The loss of 100 million trees recently in the eastern and midwestern United States was an unprecedented opportunity to study the impact of a major change in the natural environment on human health for researchers at the U.S Forest Service, Yvonne Michael, a professor of epidemiology and biostatistics, and their colleagues.

In an analysis of 18 years of data from 1,296 counties in 15 states, researchers found that Americans living in areas infested by the emerald ash borer, a beetle that kills ash trees, suffered from an additional 15,000 deaths from cardiovascular disease and 6,000 more deaths from lower respiratory disease when compared to uninfected areas. The borer attacks all 22 species of North American ash and kills virtually all the trees it infests.

The researchers analyzed demographic, human mortality and forest health data at the county level between 1990 and 2007. The findings, which hold true after accounting for the influence of demographic differences, such as income, race and education, were published in the American Journal of Preventive Medicine.

“The devastation from emerald ash borer on ash trees provided a unique natural experiment,” Michael says. “Existing research is limited by primarily cross-sectional designs and subjective measures of environment.”

Although the study shows the association between loss of trees and human mortality from cardiovascular and lower respiratory disease, the study cannot prove a causal link. The biological mechanism of action linking trees to mortality is not known but may include physical activity, social capital and/or improved air quality.

The study was conducted by the Forest Service’s Geoffrey Donovan and Michael in collaboration with David Butry, with the National Institute of Standards and Technology; and Jeffrey Prestemon, Andrew Liebhold, Demetrios Gatziolis and Megan Mao with the Forest Service’s Southern, Northern and Pacific Northwest Research Stations.

Michael’s research explores social and physical environmental determinants of health. The NIH and other organizations fund her research.