A filter unit is a complete system for air cleaning and the separation from it particles of different sizes. The particular unites are adjusted on the ventilation system of a building. It consists of a sequence of filters through which the air is filtered progressively. The special types and sequences of filters determine desirable number and type of particles to be restrained by the system. The system with a progressive sequence of filters which will end up to absolute filters, has the capability of 99.9995% of air filtration which has as a result the clearest air that can be achieved.