Here’s an interesting blog with very valuable information about the highly dubious nature of breathalyzer tests and why, frankly, they should be inadmissable in court (and this from a guy who rarely drinks and never drinks and drives). Breathalyzer tests do not measure the presence of alcohol, they measure the presence of methyl compounds. They then multiply the concentration of such compounds in your breath by 2100, which is a calculation of how much alcohol would be in an average person’s bloodstream if it is found in such concentrations in one’s breath.
It’s not difficult to see the problems with such a measurement. First, methyl compounds are found in many other substances other than alcohol and are even produced by the body itself (especially for diabetics). Thus, exposure to or ingestion of non-alcoholic substances which contain methyl compounds can give a falsely high reading. Second, the rate at which the human body metabolizes alcohol varies widely from person to person, making the average calculation highly inaccurate in many cases. Enough so that the margin for error for a breathalyzer test, according to the manufacturers, is a full 20%. And yet this is the sole evidence used to convict drunk drivers even without any confirmation that they were driving badly or a risk to anyone. So much for “beyond a reasonable doubt.”