When the user exhales into a breath analyzer, any ethanol present in their breath is oxidized to acetic acid at the anode:

CH₃CH₂OH(g) + H₂O(l) → CH₃CO₂H(l) + 4H⁺(aq) + 4e^-

At the cathode, atmospheric oxygen is reduced:

O₂(g) + 4H⁺(aq) + 4e^- → 2H₂O(l)

The overall reaction is the oxidation of ethanol to acetic acid and water.

CH₃CH₂OH(l) + O₂(g) → CH₃COOH(l) + H₂O(l)

The electrical current produced by this reaction is measured by a microprocessor, and displayed as an approximation of overall blood alcohol content