Plus our modern tendency to use antibacterial detergents to clean the house. Exposure to domestic "dirt" helps improve the immune system, because it learns how to deal with all kinds of bacteria. Kids who are kept in an ultra-clean, almost antiseptic environment are more prone to allergies than those who eat a scoop of dirt in the sandbox now and then, kiss their cats and dogs heartily and crawl on an unswept floor. Even not quite so clean air makes the body more resistant against allergies of the respiratory system, as was surprisingly discovered in Germany after reunification. Kids from eastern Germany had been exposed to more smoke from burning coal (for heating - as opposed to oil or gas heating in the western part) and they had less asthma and other respiratory diseases than kids from west Germany.
Great thread,
@Eleanor Rose
Very worth thinking about!