An air treatment unit having a frame and a source of UV light, with the frame having an air guidance assembly. Air within a primary treatment volume is guided by the frame to move radially outwardly from the primary treatment volume through a dispersion angle. Air within the primary treatment volume is exposed to UV light rays from the UV light source. The air treatment unit is changeable between: a) a first state wherein a predetermined part of a volume of space within which the air treatment unit resides is strategically blocked from direct exposure to UV light rays generated by the UV light source； and b) a second state wherein at least a portion of the predetermined part of the volume of space is directly exposed to UV light rays from the UV light source.
