Children who live in tree-lined streets have lower rates of asthma, a New York-based study suggests.

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.

US rates of childhood asthma soared 50% 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.