How Does a Breathalyser Work?

When alcohol is in the bloodstream, a fraction evaporates into the lungs' alveoli and is exhaled. Breathalysers exploit this: fuel-cell models pass breath over a platinum electrode where alcohol undergoes oxidation, generating an electric current proportional to the alcohol concentration. Infrared spectroscopy models shine infrared light through a breath sample — ethanol molecules absorb specific wavelengths, and the absorption level indicates concentration. The blood-to-breath alcohol ratio is approximately 2,100:1, meaning 2,100 mL of alveolar air contains roughly the same amount of alcohol as 1 mL of blood. Evidential-grade instruments used by law enforcement are accurate to within ±5%.