It appears that children who live in tree-lined streets have lower rates of asthma. According to Columbia University researchers, asthma rates among children aged four and five fell by 25% for every extra 343 trees per square kilometer.

Researchers said more trees may improve air quality and encouraged children to play outside. The study appears in the Journal of Epidemiology and Community Health.

Several experts think that children who are exposed to few microbes in early life are at an increased risk of asthma, as their immune systems cannot fight infection.

“Previous research looking at the influence of the environment on levels of asthma has focused on negative aspects, such as pollution and chemical exposure. This innovative report is the first to look specifically at the potentially beneficial effects of trees in urban areas and raises some interesting issues,” lead researcher Dr Gina Lovasi said, quoted by BBC News. “Despite the need for further work, this is a positive first step into a new area of research linking the environment and asthma,” she added.

New York City will plant 1 million extra trees by 2017. Here, asthma is the major cause of admission to hospital among children under 15. Experts discovered the city had an average of 613 street trees per square kilometer, and 9% of young children had asthma.