When warm air rises, it leaks out of the upper levels of houses. New air that must replace this escaped air comes from the very lower level of the home; the air that replaces the escaped air comes from the home's crawlspace and/basement.
This is called the ‘stack effect', which resembles a bottom to top air flow. About 50% of the air you breathe on the first floor of your home is air from your crawlspace. To make matter worse, if you have a hot air heating ductwork, the air mixes even more thoroughly throughout the entire home.
If there is high humidity in your crawlspace, your upstairs levels of your home will also have a higher humidity. If mold is growing in your crawlspace then your upstairs is more prone to mold. Air gets from the crawlspace up into the house by more than just air ducts. Since air is so tiny, it can seep through even the smallest of cracks into the upstairs of your house. These small cracks are usually located by holes that surround wires and pipes. And no matter how you try to clog these holes, air from your crawlspace will still enter the home.
In short, keeping your crawlspace clean and encapsualated, with a CleanSpace vapor barrier designed specifically to resist mold and the usual wear and tear, is the best thing to help the overall air quality of your home.