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Ventilation Ventilation means changing the air in a room. We need fresh air to breathe. We also need to remove bad smells and moisture. Moisture from cooking and showers can cause mold. The simplest ventilation is to open a window. But in many modern buildings, windows do not open. These buildings use mechanical ventilation. A fan sucks the old, damp air out of bathrooms and kitchens. It also brings fresh air in from outside. Some systems also recover heat from the old air to warm the new air.