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How does a breathalyzer work?

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Quick answer

A breathalyzer works because alcohol in the blood evaporates into the lungs' air sacs, reaching a stable ratio of about 1 part breath alcohol to 2100 parts blood alcohol. A sensor—usually a fuel cell or infrared detector—measures ethanol in a deep breath sample and converts that signal into a blood alcohol concentration estimate.

A breathalyzer estimates blood alcohol concentration by measuring ethanol vapour in a breath sample, because alcohol in the blood equilibrates with air in the lungs. Most police devices use a fuel cell or infrared sensor that converts ethanol concentration into a numeric reading.

Step by step

  1. Alcohol from a drink is absorbed into the bloodstream and carried to the lungs.
  2. In the lungs' alveoli, alcohol diffuses from blood into the air until equilibrium is reached.
  3. The person blows deeply into the breathalyzer, filling a sample chamber with alveolar air.
  4. A fuel cell or infrared sensor reacts specifically to ethanol molecules in the sample.
  5. The sensor's electrical or optical signal is processed and displayed as a blood alcohol concentration value.

Key things to understand

  1. When a person drinks alcohol, it enters the bloodstream and circulates through the lungs.
  2. In the lungs' air sacs, alcohol evaporates from blood into breathed air until the two concentrations balance.
  3. The ratio of breath alcohol to blood alcohol is roughly 1 to 2100, a widely used forensic constant.
  4. A breath sample is captured and passed into a sensor that responds specifically to ethanol molecules.
  5. The sensor's electrical or optical signal is converted into a blood alcohol concentration reading.

Labelled diagram

  1. Drink

    Alcohol is consumed and absorbed into the bloodstream through the stomach and intestines.

  2. Bloodstream

    Alcohol circulates in the blood and reaches the lungs.

  3. Alveoli

    In the lungs' air sacs, alcohol evaporates from blood into breathed air.

  4. Breath sample

    A deep breath carries alveolar air into the device's sample chamber.

  5. Sensor

    A fuel cell or infrared detector measures ethanol concentration in the sample.

  6. Reading

    The signal is converted into a blood alcohol concentration estimate.

How a breathalyzer estimates blood alcohol

Real-world examples

  • Police officers use handheld fuel-cell breathalyzers during traffic stops to screen drivers for alcohol.
  • In some countries, ignition interlock devices require a breath test before a car will start.
  • Workplace safety programs may use breathalyzers to test employees in safety-sensitive roles.
  • Hospital emergency rooms sometimes use breath alcohol tests for rapid screening of patients.

Common mistake

✗ Myth: A breathalyzer directly measures how drunk a person is.

✓ Fact: It estimates blood alcohol concentration from breath vapour, which can be affected by breathing pattern, body temperature, and recent mouth alcohol. A blood test is more direct and is often used for confirmation.

People also ask

What does a breathalyzer measure?
A breathalyzer measures the concentration of ethanol vapour in a person's breath. Because alcohol in the blood equilibrates with air in the lungs at a known ratio, that breath measurement is used to estimate blood alcohol concentration, the legal measure of intoxication.
Why is breathalyzer ratio 2100 to 1?
The 2100 to 1 ratio comes from the partition coefficient of ethanol between blood and air in the lungs' alveoli. At body temperature, alcohol partitions into air at about 1 part breath to 2100 parts blood. This constant is widely used in forensic breath testing, though actual ratios can vary slightly between individuals.
Can you fool a breathalyzer?
Common tricks like sucking on coins or using mouthwash do not lower a breathalyzer reading; mouthwash can even raise it. Some may briefly mask mouth alcohol, but deep-lung air still reflects blood alcohol. Police procedures include observation periods to prevent such attempts, and confirmatory blood tests are often used.
Why do breathalyzers need a deep breath?
A deep breath ensures the sample comes from deep in the lungs, where air has fully equilibrated with blood alcohol. Shallow mouth air can be contaminated by recent drinks or mouthwash, giving a falsely high reading.
Can mouthwash or breath mints cause a false positive?
Yes, some mouthwashes contain alcohol that lingers in the mouth for a few minutes. Most police procedures include a waiting period of about 15 minutes before testing to let mouth alcohol dissipate.
How accurate is a breathalyzer compared to a blood test?
Breath testing is less direct and can be affected by temperature, breathing pattern, and other substances. A blood test measures alcohol directly in the bloodstream and is often used to confirm breath results.
Do breathalyzers measure all types of alcohol?
Most are designed to respond mainly to ethanol, the alcohol in drinks. Some industrial alcohols or acetone from diabetes can interfere with certain sensor types, but modern fuel cells are more selective.

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