Ionizing photons and particles — whether delivered to the body or engineered out of the occupied space. The atlas already splits the electromagnetic spectrum by interaction mechanism (Photonic for photoacceptor chemistry, Electromagnetic for field effects), so a third split for ionization follows the same logic rather than breaking it. The family opens with a delivered row — radon therapy, the one modality in the whole atlas whose dose is actually written down, and what is written down is a dose of a Group 1 carcinogen — and a subtractive one, radiation shielding.