Wood creates particulate matter and carbon monoxide when it burns, which are both harmful to breathe. Wood-burning fireplaces need ventilation to remove these from the home.
Open fireplaces, which have no front cover, typically use simple chimneys. Chimneys allow dangerous gases and smoke to leave the home, but they vent out a majority of the heat that the fireplace generates, as well. Some fireplaces have a front cover and use ventilation to draw air from the room. These fireplaces use smaller vents attached to the wall or ceiling that allow them to retain more of the heat generated at the cost of more complex design and more frequent cleaning and maintenance requirements.