A common myth is that breath testers can be "fooled" (that is, made to generate estimates making one's blood alcohol content appear lower) by using certain substances. An episode of the Discovery Channel's MythBusters tested substances usually connected to this practice -- including breath mints, mouthwash, and onions -- and found them to be ineffective. Adding an odor to mask the smell of alcohol might fool a person, but does not change the actual alcohol concentration in the body or on the breath. Interestingly, substances that might actually reduce the BAC reading were not tested on the show. These include a bag of activated charcoal concealed in the mouth (to absorb alcohol vapor), an oxidizing gas (such as N2O, Cl2, and O3 etc.) which would fool a fuel cell type detector, or an organic interferent to fool an infra-red absorption detector. The infra-red absorption detector is especially vulnerable to countermeasures, since it only makes measurements at particular discrete wavelengths rather than producing a continuous absorption spectrum as a laboratory instrument would do.
On the other hand, products such as mouthwash or breath spray can "fool" breath machines by significantly raising test results. Listerine, for example, contains 27% alcohol; because the breath machine will assume the alcohol is coming from alcohol in the blood diffusing into the lung rather than directly from the mouth, it will apply a "partition ratio" of 2100:1 in computing blood alcohol concentration -- resulting in a false high test reading. To counter this, officers are not supposed to administer a screening device for 15 minutes after the subject eats, vomits, or put anything in their mouth.
This was clearly illustrated in a study conducted with Listerine mouthwash on a breath machine and reported in an article entitled "Field Sobriety Testing: Intoxilyzers and Listerine Antiseptic", published in the July 1985 issue of The Police Chief (page 70). Seven individuals were tested at a police station, with readings of .00%. Each then rinsed his mouth with 20 milliliters of Listerine mouthwash for 30 seconds in accordance with directions on the label. All seven were then tested on the machine at intervals of one, three, five and ten minutes.
The results indicated an average reading of .43% blood-alcohol concentration -- indicating a level that, if accurate, approaches lethal proportions. After three minutes, the average level was still .20%, despite the absence of any alcohol in the system. Even after five minutes, the average level was .11% -- well over the legal limit.
In another study, reported in 8 (22) Drinking/Driving Law Letter 1, a scientist tested the effects of Binaca breath spray on an Intoxilyzer 5000. He performed 23 tests with subjects who sprayed their throats, and obtained readings as high as .81% -- far beyond lethal levels. The scientist also noted that the effects of the spray did not fall below detectable levels until after 18 minutes.
