Digging some college Chemistry out of the back of my brain.. 2.4 GHz is the resonant frequency of the water molecule, meaning 100% of the energy is absorbed and converted to heat.
Put anything containing water into a microwave, and it heats up. Put anything without water, like paper, in a microwave, and nothing happens.
Other molecules have their own resonant frequencies. I remember reading an article about a pharma company using radio waves around 200 MHz to interfere with brain cancer growth. That 200 MHz frequency was just right to heat up some protien the cancer needed and prevent it from participating in the next reaction.
The vast majority of radio waves seem harmless, and too weak to cause much heating, but that makes you wonder.
