The stack effect refers to how the air in your house moves. Just like a chimney, warm air naturally rises in a house, traveling from the lower levels up through the living area, and exiting through the attic.
This stack effect creates a vacuum in your crawl space or basement. As the air is sucked up through the home, it brings with it all the contaminants from the basement or crawl space including mold spores, dust mite feces and radon gas. This can cause a variety of health problems and has been known to cause and agitate Asthma as well as many other breathing problems. Some of us are allergic to these particles and will suffer from living with them at all times.
A proven method to prevent crawl space damage and to improve the health of the home is to eliminate vents and seal the crawl space with a quality vapor barrier.