Mobile phones

ELF magnetic fields produced by mobile phones

We normally think of mobile phones as producing radiofrequency EMFs.  But mobile phones transmit in pulses of radiofrequency energy, which means that their batteries produce pulses of current to power the transmission, and these pulses of current produce ELF magnetic fields at similar frequencies to power frequencies.  The current may not be very high - an amp or so - but because the phone is held very close to the head, it can be a significant source of magnetic field exposure.

Jokela et al measured the current flowing from the battery of several mobile phones.  They found pulses of 1 - 2.7 A, lasting 0.7 - 1 ms, repeated every 4.6 ms (a frequency of 217 Hz).  They calculate this produces a maximum of 50 µT at 5 mm from the phone when it is transmitting on full power, less at larger distances or when the phone is operating at lower power.

Other workers have measured the magnetic field directly.  Kusler measured pulses up to 15 µT peak on the front surface and 35 µT on the back surface at 5 mm from the surface.  But other people have measured higher fields, and have been criticised for using instruments that weren't suitable because they suffered from interference from the radiofrequency radiation.