Chronicle Online e-News
Meeting to consider tree planting as antidote to urban ills is
uprooted by 'inconvenient conclusion'
http://www.news.cornell.edu/stories/June08/trees.health.
deb.html
June 11, 2008
By David Brand
deb27@cornell.edu
"I think that I shall never see/A poem lovely as a tree," wrote Joyce
Kilmer. The poet, no doubt, was talking about aesthetics. If he wrote
those lines today, chances are he would also have urban blight and
community health in mind.
As well as bringing arboreal charms to inner cities, trees help
improve air quality by reducing air temperature, removing air
pollutants and providing shade that lowers energy use in buildings,
thus reducing air-polluting emissions from power plants.
In New York City alone, it's estimated that trees remove about three
times more lung-choking air pollutants than do shrubs. It's little
wonder, then, that the city has embarked on an ambitious program to
plant 1 million new trees by 2030, and Boston is planning to plant
100,000 new trees over 20 years.
A major reason why cities increasingly are turning to trees as a
public health measure is because of the alarming rise in respiratory
and cardiovascular diseases in urban areas. Asthma, in particular,
now affects 20 million Americans. The culprit in many researchers'
view is particulate matter (PM) spewed out by diesel combustion,
power plants and smelters, to name just a few sources. But trees,
according to an increasing body of research, are a perfect antidote
by capturing fine particles in the air and potentially deflecting
them away from high-population areas.
On June 2, Cornell Cooperative Extension-New York City and Cornell's
Department of Horticulture focused on this thesis at a conference at
Weill Cornell Medical College funded by the U.S. Department of
Agriculture.
The participants heard from urban planners about their tree-planting
programs, from horticulturists about designing sites to mitigate the
effects of particulate pollution, and from medical experts about tiny
particles in the air that can get into the lungs and interact with
lung cells. Then they heard from Tom Whitlow, a researcher in
Cornell's Urban Horticulture Institute.
His "inconvenient conclusion": Increasing tree cover as a strategy
for reducing asthma "is unlikely to work." It might be disingenuous,
he said, "to suggest that planting more trees might help a
community's health" in a directly measurable way.
Whitlow described his as-yet unpublished research into the way fine
particulate matter, called PM2.5 (meaning the particles are 2.5
micrometers in diameter and smaller), is deposited on leaves. These
particles are among the U.S. Environmental Protection Agency's
designated six so-called criteria pollutants. The agency warns that
PM2.5 pollutants are unhealthy to breathe and are associated with
premature mortality and other serious health effects.
Using a wind tunnel, Whitlow circulated a plume of particles through
bunches of foliage. After measuring the change in the plume, he
concluded that there was "no leaf-area effect" and that leaves are
not good filters of PM2.5. In fact, he noted, "Deposition [on a
surface] reaches a minimum in the PM2.5 zone."
Although more research is needed, Whitlow said he has so far
monitored 16 different size fractions, both larger and smaller.
What's important, he later added, is to use "the appropriate currency
to establish cause and effect."
Participant Max Zhang, Cornell assistant professor of mechanical and
aerospace engineering, who has developed a computer model of the
changes in size of particulates in relation to their distance from
roads in New York City's South Bronx, noted the increasing evidence
of the impact on human health by the smallest particles in the air,
or nanoparticles. Planting trees near roadways has the potential to
reduce particles at the nano level, he said. "Canopy has the
potential to protect those living near roads." Zhang noted that 36
million Americans live within 300 feet of a four-lane highway,
railroad or airport.
Among those advocating on-ground studies was Nina Bassuk, Cornell
professor of horticulture. She suggested that a research program be
tied to areas in New York City where the air quality in similar
streets with and without tress could be compared to determine if
trees were able to deflect or remove a wide range of pollutants from
the air.
Jennifer Greenfeld of New York City's Department of Parks and
Recreation offered the view that "public health benefits do not trump
other considerations. Tree planting has a role to play in some of
these problems of neighborhood ills."
But Charles Lord of Boston's Urban Ecology Institute voiced a fear
that was in many minds. "I hope this is not the silver bullet that
kills tree-planting programs," he said.
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