> The only mechanism besides ionization that could harm humans [is heat]
This claim is, IMO, too strong given available evidence.
There are many chemical interactions with characteristic energies well below 1eV, or any reasonable threshold for "ionizing". Photons can couple with these interactions without ionizing anything. 4GHz range is probably fine, because the per photon energy is a small fraction of a mEv, but even then I would not rule out the possibility of multiple photons coupling to a structure without the imparted energy immediately being dispersed as heat.
Any time you have EM with low entropy/etendue, it is always theoretically possible for interactions to occur outside of the thermal regime.
This claim is, IMO, too strong given available evidence.
There are many chemical interactions with characteristic energies well below 1eV, or any reasonable threshold for "ionizing". Photons can couple with these interactions without ionizing anything. 4GHz range is probably fine, because the per photon energy is a small fraction of a mEv, but even then I would not rule out the possibility of multiple photons coupling to a structure without the imparted energy immediately being dispersed as heat.
Any time you have EM with low entropy/etendue, it is always theoretically possible for interactions to occur outside of the thermal regime.