Breath alcohol metering
PPL Hobbyist -- I believe that gas spectrometry is the usual method, not a 'fuel cell'. Breath alc is not the same as blood alc; there is an accepted equation linking the two. In the early days of road testing the law in the UK used to state blood alcohol now driving with an excess of breath alcohol is also an offence.
Nobody is suggesting that the level of alcohol is an accurate measure of incapacity but it is a perfectly sensible and practicable way to test for those who shouldn't be flying or driving. So any correlation is academic. Zero is, of course, impracticable as the body produces ethanol. Nobody is 100% free of ethanol.