One theory I have encountered is the improved efficency of modern
building techniques. As homes and offices become more energy efficient,
the are also more air tight. No longer do we open the window when it
gets hot or close it when it gets cold. By relying on automated air
cooling and heating systems, we have greatly reduced the flow of fresh
air. In turn, dust builds up and allergens have no where to go once
they enter an automated system on clothing or through entry ways.
