A heat exchanger cap configured to be worn on a human head to regulate the temperature thereof, the heat exchanger cap comprising a layer of material defining a single passageway through which a heat transfer fluid may flow and the passageway extends from a single inlet to a single outlet, characterised in that the material layer defines: a first element for covering one side of the head, a second element for covering an opposing side of the head; an intermediate joining element configured for covering the top of the head; each of the first element, second element and intermediate joining element defining a section of the single passageway through the cap; the material layer having: a first side for contact with the head; and a second side which, in use, will face away from the head; the first side has a relatively high coefficient of thermal conduction; and the second side has a relatively low coefficient of thermal conduction compared to the first side; wherein in at least the first element and the second element, the single passageway defines a first flow region located on an opposite side of a junction line to a second flow region; with each of the flow regions (92, 94) extending only part of the way from the front to the back of the cap; and the passageway has a serpentine route in each of the elements (55, 56, 57) and the serpentine route is back and forth in the direction from the nominal front of the cap to the nominal back of the cap; the passageway in the first flow region has a serpentine route back and forth over the front half of the side of the cap which corresponds to the front half of the side of the head on which it will be worn; the passageway in the second flow region has a serpentine route back and forth over the back half of the side of the cap which corresponds to the back half of the head on which it will be worn. a first element for covering one side of the head, a second element for covering an opposing side of the head; an intermediate joining element configured for covering the top of the head; each of the first element, second element and intermediate joining element defining a section of the single passageway through the cap; the material layer having: a first side for contact with the head; and a second side which, in use, will face away from the head; a first side for contact with the head; and a second side which, in use, will face away from the head; the first side has a relatively high coefficient of thermal conduction; and the second side has a relatively low coefficient of thermal conduction compared to the first side; wherein in at least the first element and the second element, the single passageway defines a first flow region located on an opposite side of a junction line to a second flow region; with each of the flow regions (92, 94) extending only part of the way from the front to the back of the cap; and the passageway has a serpentine route in each of the elements (55, 56, 57) and the serpentine route is back and forth in the direction from the nominal front of the cap to the nominal back of the cap; the passageway in the first flow region has a serpentine route back and forth over the front half of the side of the cap which corresponds to the front half of the side of the head on which it will be worn; the passageway in the second flow region has a serpentine route back and forth over the back half of the side of the cap which corresponds to the back half of the head on which it will be worn.