Indoor air is teeming with particulates, bacteria, viruses, mold spores and VOC's which can be hazardous to your health. Microbes are nasty and getting nastier - some are antibiotic resistant, others cause allergies, still others have traveled great distances to your work places or home. The world is getting more crowded, more urbanized, and more traveled. Perfect conditions for the spread of microbial pathogens.

Because particles are formed in so many different ways, they can be composed of many different compounds. Although we often think of particles as solids, not all are. Some are completely liquid; some are solids suspended in liquids. As the EPA puts it, particles are really "a mixture of mixtures".