Jam Cellphones, get cancer.
Jamming devices overpower the cell phone by transmitting a signal on the same frequency but at a higher power.
Less complex devices jam only one group of frequencies, while others jam all frequencies at once.
To jam a cell phone, all you need is a device that broadcasts on the correct frequencies. GSM and PCS use the 900, 1800, 1900 MHz (or 1.9 GHz) which (surprise!) is also in the Microwave area of the Electromagnetic Spectrum (See
http://en.wikipedia.org/wiki/Microwave).
Cellphones are manufactured to pass EPA regulations and are "considered safe", but Cellphone jammers are very basic devices which are not regulated or tested safe in any way and are sold to people at ever increasing power ratings.
They generate "noise" (i.e. : A Wide signal across many frequencies) to jam the signals of phones, and an RF Amplifier boosts the signal. Use it you and the people around you are awash in high levels of microwave radiation, much higher than what is found in normal background radiation and much more than from a single cellphone, wifi router, or other electronic device.
If I saw some idiot on the bus playing with this thing I would first report him to the police then sue him for endangering my life, for carrying and using a potentially dangerous, highly polluting unlicensed device for no reason except to fuck with my phone call that I am paying for.