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Washington Post: How planting trees in cities can save thousands of lives.

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November 2, 2016

Yet another study has reaffirmed the idea that living near nature is good for human health — and can even save lives. A new paper, published Monday by the Nature Conservancy, suggests that planting trees in cities can result in cooler temperatures and reduced air pollution for millions of urban residents. The paper was released at the annual meeting of the American Public Health Association, but it has not been published in a peer-reviewed journal.

The study indicates that a global investment of $3.2 billion throughout 245 of the world’s largest cities — that’s about $4 per resident — could reduce pollution-related mortalities by anywhere from 2.7 to 8.7 percent, saving up to 36,000 lives every year. This level of investment could also reduce temperatures on the hottest days of the year for millions of people, save up to 48 billion kilowatt-hours of electricity annually and avoid up to 13 million tons of carbon dioxide emissions each year.

“Cities usually think of trees just as aesthetic ornamentation,” said lead study author Rob McDonald, lead scientist for The Nature Conservancy’s Global Cities program. “But we wanted to show there’s a lot of benefits. Cities should be thinking about their public health goals as connected to and integrated with their urban goals.”

Previous research has already shown that trees are capable of both cooling and cleaning the air in urban spaces. They provide shade and are able to redirect some of the sun’s energy into their own biological processes, lowering the ambient temperature. And they can remove harmful particulate matter from the air around them.

These are important benefits, both in terms of economics and human health. Previous research estimates that exposure to particulate matter leads to about 3 million premature deaths around the world each year. Excessive heat can also lead to heat stroke and even death, declines in productivity and spikes in air conditioning-related energy use.

The new study focused on 245 cities around the world, which are home to about 910 million people, relying on previous research to assess the impact of trees on ambient temperature and air pollution. The paper suggests that existing urban trees already reduce air pollution by at least 1 microgram of fine particulate matter per cubic meter for about 52 million people worldwide. And more than 68 million people are benefiting from a reduction in summer temperature of up to 3.6 degrees Celsius, thanks to the trees.

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