The Stack Effect
The way air moves throughout the home is called the "Stack Effect". Warm air moves upward through the home. As air leaves from the upper floors and attic, it pulls from below, including your basement or crawl space. As a result, whatever is in the air will also be in your home. If you have foundation vents in your crawl space, the problem is compounded. Humidity flows into the crawl space from the outside air. This air is then pulled upward into the home to fill the vacuum caused by the stack effect. The end result: The air you breathe inside your home, comes from your crawl space.