The smell sometimes observed is the result of the UV-C light breaking down Volatile Organic Compounds (VOCs) in the air. More specifically, 254nm UV-C like ours breaks down keratin-containing compounds (e.g. from dead skin and hair) in the air and on surfaces into sulfuric compounds, which is what produces the smell. Put differently, that's what "clean" smells like. Congratulations, this is officially the cleanest the air in this room has ever been!