Ironically, it's not just the pollutants that are affecting us. It may be too much cleanliness. A prevalent theory among allergists is known as the hygiene hypothesis. While it's true that industrialization brings with it better health care and fewer serious childhood infections, it also brings an obsession with cleanliness. We are not exposed to dirt at a young enough age to give our immune systems a good workout. Also, because of the high cost of energy, more homes are built with better insulation-insulation that seals in mold and dust.