Since ancient times, observers praised the health benefits of “being in nature.”  Only recently, have scientists estimated the extent to which exposure to nature improves health outcomes. But their studies focus on comparisons between neighborhoods with more — or fewer — trees.  Up till now, no one had the opportunity to conduct a “before and after study” — that is, what happened to community health when existing trees disappeared.

Purpose

A natural experiment, which provides stronger evidence of causality, was used to test whether a major change to the natural environment—the loss of 100 million trees to the emerald ash borer, an invasive forest pest—had influenced mortality related to cardiovascular and lower-respiratory diseases.

Methods

Two regression models were used to estimate the relationship between emerald ash borer presence and county-level mortality from 1990 to 2007 in 15 U.S. states, while controlling for a wide range of demographic variables. Data were collected from 1990 to 2007, and the analyses were conducted in 2011 and 2012.

Results

Mortality related to cardiovascular and lower-respiratory-tract illness increased in counties infested with the emerald ash borer.  As the infestation spread, the incidence of disease increased. Across the 15 states in the study area, loss of ash trees was associated with an additional 6,113 deaths related to respiratory illness, and 15,080 to cardiovascular causes.

Conclusions

The presence of trees can make communities healthier.  Their absence?  Increased health problems and more deaths.