Less asthma in tree-lined streets6th May 2008
Researchers in New York have found a link between the amount of trees lining urban streets and a lower incidence of asthma in children.
The study, carried out by a team at Columbia University, found that children who live in tree-lined streets have lower rates of asthma.
They looked at asthma rates among children aged four and five, and found that they fell by a quarter for every extra 343 trees per square kilometre.
While it was not immediately clear exactly how the trees might influence the children's health, researchers speculated that more trees might aid air quality or simply encourage children to play outside.
The study, published in the Journal of Epidemiology and Community Health, was welcomed by asthma campaigners as a positive first step into a new area of research into the link between asthma and the environment.
Childhood asthma rates in the United States rose by 50% between 1980 and 2000, with particularly high rates in poor, urban communities.
The condition is the leading cause of admission to hospital in New York City for children under 15 years of age.
The city has an average of 613 street trees per square kilometre, and 9% of its young children have asthma.
The link was clearly demonstrated even after allowing for pollution sources, socioeconomic status and density of population.
However, once these factors were taken into account, the number of trees in a street did not appear to have any impact on the number of hospital admissions of children for severe asthma.
The exposure theory of asthma suggests that today's society is too clean, and that children who are deprived of early contact with pathogens do not develop a healthy immune response capable of distinguishing between microscopic intruders.
Their immune systems do not get the practice they need at fighting infection, proponents of the theory say.
Playing outside in a tree-lined street might therefore maximise the odds that children will be exposed to microbes. Trees, however, are also a source of pollen, which may potentially exacerbate asthma symptoms in vulnerable children.
Lead researcher Dr Gina Lovasi said there might be some other factor improving the health of children in areas with more trees, which could be more abundant in areas that were well maintained in other ways.
Leanne Male, assistant director of research at the charity Asthma UK, said previous research looking at the influence of the environment on levels of asthma had focused on negative aspects, such as pollution and chemical exposure.
This report was the first to look specifically at the potentially beneficial effects of trees in urban areas. But she said other factors, like whether or not the families concerned kept pets, needed to be examined as well.
New York City has announced plans to plant a million new trees by 2017.
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