Abstract: A humidifier for humidifying a first fluid via a second fluid a plurality of membranes and a plurality of spacers. The plurality of membranes may be arranged following one another in a stacking direction. The plurality of spacers may include a plurality of first spacers and a plurality of second spacers arranged alternately between the plurality of membranes in the stacking direction. The plurality of spacers may space the plurality of membranes apart with respect to one another. A first height of the plurality of first spacers extending in the stacking direction may be smaller than a second height of the plurality of second spacers extending in the stacking direction. A ratio of the second height to the first height may be at least 1.2.