You have to blow into a breathalyzer for roughly four to ten seconds of steady, continuous exhalation, though the exact time depends on the device and your lung capacity. The machine isn’t counting seconds. It’s waiting for enough deep lung air to measure, and most evidentiary devices require a minimum sample volume of about 1.1 to 1.5 liters before they’ll accept the reading. Blow until the device tells you to stop.
Why the Machine Needs That Long
A breathalyzer doesn’t analyze whatever air comes out of your mouth first. It needs deep lung air, because alcohol concentration in that air most closely mirrors what’s in your blood. The first air out of your mouth is shallow and can be contaminated by whatever’s sitting in your throat or mouth. To get past that and reach the sample the device actually wants, you have to keep blowing.
The operator will typically tell you to take a deep breath and exhale steadily without stopping. Four seconds is on the short end. Ten is on the longer end. Somewhere in that range, the machine will beep or click to signal it has enough.
What Happens If You Stop Too Soon
If you stop early, blow too softly, or lose the seal around the mouthpiece, the device rejects the sample. You’ll be asked to try again. A complete evidentiary breath test generally requires two valid samples taken a couple of minutes apart, and if the two readings don’t correlate closely enough, the operator may run additional tests or switch to a blood draw.
Repeated incomplete samples can also be treated as a refusal, which carries its own penalties. So a half-hearted blow isn’t a middle path. It can end up in the same category as saying no.
Roadside Handheld vs. Station Machine
The blow procedure is similar for both, but the two tests aren’t the same in the eyes of the law. The handheld device an officer uses during a traffic stop is a preliminary alcohol screening tool, sometimes called a portable breath test or PBT. It helps the officer decide whether there’s probable cause to arrest you, and the PBT result alone typically isn’t admissible in court to prove intoxication.
The evidentiary breath test is the one that matters at trial. It’s administered at the station or in a mobile unit using a larger, calibrated machine like an Intoxilyzer or Draeger, operated by a certified technician. When people picture “the breathalyzer” in a legal sense, this is it.
The Wait Before You Blow
Before the evidentiary test, the operator is generally required to observe you for a waiting period, typically 15 to 20 minutes. Federal workplace alcohol testing protocols require a minimum 15-minute wait before a confirmation test, and state DUI protocols follow a similar standard, though exact times vary by jurisdiction.1U.S. Department of Transportation. Back to Basics for Breath Alcohol Technicians/Screening Test Technicians
The point is to let any residual alcohol in your mouth (from a recent drink, a belch, or reflux) dissipate so the machine measures deep lung air rather than a concentrated pocket of vapor. During the wait, you can’t eat, drink, smoke, or put anything in your mouth. If you burp noticeably or vomit, the clock usually restarts.
What Can Skew a Valid Blow
Even a technically correct blow can produce an unreliable reading. The most common causes:
- Gastroesophageal reflux, which pushes stomach contents back up into the throat where the device can pick them up as if they were deep lung air.
- Diabetes and low-carb diets, which produce acetone; some breathalyzer models, particularly older fuel cell sensors, can mistake acetone for ethanol.
- Mouthwash, cough syrups, and oral sprays that contain alcohol. The observation period is meant to catch this, but doesn’t always eliminate the problem.
- Environmental contamination from paint fumes, cleaning solvents, or other volatile compounds.
- Temperature extremes and missed calibration. Defense attorneys frequently challenge results on calibration records alone.
None of these guarantees a false reading. They create the kind of doubt a defense attorney can use to challenge one.
When You Physically Can’t Blow Long Enough
Some people genuinely can’t produce enough air to satisfy the machine. Asthma, COPD, emphysema, advanced age, and a history of heavy smoking can all make it impossible to hit the minimum volume. Research has documented cases where people made genuine attempts to blow but were still charged with refusal because they couldn’t meet the threshold.2PubMed. Small Samples, Big Problems – The Inability to Provide a Breath Sample
If you have a respiratory condition, tell the officer before the test starts and ask for a blood draw instead. The officer has discretion to allow it, and medical records go a long way if the issue ends up in court. Without documentation, “I couldn’t blow hard enough” looks identical from the officer’s chair to “I didn’t want to.”
What Happens If You Refuse to Blow
Every state has an implied consent law. By driving on public roads, you’ve already agreed to submit to chemical testing if an officer has lawful grounds to suspect impaired driving.3NHTSA. BAC Test Refusal Penalties Consent kicks in the moment you’re lawfully arrested on suspicion of DUI.
Refusing doesn’t make a DUI go away. All states except one have separate penalties for test refusal, and in at least a dozen states, refusal itself is a criminal offense. The most common consequence is administrative license suspension through the DMV, separate from anything the criminal court does. First-refusal suspensions typically run six months to a year, with longer terms for repeat refusals. In many jurisdictions, the suspension for refusing is longer than the one you’d face for failing.
Your refusal can also be introduced as evidence at trial. Prosecutors argue, and juries often accept, that refusal suggests you knew you were over the limit. The case then gets built with field sobriety results, officer observations, dashcam footage, and the refusal itself.
One narrow boundary worth knowing: the mechanics above apply to breath testing. Blood testing sits in a different legal category, and the Supreme Court has held that police generally need a warrant or actual consent for a blood draw.4Justia Law. Birchfield v. North Dakota For the breath test at the station, no warrant is required.