It's a world we barely see, but it is teeming with life. In the dust of a typical house, carpet beetles, mites, silverfish, and other creatures live and die, producing new generations every few months. Mold, bacteria, and yeast lurk undetected in heating and cooling systems. Debris dispersed into the air from these organisms can cause runny noses, itchy eyes, coughing, headaches, and breathing difficulties. Some people, especially those made highly...