What ever enters your crawl space enters your home. Most times in older homes the holes drilled in the floor for your duct work, plumbing, gas lines and electrical are not properly sealed. This creates a natural draft and circulation of air through those unsealed holes into your living space. If the air in the crawl is unhealthy it will make its way into your home. Especially when your heating and air system turns on and creates a vacuum through out you home, pulling air from every drafty place.