We've all heard that nature is good for our health. I've always taken such advice as probably good, but also probably hard to measure in any sort of accurate way. It turns out I was wrong. Scientists have measured it, and the advice is right. Trees, specifically, are very good for our health.
The study I discovered this past week was led by scientists in the U.S. Forest Service. I didn't know the U.S. Forest Service had a research group; this one is called the Northwest Research Station. They basically do scientific studies of trees and publish the results.
There are many good biological reasons to believe trees are good for our health. Trees clean the air and are thought to reduce stress. However, measuring trees' influence is hard because it is expensive, experimentally, to add millions of trees to an area.
The study took advantage of a natural experiment: the emerald ash borer. The emerald ash borer is a small bug, native to Asia, that kills ash trees. It migrated to the U.S. and was discovered in Detroit in 2002. It has been spreading ever since. Fort Wayne has lost almost all of its ash trees to the bug. Its spread has been carefully tracked because it is so devastating.
The emerald ash borer spreads somewhat slowly on its own. It can jump counties, however, by living in firewood, which people sometimes move large distances. You may have seen the bumper stickers: “Don't move firewood, it BUGS me.” As a result, there are some counties that are isolated pockets of the bug, such as Wayne County in Missouri or Hennepin county in Minnesota.
All that data, especially the isolated counties, helped the U.S. Forest Service scientists measure how trees affect our health. That is, they aimed to measure how the loss of ash trees reduces overall health.
The U.S. Forest Service scientists measured several properties of emerald ash borer-affected counties: the size of the tree canopy (leafy branches), the number of years the borer had infected the county's trees and the percentage of a county's trees that were ash. To measure the influence on people's health, the scientists measured the deaths due to lower-respiratory-tract disease (such as pneumonia) and cardiovascular (heart) disease as well as accidental deaths.
All three are common causes of death, which makes for generally better statistics. Respiratory diseases have been connected to tree cover because some trees remove a significant amount of particulates from the air, making the air healthier for us. Heart disease was chosen because having more trees appears to encourage people to spend more time outside, getting exercise, as well as reducing stress. Increasing exercise and reducing stress are both associated with reducing incidence of heart disease.
Including accidental death in the study strikes me as an especially clever trick. There is no reason to connect trees with accidental death. The scientists thus expected the loss of ash trees to have no effect on accidental deaths, but to increase the incidence of the other two causes of death (cardiovascular and lower-respiratory-tract diseases).
They measured other things as well, such as median income and education level. Of course, income and such are associated with better health, so they accounted for such differences.
After all the statistics, they found that trees really do matter. As the ash borer infested a county, the scientists estimated that annual mortality increased by about 24 deaths per 100,000. About a third of the increase was due to respiratory diseases, two thirds to cardiovascular disease. To give us more confidence in the result, they found no relation between the ash borer and accidental deaths. Total mortality in the United States, due to all causes, is about 800. An increase of 24 is important.
So in the end, trees matter. We breathe better air, spend more time outside and feel less stress when we are surrounded by trees. It's a shame we lost all the ash trees, but at least we understand ourselves a little better.
Christer Watson, of Fort Wayne, is a professor of physics at Manchester University. Opinions expressed are his own. He wrote this for The Journal Gazette, where his columns appear the first and third Tuesday of each month.