The HVAC system in this crawlspace is drawing in air from moist, mold spore-ridden area and disbursing that into the rest of the house. This has been known to cause an increase in allergies, onset of asthma or asthma-like problems such as reactive airway dysfunction in children and adults. The best way to get nice, clean air into your home is to start with the crawlspace. Create a sealed, conditioned space there and your entire family will breathe better.