The primary purpose of an air exchanger, as the name might suggest, is to change air. Specifically, an air exchanger is used to bring in fresh, natural air from outside and to exchange it for air that's grown stale on the inside of a building. Put another way, an air exchanger ventilates a room or building, in a much more dramatic way than simply opening a window can. These products are installed in the walls of a house, but they may also be installed in existing heating or air conditioning systems as an add on to bring fresh air into the ducts.
Air exchangers work much like an automatic heating system. When the exchanger detects a higher than optimal humidity level, it begins to expel the excessively humid air to the outdoors. The air exchanger also brings in the cooler outdoor air through a second pathway that runs parallel to the first, without allowing the two air streams to interact. As a result, the outgoing, humid air may heat the incoming air, helping to keep a temperature balance in addition to keeping a constant flow of fresh air in the home.
Air exchanges also have filtration systems built into them. As the humid air from the inside moves out, it passes through a filter that captures the dust, pollen, mold, and other pollutants. Air is also filtered when it is brought in from the outside, going through the same filters which will capture the same pollen, spores, etc. that come from the outdoors. This ensures that those pollutants won't be floating around in the air, which keeps a safer, healthier environment in addition to providing constant, necessary ventilation.