The essence of the idea of giving bactericidal and fungicidal properties to ducts of ventilation systems made of tubular elements, inner surface of which are covered with a thin film of a metal oxide, preferably silver oxide or titanium oxide or zinc oxide with antiseptic properties according to the invention consists in that the air drawn in at the inlet to a ventilation system duct (1) is exposed to ozone-generating UV radiation generated by a strong source (3) emitting waves with wavelength below 240 nm, and in further part of the duct along the whole length of the ventilation system the air is exposed to UV radiation from a weaker source (7) generating such radiation with wavelength above 240 nm and arranged preferably at equal distance L 2 from each other, or the air present in the ventilation system duct (1) is exposed only to UV radiation generated by weaker sources (7) emitting radiation with wavelength above 240 nm that are also arranged preferably at equal distance L 4 from each other along the whole length of the system, or the air present in the ventilation system duct (1) is exposed only to UV radiation generated by uniform distribution units, with each of such units comprising a LED (14) and a light diverging lens (15) and generating radiation with wavelength preferably above 240 nm and also arranged preferably at equal distance L 4 from each other along the whole length of the ventilation system.
