U.S. rates of childhood asthma soared 50 percent between 1980 and 2000, with particularly high rates in poor, urban communities.

In New York City, asthma is the leading cause of admission to hospital among children under 15. The researchers found the city had an average of 613 street trees per two square miles, and nine percent of young children had asthma. The link between numbers of trees and asthma cases held true even after taking into account sources of pollution, levels of affluence and population density, the researchers said.

This innovative report is the first to look specifically at the potentially beneficial effects of trees in urban areas and raises some interesting issues. However, there are a number of other factors that have not been considered, for example whether the families involved have pets. Despite the need for further work, this is a positive first step into a new area of research linking the environment and asthma. New York City is planning to plant one million extra trees by 2017.