One of the dirty little secrets in DUI law enforcement is that breathalyzers are susceptible to error caused by radio frequency interference (RFI), sometimes called electromagnetic interference (EMI). Put simply, any electronic device in the vicinity of the breathalyzer can emit electrical energy which can interfere with the circuitry of the machine, causing false test results. (A common example of the problem can be found in restaurants, where signs saying "Warning: Microwave in Use" alert customers to the danger of radio frequency interference with heart pacemakers.)
The police station where the tests are usually given is, of course, a veritable jungle of devices emitting electromagnetic energy — computers, cell phones, fax machines, police dispatch transmitters, teletypes, AM-FM radios, copy machines, hand-held "walkie-talkies", radar units, security cameras, microwaves, electronic locks, transmitters in police cars in the parking lot, fluorescent lighting, and so on….And in the middle of all of this sits the breathalyzer.
The problem is not a new one. In 1983, the National Bureau of Standards quietly prepared a preliminary report on tests performed on the various breath testing devices used by police agencies nationwide (Effects for the Electromagnetic Fields on Evidential Breath Testers). Each of the 16 models tested were subjected to four different frequencies typically present in the standard police environment. Of the 16 units tested, 6 showed minimal interference; 10 of the 16 showed substantial susceptibility on at least one frequency. The report characterized the potential effect of RFI on the testing of alcohol as "severe". Those conducting the study noted that the local Washington D.C. Metropolitan Police Department was complaining that breathalyzers were giving erroneous breath alcohol readings in the presence of radio transmissions.
In a field demonstration of the RFI problem for representatives of NBS and the National Highway Traffic Safety Administration, D.C. officers using a breathalyzer in a mobile van showed how handheld radios radically affected the analysis of breath samples. To avoid a loss of public confidence in breathalyzers, the report was kept confidential — until attorney Don Nichols of Minneapolis successfully filed a legal action under the Freedom of Information Act.
Manufacturers of the various breath testing machines, which had long claimed RFI was simply the invention of defense lawyers, suddenly started offering "RFI detectors" as an option on their products. Predictably, these "detectors" have proven relatively ineffective. First, as repeated tests have demonstrated, there are segments of the frequency band to which the detectors are blind. Second, the detectors are rarely calibrated correctly, if at all. This type of calibration must be done at the factory, but most law enforcement agencies are unwilling to take their machines out of service. Instead, the detector is "calibrated" by a police officer simply holding a hand-held radio next to the machine; if the detector is activated, it is considered "calibrated". Of course, this only indicates that the detector worked one time at the one frequency.
Further, the "calibration" is rarely done during an actual capture and analysis — that is, during actual operating conditions — and so the all circuits are not tested during all phases of the operation. The only real guard against false blood alcohol readings due to RFI is to require duplicate analysis — that is, running two separate tests. This does not eliminate RFI, as a constant source of electromagnetic energy can cause duplicate false results, but it reduces the likelihood. Many states now require duplicate breath tests; many others, however, still do not.