Since 2002, the emerald ash borer has killed 100 million ash trees across the Midwestern United States. Scientists explored the impact of losing these urban trees on human health. This is the first study to use a natural experimental design to look at the public health benefits of trees. The beetle invasion spreads over space and time, enabling researchers to weed out other possibly confounding influences on human health outcomes. They found that the deaths of these trees were associated with increased death rates in humans through cardiovascular and lower-respiratory mortality. Specifically, the emerald ash borer is associated with more than 6,000 additional lower-respiratory deaths, and over 15,000 additional cardiovascular deaths in counties with infestations. This study has raised awareness that trees can be considered part of a city's public health infrastructure; and, planting trees is something cities can do fairly easily. The Arbor Day Foundation has acknowledged the importance of these study findings and has used them in campaigns to spread the word about the value of trees.