What is the effect of heat penetrating the head when a handset is used?
Biological material is weakly conductive to heat. The heating effect would be a maximum of one twentieth of a degree centigrade in 6 minutes. In practice the effect would be less, as the power is often reduced by adaptive power control, and transmissions are very rarely as long as six minutes. Human body temperature varies during the day by around one degree centigrade so most people believe there is no impact on health caused by heating. If users hold the handset against their face they may feel some warmth but this is caused by the battery in the handset being slightly warmer than room temperature, and not by radio frequency emissions.
