NEW YORK (Reuters) -- 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 U.S. 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 hospitalization rates for 1999.
The wealthy Upper East Side of Manhattan was the greenest neighborhood in the city, with 1,675 trees per square kilometer, or nearly seven trees an acre, while the impoverished Hunt's Point-Mott Haven neighborhood in the Bronx was the city's barest, with only 109 trees per square kilometer or less than half a tree per acre.
As the density of trees in a neighborhood rose, asthma prevalence fell, even after the researchers accounted for the percentage of residents living below the poverty line, a neighborhood's proximity to pollution sources such as busy truck routes, and other relevant factors.
An increase of 343 trees per square kilometer, 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 neighborhood with 2.5 trees per acre, but just 6% in a neighborhood with four trees per acre.
Rates of asthma hospitalization tended to be lower in neighborhoods 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 neighborhood health as the project progresses.