How does breath alcohol testing work?

Breath alcohol testing measures the alcohol concentration in exhaled breath (BrAC), which serves as a proxy for blood alcohol concentration (BAC). It is the federally approved method for DOT alcohol testing under 49 CFR Part 40 and is widely used in workplace, post-accident, and law-enforcement contexts. Results are immediate, and the detection window matches the elimination kinetics of alcohol — typically hours.

How breath testing works

Alcohol in the bloodstream diffuses across the lung's alveolar membrane into the air being exhaled. Breath alcohol devices — calibrated evidential breath testers (EBTs) — measure that exhaled concentration and apply a blood-to-breath ratio (typically 2,100:1 in U.S. devices) to express the result as BrAC, which approximates BAC.

The DOT framework

Under 49 CFR Part 40, DOT-regulated alcohol testing uses certified Breath Alcohol Technicians (BATs) and either an EBT or an Approved Screening Device (ASD). The procedural sequence is:

  1. Initial screening test.
  2. If BrAC ≥ 0.02, a 15-minute deprivation period followed by a confirmation test on an EBT.
  3. A confirmation BrAC ≥ 0.04 is the regulated threshold above which a covered employee cannot perform safety-sensitive functions.

A BrAC between 0.02 and 0.04 on a confirmation test triggers removal from safety-sensitive duty for a minimum period, but is not, on its own, a violation that triggers the formal return-to-duty process under Part 40.

Detection window

  • Alcohol on breath: ~Hours; eliminated at ~0.015–0.020 BrAC per hour after drinking ends.
  • EtG / EtS (longer alcohol detection): Done in urine, not breath — see the EtG glossary entry.

At a glance: pros and cons

Pros

  • Federally approved for alcohol testing
  • Immediate result
  • Non-invasive collection
  • Strong correlation with BAC and impairment

Limits

  • Alcohol only — does not detect drugs
  • Very short detection window
  • Requires properly calibrated devices and trained BATs

Frequently asked questions

Related reading

Test type

Blood drug testing

The gold-standard BAC reference.

 Glossary

EtG (ethyl glucuronide)

Urinary metabolite for longer alcohol windows.

 Workplace

Building a testing program

How to set up drug and alcohol testing under DOT.

Sources & references

drugtest.co content is sourced from primary regulatory and clinical references. We do not cite gray-market or "how to pass" sources.

  1. 49 CFR Part 40 — Procedures for Transportation Workplace Drug and Alcohol Testing Programs — U.S. Department of Transportation