From Lindsay Abrams:
The blight was first detected in June 2002, when the trees in Canton, Michigan, got sick. The culprit, the emerald ash borer, had arrived from overseas, and it rapidly spread — a literal bug — across state and national lines to Ohio, Minnesota, Ontario. It popped up in more distant, seemingly random locations as infested trees were unwittingly shipped beyond the Midwest.
Within four years of first becoming infested, the ash trees die — over 100 million since the plague began. In some cases, their death has an immediate impact, as they fall on cars, houses, and people. In the long term, their disappearance means parks and neighborhoods, once tree-lined, are now bare.
Something else, less readily apparent, may have happened as well. When the U.S. Forest Service looked at mortality rates in counties affected by the emerald ash borer, they found increased mortality rates. Specifically, more people were dying of cardiovascular and lower respiratory tract illness — the first and third most common causes of death in the U.S. As the infestation took over in each of these places, the connection to poor health strengthened.
The “relationship between trees and human health,” as they put it, is convincingly strong. They controlled for as many other demographic factors as possible. And yet, they are unable to satisfactorily explain why this might be so.