Moisture
Whether it's from standing water after a rainstorm or just the dirt floor, evaporating moisture can damage wood and insulation and eventually make its way into your home. Mold in the crawl space and upstairs living areas is a common problem caused by excess moisture.
Pests
Moisture in the crawl space is a magnet for spiders, mice, rats, snakes, termites and other nasty pests. Because the crawl space is usually left undisturbed, this provides a safe haven for critters looking for a cozy home. What's more, much like our kids, critters don't usually clean up after themselves. Pathogens from rodent droppings ad urine can affect the air quality inside your home.
Ducts in the crawl space
Ducts that are well-sealed shouldn't be an issue, but the reality is that ducts in the crawl space are usually poorly installed and unsealed. Ducts carrying cold air through a humid crawl space can mean condensation problems.
Ducts in the crawl space
If the ducting for your HVAC system is running through the crawl space, it acts as a highway, carrying all the dust, moisture, rodent droppings, and chemicals from the crawl space directly into your living space and your air.