The first thing to come to my mind is water flows down and mold/mildew would be more present in lower floors. Air pollution is a killer.
If I told you the people on the north side of my grandma’s nursing home died before the people who lived on the south side, you’d have some theory about sunlight exposure and excessive mold growth on the north side of the building. Air pollution kills!
But then it turns out the north side is just where they put the sicker people.