The term “alcohol” to a chemist means an organic compound that contains an OH group, but as far as the public is concerned “alcohol” refers to one specific compound, namely, ethanol. It is ethanol that we consume in wine or beer, and when we measure blood alcohol content (BAC), we’re really measuring blood ethanol content.
Breath analyzers (Breathalyzer is a brand name) contain an anode (negatively charged electrode) and a cathode (positively charged electrode). When you blow into a breathalyzer, the ethanol in your breath reacts with water from the air at the anode and is oxidized to form acetic acid (like in vinegar).
Meanwhile, at the cathode, oxygen from the atmosphere is reduced to form water. These two coupled reactions produce an electrical current between the electrodes that’s proportional to the amount of ethanol present in your breath. So, breathalyzers don’t truly measure blood alcohol content (which can only be done with a blood test) but estimate it based on the ethanol in your breath.
There are a few situations in which a breathalyzer may fail to measure BAC accurately. Notably, individuals with higher-than-normal levels of acetone in their breath may have it detected as ethanol. This could include diabetics, those on fasting diets, or those adhering to a ketogenic diet. There are a few other substances that could interfere with the chemistry of a breathalyzer, but not ones that you’re too likely to have in your bloodstream, thankfully.
