It's long been known that trees help clean the environment, but until recently their exact value hasn't been well understood. Initiatives like i-Tree and OpenTreeMap have begun to help visualize all of the good that trees do for us, and now a new paper goes even further.
As well as quantifying the value of a tree in taking pollution out of the air, it also sums the actual health benefits to humans. The upshot is that, in 2010, America's trees saved 850 lives and prevented about 670,000 cases of severe respiratory problems, according to the research.
The study, which is published in the journal Environmental Pollution, uses hourly weather and pollution data together with tree cover data to estimate pollution removal for each county. It then uses census data and a model from the Environmental Protection Agency to estimate the combined health-effect value in removing four pollutants: nitrogen dioxide, ozone, sulfur dioxide, and particulate matter smaller than 2.5 microns.
In 2010, trees absorbed between 9 and 23.2 million tons of pollution, with a health value of between $1.5 and $13 billion, the research says. Because most trees are in rural areas, the majority of the pollution reduction was outside cities. But in health terms, cities saw the biggest benefit: $4.7 billion compared to $2.2 billion for the countryside.
California, Texas and Georgia saw the greatest removal amounts by volume. Florida, Pennsylvania and California saw the greatest benefits in terms of health improvements.
Lead author David Nowak, who heads the i-Tree project at the U.S. Forest Service, says the paper is the first to estimate the value of trees to human health across the country. "We have made estimates of pollution removal in urban areas nationally in the past, but this paper expands estimates to all trees (urban and rural) in the conterminous US and specifically links impacts to human health impacts and values," he writes in an email.
Mapping and valuing trees helps advocates make the case for greening cities, so that the argument isn't so much "trees are nice" but "trees provide X dollar benefits." If we're going to have more trees, we first need to understand what they're worth.