Abstract: System for the circulation of filtered air inside the wind turbine (1) of a quality which is free from particles of sand and dust, for the ventilation of components in the turbine's interior. The system (6) is hardly invasive, is installed at the door (3) which leads inside the tower (2) of a wind turbine (1), or, alternatively, this door (3) may be substituted by a door equipped with the system (6). The system (6) allows for regulating the air flow input, as well as for varying the efficiency of the filtration depending on the conditions demanded by the place where the wind turbine is located (1).