Collecting device (1, 101, 201) for collecting and sucking mercury-contaminated air, in particular mercury-contaminated room and breathing air, from an environment of a patient's head during dental treatment of the patient, the collecting device (1, 101, 201) comprising a collecting device interior space (2, 102, 202), an inlet (3, 103, 203) connected to the collecting device interior space (2, 102, 202) for admitting air into the collecting device interior space (2, 102, 202), and a connection (4, 104, 204) connected to the collecting device interior space (2, 102, 202) for connecting a suction device for sucking the air from the collecting device interior space (2, 102, 202), wherein, in a working orientation of the collecting device (1, 101, 201), the inlet (3, 103, 203) forms an upwardly pointing opening through which air from outside the collecting device (1, 101, 201) can descend into the collecting device interior space (2, 102, 202), wherein the collecting device (1, 101, 201) has a cutout (5, 105, 205), in particular a neck cutout, which, in the working orientation of the collecting device (1, 101, 201) extends from an upper side of the collecting device (1, 101, 201) to a lower side of the collecting device (1, 101, 201), so that the collecting device (1, 101, 201) can be arranged around a neck of the patient in the working orientation, so that the neck of the patient is at least partially located in the cutout (5, 105, 205), characterized in that the connection in the collecting device interior space has a transition region in which the connection widens into the collecting device interior space, the transition region being covered with a cover which has clearances arranged rotationally symmetrically, the clearances having side walls lying closest to the center of the cover with respect to the cover, and side walls lying away from the center of the cover, the clearances having substantially radially extending side walls with respect to the cover each connecting the inner side wall to the outer side wall of the respective clearance, wherein the substantially radially extending side walls are each angled with respect to an axis of rotational symmetry of the arrangement of the clearances, and wherein, viewed from the axis of rotational symmetry of the arrangement of the clearances with respect to the respective substantially radially extending side wall, the substantially radially extending side walls are inclined from below a first side upwardly toward the other side in order to create a vortex in the aspirated air flow so that air can be sucked out of the collecting device interior space more efficiently. wherein, in a working orientation of the collecting device (1, 101, 201), the inlet (3, 103, 203) forms an upwardly pointing opening through which air from outside the collecting device (1, 101, 201) can descend into the collecting device interior space (2, 102, 202), wherein the collecting device (1, 101, 201) has a cutout (5, 105, 205), in particular a neck cutout, which, in the working orientation of the collecting device (1, 101, 201) extends from an upper side of the collecting device (1, 101, 201) to a lower side of the collecting device (1, 101, 201), so that the collecting device (1, 101, 201) can be arranged around a neck of the patient in the working orientation, so that the neck of the patient is at least partially located in the cutout (5, 105, 205), characterized in that the connection in the collecting device interior space has a transition region in which the connection widens into the collecting device interior space, the transition region being covered with a cover which has clearances arranged rotationally symmetrically, the clearances having side walls lying closest to the center of the cover with respect to the cover, and side walls lying away from the center of the cover, the clearances having substantially radially extending side walls with respect to the cover each connecting the inner side wall to the outer side wall of the respective clearance, wherein the substantially radially extending side walls are each angled with respect to an axis of rotational symmetry of the arrangement of the clearances, and wherein, viewed from the axis of rotational symmetry of the arrangement of the clearances with respect to the respective substantially radially extending side wall, the substantially radially extending side walls are inclined from below a first side upwardly toward the other side in order to create a vortex in the aspirated air flow so that air can be sucked out of the collecting device interior space more efficiently.