Wireless/microwaves disrupt pacemakers/aircraft how?

August 13, 2008 10:26 AM

Pilots and medical personnel: Exactly why are cell phones and wireless devices prohibited at takeoff/landings and in ICUs (etc.). Are there known consequences on certain equipment? And why, precisely, are microwaves bad for pacemaker wearers? I'm interested in knowing exactly what mechanisms are/could be affected, how and why.

This question was spurred by the pacemaker remote hacking thread.

posted by spock to science & nature (15 comments total)

A pilot's answer.
posted by nitsuj at 10:30 AM on August 13, 2008

The only interactions I have ever witnessed between my GSM cell phone and any radios on an aircraft was buzzing in the speakers in my headset, but that's a problem that GSM causes on most speakers in airplanes or not. So, the danger there would be being unable to hear commands from air traffic control or other radio communications.

Really, the cell phone ban is a bit of an overreaction, I think. The real trick would be getting reception at altitude - antennae on cell towers are pointed down (or horizontally, at least) and are not designed to pick up signals from above. Add to that the speed with which you are traveling (and hopping from one tower to another) and I can't imagine getting a decent signal would be easy.
posted by backseatpilot at 10:33 AM on August 13, 2008

Cell phones and wireless devices have the potential to interfere with communication equipment on a plane although it's never been proven that they actually do anything. It's a ‘better safe than sorry’ mentality that keeps you from using cell phones during the entire flight. I, personally, have made cell calls from inside helicopters (military only, so shielding standards may be different) and there's not a pilot at my command that hasn't checked their blackberry during a long ferry flight. No interference or issues have ever been reported.

The whole thing where they make you turn off everything, including stuff that doesn't transmit, during takeoff and landing is about emergency readiness. Those are the most likely times for some kind of emergency that might require instructions over the PA or a crash that would require evacuation and they want you to not be rocking out to your ipod or working on your laptop if that happens.
posted by macfly at 10:37 AM on August 13, 2008 [1 favorite has favorites]

EMI == Electromagnetic Interference. All wires are antenna. If a transmitter is too close to certain kinds of equipment, it can induce currents in wires in that equipment. It's highly unlikely to destroy the equipment but it could cause it to malfunction.

Those rules were put in place during the early years of cell phones, back when AMPS phones were permitted to transmit at a power level of 2 watts. Nowadays, the FCC rule is that the max is 200 milliwatts average but for GSM that means max 1 watt at a 20% duty cycle. That can definitely induce parasitic current flow in nearby equipment. (CDMA phones, IS-95 or CDMA2K or UMTS, never transmit more than 200 milliwatts.)

Where that's going to be the worst problem is cases where the equipment operates at extremely low power levels or uses big amplifiers. In the case of a pacemaker the danger is that the EMI could make the circuitry get "confused". It wouldn't blow any FETs, but it could prevent it from operating correctly.

This is the same kind of problem as is suffered by people who post here periodically asking what they can do about the noise in their amplified computer speakers caused by their GSM cell phones.

WIth many kinds of medical equipment, the problem is amplifiers. For instance, a heart monitor is relying on sensors taped to your skin which are picking up very minute electrical changes, which are then amplified and displayed on a scope. The wires going to the sensors are good antennas, and the induced EMI from a cell phone could swamp the signal from the patient's heart and render the heart monitor useless.

Re Microwave ovens and pacemakers: Where cell phones limit at about 1 watt, as you know a microwave oven often is more than a thousand watts. The window on the front has a screen, which is a diffraction grating that stops nearly all leakage of microwaves. What little gets out is safe for normal people, but it could be enough to disrupt activity of a pacemaker.

Your question implies that you think that microwaves are somehow special in this regard. They aren't. Any kind of RF can do these things. If I had a pacemaker I wouldn't want to own a CB radio, either. But for a small, low-current device like a pacemaker microwaves are more of a concern than a long wavelength device like a CB. (An ideal antenna is half a wavelength. Half a wavelength of CB is five and a half meters. Half a wavelength of 1900 MHz cellular is seven and a half centimeters. So the wires inside a pacemaker will pick up much more EMI from 1 watt of cellular than from 20 watts of CB.)

