Don’t be mislead by a common misconception started in the 90′s that wired headsets, that the headset that came with your cell phone is a safe alternative to placing a phone to your head. This is simply not true at all! Ordinary headsets use a wire to deliver sound to an electronic earpiece that can deliver electromagnetic radiation into your head directly through your ear canal.
The energy of electromagnetic radiation is determined by its frequency; ionizing radiation is high frequency, and therefore high energy, whereas non-ionizing radiation is low frequency, and therefore low energy. The NCI fact sheet Electromagnetic Fields and Cancer lists sources of radiofrequency radiation. More information about ionizing radiation can be found on the Radiation page.