Dry earth is a reasonably good thermal insulator, but over several weeks of habitation, a shelter will become dangerously hot. The simplest form of effective fan to cool a shelter is a wide, heavy frame with flaps that swing in the shelter's doorway and can be swung from hinges on the ceiling. The flaps open in one direction and close in the other, pumping air. (This is a Kearny air pump, or KAP, named after the inventor, Cresson Kearny.)
Unfiltered air is safe, since the most dangerous fallout has the consistency of sand or finely ground pumice. Such large particles are not easily ingested into the soft tissues of the body, so extensive filters are not required. Any exposure to fine dust is far less hazardous than exposure to the fallout outside the shelter. Dust fine enough to pass the entrance will probably pass through the shelter. Some shelters, however, incorporate NBC-filters for additional protection.