Crawl space encapsulation
Do you have a crawl space? If your home doesn't sit on top of concrete, it's that area under the first floor of your home. It usually has one or two doorways to get to it.
Most crawl spaces are one foot in height or more, and the floor is soil. Check to see if yours has a vapor barrier across the floor (a sheet or sheets of poly plastic).
That vapor barrier is important as it helps keep excess moisture and humidity out of the crawl space and slows the growth of mold on the exposed wood.
As heat rises in your home through the roof and upper regions, cooler air from the crawl space gets sucked into the home. Tiny particles of mold, fecal matter from dust mites, and decomposition odors can come up with this cooler air aggravating asthma, allergies, and other health concerns.