Urban trees linked to cutting childhood asthma

US researchers suggest that urban trees have a wider role to play than just an aesthetic one. They are undertaking a study into claims that tree-lined avenues can cut the number of children who develop asthma.

City officials are going to embark on a 10-year project to see if increasing the number of trees per square kilometre leads to a decline in the number of children suffering from the lung condition.

It appears as if the initial hypothesis is based on observations that a wealthly New York neighbourhood, with a high ratio of trees , had a  low  level of asthma among  young residents; whereas a poor district (with considerably less trees) had a much higher percentage of cases of childhood asthma.

Columbia University researchers found that asthma rates among children aged four and five fell by 25% for every extra 343 trees per square kilometre.

They believe more trees may aid air quality or simply encourage children to play outside, although they say the true reason for the finding is unclear.

The study appears in the Journal of Epidemiology and Community Health.

Here is the report from Reuters:

NEW YORK – City blocks boasting plenty of trees aren’t only more pleasing to the eye; they may be healthier for children’s lungs, according to research conducted in New York City.
Four- and five-year-olds living along the city’s greenest streets were less likely to have asthma than young children living in sparsely planted neighborhoods, Dr. Gina S. Lovasi and colleagues from Columbia University found. “We think that trees might have a beneficial effect on air quality — affecting air quality right at the street level,” Lovasi told Reuters Health. While the effects were independent of poverty and pollution, the researcher added, its possible street trees may simply be a stand-in for a healthful environment. “We’re not confident that it’s the trees themselves that are what’s driving this.”

Asthma rates have risen sharply in the US since 1980, and inner cities have been hit particularly hard, Lovasi and her colleagues note in their report. Trees could cut asthma risk by cleaning the air and encouraging kids to play outdoors, they add; but the pollen they release could also contribute to asthma attacks. To investigate, the researchers compared a census of New York City’s half-million street trees from 1995 to statistics on asthma prevalence and hospitalisation rates for 1999.

The wealthy Upper East Side of Manhattan was the greenest neighbourhood in the city, with 1,675 trees per square kilometre, or nearly seven trees an acre, while the impoverished Hunt’s Point-Mott Haven neighbourhood in the Bronx was the city’s barest, with only 109 trees per square kilometre or less than half a tree per acre.

As the density of trees in a neighbourhood rose, asthma prevalence fell, even after the researchers accounted for the percentage of residents living below the poverty line, a neighbourhood’s proximity to pollution sources such as busy truck routes, and other relevant factors.

An increase of 343 trees per square kilometre, or roughly 1.5 trees per acre, translated to 29% lower asthma prevalence. For example, asthma prevalence among 4- and 5-year-olds would be 9% in a neighbourhood with 2.5 trees per acre, but just 6% in a neighbourhood with four trees per acre.

Rates of asthma hospitalisation tended to be lower in neighbourhoods with more street trees, but the relationship wasn’t statistically significant; nevertheless, this suggests that trees aren’t a major contributor to asthma attacks, Lovasi said.

A “natural experiment” set to take place over the next decade will help to answer the question of whether street trees really do make for healthier kids; New York City Mayor Mike Bloomberg has launched an effort to plant a million new trees by 2017, and Lovasi and her colleagues are now working with the city government to study neighbourhood health as the project progresses.

Story by Anne Harding


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