"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.