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Those familiar with the fundamental concepts of quantum theory will remember our old friend Planck's law, which describes the essential concept of the quanta of electromagnetic energy. Planck's law states that the quantum of energy, E, associated with an electromagnetic field is given by E = hv, where h is Planck's constant and v is the frequency of the electromagnetic radiation. If you express Planck's constant in joule-seconds and the frequency in hertz, the energy units fall out in joules.

Anything in the frequency of x-rays and above produces ionization directly through absorption or scattering of the incident photon, which is harmful, in sufficient quantities, to biological systems, because of the inhibiting of essential biochemical reactions, and the breakdown of long-chain molecules. Direct ionization requires incident energy quanta sufficient to remove electrons from the atoms they are bound to. Below a given threshold, ionization will not occur, but thermal effects, as noted in earlier posts, can be significant if field strengths are high enough. RF can produce ionization (like in the ion bottles of accelerators) but that is essentially a thermal effect. The free electrons are accelerated to high velocities by the oscillating field and collisions eventually strip other electrons from their associated atoms.