As homes have become more efficient thanks to weather stripping and insulation, they have also become more airtight. This means many homes have stale air. In addition oven/kitchen, bathroom, furnaces and hot water tanks all exhaust air from our homes. Increasingly, a negative pressure is created as more air is exhausted than is being brought into the house. Opening a window is not the best option on either a cold winter day or a hot humid summer day.