Unlike the average air purifier, the Airocide doesn't rely on a fiberglass filter that has to be changed. Instead, this perforated rectangle implements technology used by NASA to neutralize ethylene gas aboard the International Space Station: A light inside activates a photocatalytic reaction on glass rings coated with titanium dioxide and other materials. Particles and spores including dust, bacteria, pollen, mold, and pet dander bond to the glass, then vaporize. [$800; airocide.com]