Have you ever noticed those "Warning: Microwave in Use" signs in restaurants? It’s for folks who have heart pacemakers: There is a risk that the electromagnetic interference (EMI) from the microwave will interfere with the electronic circuitry in the customer’s pacemaker and cause it to malfunction. This phenomenon, often called radio frequency interference (RFI), can be a recurring problem with any instrument containing electronic circuitry.
Now try to think of some place in your neighborhood that is chock full of electronic gizmos constantly transmitting RFI 24 hours a day. How about a police station? Powerful dispatch radio transmitters, radio transmitters in squad cars in the parking lot, walkie-talkies in every officer’s belt, cell phones, computer cathode ray tubes, microwave relays, electronic door locks, microwave ovens, fluorescent lighting — a veritable jungle of RFI. Now let’s put a Breathalyzer smack in the middle of this police station. An instrument filled with sensitive electronic circuitry that has to analyze tiny amounts of alcohol in breath to an accuracy of one tenth of a percent…..
Just a theory of some DUI defense attorney? Consider a report from the National Bureau of Standards, under contract with the National Highway Traffic Safety Administration to conduct accuracy testing on breath machines (referred to in the report as "Evidential Breath Testing" devices, or "EBTs"):
The Washington D.C. Metropolitan Police Department reported to NHTSA that EBTs were found to display erroneous BAC [blood-alcohol content] readings in the presence of electromagnetic fields from radio transmission…. Representatives of NHTSA and NBS were given a demonstration by police officers who routinely conduct breath testing using an EBT in a mobile van. One police officer operated his handheld radio within 1 foot of the EBT and demonstrated that the electromagnetic field could severely affect the analysis of alcohol samples.
The National Bureau of Standards subsequently conducted the testing and subsequently reported that
These results show that EMI is a potential problem with many of the EBT units currently in use….The states may have to take interim measures to determine the extent of their individual problems with EMI affecting EBTs.
The reaction by the federal government to this report was, perhaps, predictable. Afraid that it would undermine public confidence in law enforcement methods, the government classified the document and then buried it. However, it was later resurrected by a Minneapolis DUI law firm’s Freedom of Information Act lawsuit. Most manufacturers of breath machines today quietly offer an "RFI detector" as an option in their products. Unfortunately, these "detectors" are unreliable and, in any event, are rarely purchased by law enforcement agencies.
Note: all 50 states now make it a crime to drive with a blood-alcohol level of .08% or higher. In most cases, the only evidence of this comes from the breath machine. The breath cannot be re-analyzed. The machine cannot be cross-examined.