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A Branch of Medicine

New research finds areas that experience a massive die-off of trees also suffer from an increase in cardiovascular disease.
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(Photo: waldopepper/Flickr)

(Photo: waldopepper/Flickr)

When it comes to avoiding a heart attack, you know the drill. Exercise regularly. Don't smoke. Maintain a healthy weight.

And, apparently, live in a neighborhood with plenty of trees.

That's the implication of newly published research, which finds older women living in counties where an insect infection killed off a large number of trees were more likely to suffer a heart attack, stroke, or fatal coronary heart disease.

"Our results suggest that trees offer a protective effect for cardiovascular disease," a research team led by Geoffrey Donovan of the United States Department of Agriculture's Forest Service writes in the journal Health & Place.

In recent years, there have been many studies showing that exposure to nature—or even hints of nature, like a potted plant—can have a positive impact on one's health. But drawing definitive conclusions from such studies can be tricky, since they inevitably focus on a single point in time.

"Trees offer a protective effect for cardiovascular disease."

Donovan and his colleagues took advantage of a natural disaster that allowed them to look at the impact of green space—or the lack thereof—over a period of years.

In 2002, an invasive tree pest known as the emerald ash borer made its way into this country, appearing first in Detroit and then spreading to much of Michigan, Ohio, and Indiana. The beetle, native to eastern Russia and northern China, attacks ash trees, which are common in many urban areas (representing almost 11 percent of trees in Chicago).

Infested trees die relatively quickly, in three to seven years after infestation. Their deaths have "a major impact on the urban natural environment," the researchers note—a terrible outcome, but one that allowed them to assess how a loss of trees affected health outcomes in a given area.

Donovan originally used this approach in a 2013 study, which concluded that, from 2002 to 2007, "infestation was associated with an additional 15,000 deaths from cardiovascular disease, and an additional 6,000 deaths from lower-respiratory disease across 15 states."

While those results caught people's attention, they were based on country-level mortality figures. The researchers wanted to refine their work by tracking the health outcomes of actual individuals.

In this new study, they do just that, using data from the Women's Health Initiative, a large-scale report that looked at the three leading causes of death in post-menopausal women (cardiovascular disease, cancer, and osteoporosis). For that report, a diverse group of women between the ages of 50 and 79 were recruited in 1991 and followed through 2010.

The researchers looked at the health trajectories of approximately 156,000 women, 14,518 of whom experienced an episode of cardiovascular disease. (That diagnosis was applied to those who suffered a heart attack, stroke, or death from coronary heart disease.)

Those figures were compared to the number of years since the infestation was first detected in the county where each woman lived. By 2010, 245 counties had been invaded by the tree-killing beetle.

The key result: Women living in a county infested with the emerald ash borer had a 25 percent increased risk of cardiovascular disease.

Donovan and his colleagues note there are at least four possible mechanisms linking the natural environment with human health: reducing risk factors such as air pollution; encouraging exercise; increasing social contacts (with more people spending more time outdoors); and reducing stress.

The researchers aren't sure what combination of factors contributed to their results, but they note that the link between infestation and poorer cardiovascular health held firm "even after controlling for recreational energy expenditure."

That finding suggests that, "at least in post-menopausal women, the relationship between trees and cardiovascular disease is not solely mediated by exercise."

The researchers include several caveats, including the fact that they had no way of knowing how much time any particular women in the study spent outdoors. Still, they have shown that tree loss has an aggregate effect on cardiovascular disease risk.

Perhaps we shouldn't think of them as trees, but rather as branches of medicine.

Findings is a daily column by Pacific Standard staff writer Tom Jacobs, who scours the psychological-research journals to discover new insights into human behavior, ranging from the origins of our political beliefs to the cultivation of creativity.