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Columbia University researchers have completed a study that suggests that growing up on a tree-lined street is good for a child's health. Why, because asthma rates are lower in kids who grow up in leafy green neighbourhoods.

The CTV story says that the asthma rates among four- and five-year-olds in New York City fell by almost a quarter for every increase in tree density that measured 343 more trees per square kilometre.

The researchers examined asthma rates among 4 and 5 year olds, as well as asthma-related hospital admissions for kids up to age 15, in New York City and analyzed information from 42 health-service districts.

Next, they compared the information to city data from those same districts that outlined the number of trees, sources of pollution, population density and the ethnic makeup of the area.

Tree density did not affect the rates of asthma-related hospital admissions among older children when the researchers included other factors such as the children's proximity to sources of pollution, and their neighbourhood's population density and level of affluence. The study was published in the Journal of Epidemiology and Community Health.

Trees may help decrease asthma rates because kids may be more inclined to play outside when there are a lot of trees in their neighbourhoods. Trees improve air quality by absorbing carbon dioxide and release oxygen.

Data on the prevalence of asthma among children aged 4-5 years and on hospitalisations for asthma among children less than 15 years old were available for 42 health service catchment areas within New York City.

Street tree counts were provided by the New York City Department of Parks and Recreation. The proximity to pollution sources, sociodemographic characteristics and population density for each area were also measured.

"Our cross-sectional and ecological study does not permit inference that trees are causally related to the prevalence of childhood asthma at the individual level," the authors wrote.

"Future studies may be more robust if they are able to measure and control for characteristics of the home environment, such as the presence of allergens."

In the United States, asthma rates in children have grown 50 per cent between 1980 and 2000, particularly in poorer, urban communities; while in New York City, asthma is the main cause of hospital admission among kids under age 15.

In Canada, asthma affects about three million people.