The authors measured maximum temperature rises on the side of the face after 6 minutes of continuous cell phone operation. They used two models of analogue phones (835 MHz) and three early models of GSM digital phones (900 MHz). For the GSM phones the highest recorded increase was 2.3°C, and for analogue it was 4.5°C. Further measurements with phones at inoperative mode, transmitting at full power, and in stand-by mode, revealed that most of the temperature rise was due to heat conduction from the handset, rather than due to RFR.