From the site of the Swiss Federal Office for Public Health, section on "Mobile Phones":
3. Health effects
Verified effects
Low-frequency magnetic fields
Low-frequency magnetic fields penetrate into the human body, where they induce electrical fields and currents. In the presence of very strong magnetic fields, these currents can give rise to acute nerve stimulation [1]. In order to rule out such effects, the International Commission on Non-Ionising Radiation Protection (ICNIRP) has set its recommended exposure limits for magnetic fields at a level that is lower by a factor of 50 than the threshold value for the stimulation of the central nervous system by induced currents. Under certain circumstances, the low-frequency magnetic fields generated by the electronics and the battery currents of mobile phones can exceed these recommended exposure limits [6].