouthwash, burping, or acid reflux can dramatically inflate readings. This is why the deprivation period exists — but if it wasn't properly observed, the result may be compromised.
• Partition ratio variability: the assumed 2100:1 ratio varies in the real population from approximately 1700:1 to 2400:1. A person with a lower ratio will produce a breath reading higher than their actual BAC warrants — potentially crossing a legal threshold when their true BAC is below it.
• Ketosis and diabetes: fat metabolism produces acetone, a ketone that some older semiconductor devices can misidentify as ethanol. Modern fuel cell machines are largely specific to ethanol, but this remains a documented source of error with certain equipment.
• Fever: elevated body temperature increases the volatility of alcohol from blood into breath. Each 1°C of fever above normal can increase the breath alcohol reading by approximately 8%, even if true BAC is unchanged.
• Radio frequency interference (RFI): police radios and other radio transmitters near older breathalyzer models have caused documented measurement errors. Modern devices include RFI filters, but older equipment may still be vulnerable.
• Calibration failure: evidential breathalyzers must be regularly calibrated against a known standard. A device that hasn't been properly maintained will produce unreliable results — but calibration records are legally obtainable through discovery.

Your Legal Rights During a Breathalyzer Stop

Knowing your rights before you're in the situation is far more useful than trying to recall them under pressure.

• Right to remain silent: you are not required to answer questions about where you've been, whether you've been drinking, or how much. Politely declining to answer is within your rights.
• Implied consent: in every US state, by holding a driver's license and operating a vehicle, you have legally pre-consented to chemical testing. Refusing a breathalyzer test carries automatic consequences — typically license suspension ranging from 6 months to 3 years depending on the state, and refusal can be introduced as evidence of consciousness of guilt in court.
• Right to request a blood test: in most states, you can request a blood draw as an alternative to breath testing. Blood tests are more accurate, harder to challenge on partition ratio grounds, and the sample can be independently tested by your own expert.
• Right to independent testing: after submitting to police testing, you have the right to request an independent chemical test at your own expense. This right must be facilitated by law enforcement — interfering with it can result in evidence suppression.
• Right to challenge the test: evidential breathalyzer results are not unassailable. Through the discovery process, you or your attorney can obtain calibration records, maintenance logs, and the officer's certification — all of which are grounds for challenge if deficiencies exist.

Challenging a Breathalyzer Result

DUI attorneys regularly challenge breathalyzer results, and borderline cases (readings of 0.08%–0.10%) are frequently contested successfully. The most common grounds include:

• Calibration records: if the device was not calibrated on schedule, or calibration records are incomplete or missing, results may be suppressed.
• Operator certification: the administering officer must be trained and certified on the specific device used. Lack of certification can invalidate results.
• Partition ratio defense: if a blood test shows a significantly different result from the breath test, an expert can argue that the defendant's individual partition ratio differs from the assumed 2100:1 standard.
• Mouth alcohol defense: if the deprivation period was not properly observed — or if the defendant had a recent belch, used mouthwash, or has acid reflux — a defense expert can argue mouth alcohol contaminated the sample.
• Rising BAC defense: alcohol continues to absorb into the bloodstream for 30–90 minutes after your last drink. If you drank shortly before driving, your BAC may have been below the legal limit while you were driving but above it by the time you were tested — potentially 30–60 minutes later.
• Medical conditions: documented conditions that cause elevated breath acetone (Type 1 diabetes, ketogenic diet, prolonged fasting) may warrant expert testimony challenging the specificity of the device used.

Breathalyzer Accuracy in Practice

When properly calibrated and administered by a certified operator on a modern fuel cell device, evidential breathalyzers are accurate to approximately ±0.005% BAC. That is a reasonably tight margin for a field instrument.

However, systematic biases — especially partition ratio variability — mean that a population-level comparison of breath readings to blood draws shows divergence of 15–23%. Courts accept breathalyzer results as reliable evidence under the general assumption that these devices are scientifically sound, and they are — but that doesn't mean every individual result is accurate.

If your reading is close to the legal limit, the smartest move is to request a blood test. Blood draws are the gold standard — more accurate, independently testable, and much harder to challenge on device or operator grounds.

Frequently Asked Questions