A lateral aerator (2; 12) arranged or intended to be arranged in a longitudinal wall (1a; 11a) of a ventilation duct (1; 11) along which, in use, air flows, the aerator (2; 12) comprising at least three outlet openings (5, 5'; 15, 15') aligned with respect to an axis (Δ) of the duct, in the mean direction of the air flow inside the ventilation duct (1; 11), and each formed by a slot (3, 3'; 13, 13') and a fixed fin (4, 4'; 14, 14'), the slots (3, 3'; 13, 13') being part of a cylindrical or planar surface which is parallel or intended to be parallel to said longitudinal wall (1a; 11a), each fin (4, 4'; 14, 14') extending from the downstream edge (3a, 3'a), with respect to said mean direction of flow, of the respective slot (3, 3'; 13, 13'), so as to protrude into the ventilation duct (1; 11), the fins (4, 4'; 14, 14') each having a so-called guide face (4a, 4'a; 14a, 14'a) which is oriented or intended to be oriented in a direction opposite to said mean direction of flow, such that each fin (4, 4'; 14, 14') is able to deflect some of the air flowing inside the ventilation duct (1; 11) and guide it to the respective slot (3, 3'; 13, 13') through which the air exits the ventilation duct (1; 11), the aerator (2; 12) being characterised in that: - a central axis (D TH) of the guide face (4a, 4'a; 14a, 14'a) of each fin (4, 4'; 14, 14'), with said central axis being defined as a segment connecting the middle points of the fixed edge and free edge of the fin, is tilted, with respect to the axis (Δ) of the duct, towards the upstream edge (3b, 3'b) of the respective slot (3, 3'; 13, 13'), - the central axis (D TH) of the guide face (4a, 4'a; 14a, 14'a) of each of the first and last fins (4, 4'; 14, 14'), namely first-rank (4; 14) and last-rank (4'; 14') fins, respectively, with respect to the axis (Δ) of the duct, is tilted at a tilt angle ranging between 10° and 35° and between 60° and 85°, respectively, - the tilt angle of the central axes (D TH) of the guide faces (4a, 4'a) from the second fin to the penultimate fin (4; 14), namely fins (4; 14) from the second rank to the penultimate rank with respect to said mean direction of flow, gradually increases, and - the width of the guide face (4a, 4'a; 14a, 14'a) of each of the second fin (4; 14) to the last fin (4'; 14') is equal to or larger than that of the fin (4; 14) immediately upstream. - a central axis (D TH) of the guide face (4a, 4'a; 14a, 14'a) of each fin (4, 4'; 14, 14'), with said central axis being defined as a segment connecting the middle points of the fixed edge and free edge of the fin, is tilted, with respect to the axis (Δ) of the duct, towards the upstream edge (3b, 3'b) of the respective slot (3, 3'; 13, 13'), - the central axis (D TH) of the guide face (4a, 4'a; 14a, 14'a) of each of the first and last fins (4, 4'; 14, 14'), namely first-rank (4; 14) and last-rank (4'; 14') fins, respectively, with respect to the axis (Δ) of the duct, is tilted at a tilt angle ranging between 10° and 35° and between 60° and 85°, respectively, - the tilt angle of the central axes (D TH) of the guide faces (4a, 4'a) from the second fin to the penultimate fin (4; 14), namely fins (4; 14) from the second rank to the penultimate rank with respect to said mean direction of flow, gradually increases, and - the width of the guide face (4a, 4'a; 14a, 14'a) of each of the second fin (4; 14) to the last fin (4'; 14') is equal to or larger than that of the fin (4; 14) immediately upstream.