Why am I not allowed to use my cell phone in airplanes or hospitals?

Most of us experience electromagnetic interference on
a fairly regular basis. For example:

* If I put my cell phone down on my desk near the
computer, I can hear loud static in my computer's speakers
every time the phone and the tower handshake. In the same
way, my car's tape player produces loud static whenever I
make a call on my cell phone.
* When I dial a number on my home's wireless phone, I can
hear the number being dialed through the baby monitor.
* It is not uncommon for a truck to go by and have its CB
radio overwhelm the FM station I am listening to.
* Most of us have come across motors that cause radio or
TV static.

None of these things, technically, should be happening. For
example, a truck's CB radio is not transmitting on the FM
radio bands, so my radio should never hear CB signals.
However, all transmitters have some tendency to transmit at
lower power on harmonic side bands, and this is how the FM
radio picks up the CB. The same thing holds true for the
wireless phone crossing over to the baby monitor. In the case
of the cell phone affecting the computer's speakers, the wire
to each speaker is acting like an antenna, and it picks up
side bands in the audible range.

These are not dire problems -- they are just a nuisance. But
notice how common they are. In an airplane, the same
phenomena can cause big trouble.

An airplane contains a number of radios for a variety of
tasks. There is a radio that the pilots use to talk to ground
control and air traffic control (ATC). There is another radio
that the plane uses to disclose its position to ATC
computers. There are radar units used for guidance and
weather detection, and so on. All of these radios are
transmitting and receiving information at specific
frequencies. If someone were to turn on a cell phone, the
cell phone would transmit with a great deal of power (up to
3 watts). If it happens to create interference that overlaps
with radio frequencies the plane is using, then messages
between people or computers may be garbled. If one of the
wires in the plane has damaged shielding, there is some
possibility of the wire picking up the phone's signals just
like my computer's speakers do. That could create faulty
messages between pieces of equipment within the plane.

Many hospitals have installed wireless networks for equipment
networking. For example, look at the picture of the heart
monitor in How Emergency Rooms Work. The black antenna
sticking out of the top of the monitor connects it back to
the nursing station via a wireless network. If you use your
cell phone and it creates interference, it can disrupt the
transmissions between different pieces of equipment. That is
true even if you simply have the cell phone turned on -- the
cell phone and tower handshake with each other every couple
of minutes, and your phone sends a burst of data during each
handshake.

The prohibition on laptops and CD players during takeoff and
landing is addressing the same issue, but the concerns here
might fall into the category of "better safe than sorry." A
poorly shielded laptop could transmit a fair amount of radio
energy at its operating frequency, and this could,
theoretically, create a problem.