<P> Some natural and volatile interfering compounds do exist, however . For example, the National Highway Traffic Safety Administration (NHTSA) has found that dieters and diabetics may have acetone levels hundreds or even thousands of times higher than those in others . Acetone is one of the many substances that can be falsely identified as ethyl alcohol by some breath machines . However, fuel cell based systems are non-responsive to substances like acetone . </P> <P> Substances in the environment can also lead to false BAC readings . For example, methyl tert - butyl ether (MTBE), a common gasoline additive, has been alleged anecdotally to cause false positives in persons exposed to it . Tests have shown this to be true for older machines; however, newer machines detect this interference and compensate for it . Any number of other products found in the environment or workplace can also cause erroneous BAC results . These include compounds found in lacquer, paint remover, celluloid, gasoline, and cleaning fluids, especially ethers, alcohols, and other volatile compounds . </P> <P> Breath analyzers assume that the subject being tested has a 2100 - to - 1 partition ratio in converting alcohol measured in the breath to estimates of alcohol in the blood . If the instrument estimates the BAC, then it measures weight of alcohol to volume of breath, so it will effectively measure grams of alcohol per 2100 ml of breath given . This measure is in direct proportion to the amount of grams of alcohol to every 1 ml of blood . Therefore, there is a 2100 - to - 1 ratio of alcohol in blood to alcohol in breath . However, this assumed partition ratio varies from 1300: 1 to 3100: 1 or wider among individuals and within a given individual over time . Assuming a true (and US legal) blood - alcohol concentration of . 07%, for example, a person with a partition ratio of 1500: 1 would have a breath test reading of . 10%--over the legal limit . </P> <P> Most individuals do, in fact, have a 2100 - to - 1 partition ratio in accordance with William Henry's law, which states that when the water solution of a volatile compound is brought into equilibrium with air, there is a fixed ratio between the concentration of the compound in air and its concentration in water . This ratio is constant at a given temperature . The human body is 37 degrees Celsius on average . Breath leaves the mouth at a temperature of 34 degrees Celsius . Alcohol in the body obeys Henry's Law as it is a volatile compound and diffuses in body water . To ensure that variables such as fever and hypothermia could not be pointed out to influence the results in a way that was harmful to the accused, the instrument is calibrated at a ratio of 2100: 1, underestimating by 9 percent . In order for a person running a fever to significantly overestimate, he would have to have a fever that would likely see the subject in the hospital rather than driving in the first place . Studies suggest that about 1.8% of the population have a partition ratio below 2100: 1 . Thus, a machine using a 2100 - to - 1 ratio could actually overestimate the BAC . As much as 14% of the population has a partition ratio above 2100, thus causing the machine to under - report the BAC . Further, the assumption that the test subject's partition ratio will be average--that there will be 2100 parts in the blood for every part in the breath--means that accurate analysis of a given individual's blood alcohol by measuring breath alcohol is difficult, as the ratio varies considerably . </P>

What volume of breath does a breathalyzer collect