How do alcohol breath tests work?

There’s no way around it: drinking and driving don’t mix. But there has to be a way to determine one’s level of sobriety, often fixed by law at a certain amount of alcohol in the blood. That’s why anyone so childish as to ignore common sense will be pulled over by the police and asked to play blow up the balloon.

The first breath tests, introduced in the late 1930s and in common use by the 60s, actually used balloon-like bags containing chemicals that were turned into vinegar by breath alcohol. If what was left in the bag after you breathed into it could go on a salad, you were in trouble. More recently they have you breathe into an electronic device in which alcohol on your breath fuels an electric current. If you are sufficiently lit up, the machine will be, too.

Source: How in the World? A Fascinating Journey through the World of Human Ingenuity by Reader’s Digest