A good, vented fireplace will do two things to combat these problems. It uses outside air to fuel combustion. It then vents the resulting fumes and pollutants back outside.
Direct vent fireplaces can expel virtually all of the moisture, carbon monoxide, and other unhealthy byproducts, leaving your indoor air just as clean as when you lit the fire. Newer, more sophisticated models can actually improve your indoor air quality. They can bring fresh air in while expelling the stale and unhealthy. A direct vent fireplace with a heat recovery ventilator will draw fresh air into your home, creating a more comfortable and healthier environment for you and your family. The same ventilator will act as a dehumidifier, actually removing some moisture from the indoor air, thus reducing the risk of mold and mildew.