Is any of this inevitable, or even likely? No, in fact, but it could happen, and why take the chance? The precaution is only a minor inconvenience, and the consequences of EMI could be catastrophic.
posted by Class Goat at 10:47 AM on August 13, 2008 [2 favorites has favorites]

For what it's worth, cell phone are not banned in ICU's, at least any that have updated their policies in the past several years. I have been at the same hospital since the late 1980's and I don't think we have ever banned cell phones. When they first came out I did see signs banning them in other hospitals based on the fear of potential interference with monitoring equipment, but as they became more common and it became obvious it was not a problem every hospital I have seen in recent years has allowed cell phones in ICU's. Cell phones are actually the preferred method of communication in the OR where I work because you don't have to wait for someone to return a page, and we have even more electronic equipment than an ICU. Even back in 1996 ECRI was relaxing its recommendations on cell phone prohibition in hospitals.
posted by TedW at 11:22 AM on August 13, 2008

Mythbusters did a bit about this, and tried testing different phones to see if they could get any sort of a response in avionics. The answer was basically "maybe." To the best of everyone's knowledge, no plan has crashed because someone was ordering takeout. However, since new phones are rolled out daily, prudence has carried the day and they've gone the safe route of prohibiting them altogether.
posted by craven_morhead at 11:23 AM on August 13, 2008

As you can see, the available information is inconclusive, but the vast majority of aircraft accidents happen during take-off and landing, so banning the use of electronic devices during take-off and landing seems like sensible risk management until we know more about the issue. Especially as fly-by-wire....heck...everything-by-wire becomes the norm. Is it REALLY such a bother to have to put away your toys for the first and last 15 minutes of the flight?
posted by randomstriker at 11:49 AM on August 13, 2008

The question had nothing to do with whether anything was a bother or not.
posted by spock at 11:59 AM on August 13, 2008 [1 favorite has favorites]

For what it's worth, cell phone are not banned in ICU's, at least any that have updated their policies in the past several years.

The ICU my dad was in 18 months ago banned cell phones. When he later got moved onto the cardiac unit, they were still supposed to be turned off. A nurse came into his room one day to ask if we had forgotten a phone, because his heart monitor readings were coming through all funny at the nurse's station. She was right, we had left a cellphone on. She said it happens fairly frequently - the phone doesn't mess up his heartbeat or anything, it just messes up the signal that they use to monitor it from a distance. On the other hand, we could use the wireless internet in his room with no problem, and I have no idea why that's considered ok.
posted by vytae at 2:05 PM on August 13, 2008

On the other hand, we could use the wireless internet in his room with no problem, and I have no idea why that's considered ok.
posted by vytae at 5:05 PM on August 13 [+] [!]

Different parts of the spectrum. If their monitors work in the GSM spectrum, cell phones stand a chance of interfering, while WiFi does not.

I used to have a cordless phone (base attached to a normal land line), which continually picked up the signal from a neighbor's baby room monitor. Presumably, it worked the other way, too... Now imagine that, instead of being two voice-lines, the baby monitor was reading medical data from the monitors on the patient, and you can see why it would be a problem.

Better shielding of the medical equipment is the answer, but it wasn't necessary 20 years ago when much of the equipment was made.

However, I think the avionics fear is just that - fear. A momentary ripple in a fuel sensor isn't going to cause a pilot to make any startling decisions.
posted by IAmBroom at 2:28 PM on August 13, 2008

Cell phones banned in all British Hospitals I think?
posted by A189Nut at 2:32 PM on August 13, 2008

The whole thing where they make you turn off everything, including stuff that doesn't transmit, during takeoff and landing is about emergency readiness. Those are the most likely times for some kind of emergency that might require instructions over the PA or a crash that would require evacuation and they want you to not be rocking out to your ipod or working on your laptop if that happens.

They didn't in the past, though. I remember being a young girl and listening to my Walkman during takeoff on a small propeller plane -- no one cared. The blanket prohibition surprised me when it came along.
posted by Countess Elena at 5:24 PM on August 13, 2008

I worked on an experiment that sat in a largish constant magnetic field (10 Gauss, twenty times the earth's field). We had to put a "no pacemakers" sign on the door, showing the boundary of the high-field region. One of the safety inspectors commented that the rule was a little silly, since anyone with a pacemaker affected by such a field has either (a) gotten a new pacemaker in the past ten years or (b) died. Another datum that rules may be in flux in different places.

You have some nice answers above about why microwave frequency fields are different.
posted by fantabulous timewaster at 5:32 PM on August 13, 2008

The window on the front has a screen, which is a diffraction grating that stops nearly all leakage of microwaves.

pedantfilter: the screen on the door of a microwave oven is not a diffraction grating, it's just a metal screen with holes in it. the wavelength of microwave-oven radiation is about 12cm, and a diffraction grating for that kind of radiation would have to have similarly-sized structure.

the inside of a microwave is all metal, including the door, which makes it act as a faraday cage and shields the outside from any fields inside. if the holes in the door are much smaller than the wavelength of the radiation inside (they are) then the perforated door will behave just like an unperforated sheet of metal for the purposes of microwave shielding.
posted by sergeant sandwich at 5:46 PM on August 13, 2008

In my old ICU, we banned cell phones for the patients and their families, but nearly every staff member had a cell phone in their pocket, and the doctors were frequently talking on theirs. My personal theory is that people on cell phones are annoying. I think we kick them outside the unit so they don't stress out the patients (or the staff for that matter).
posted by nursegracer at 7:53 PM on August 13, 2008