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Children who live in streets lined with trees are less likely than others to develop asthma, research has found.
Trees may help prevent the condition either by improving air quality in traffic-choked streets or by encouraging children to play outside, the American researchers say.
The incidence of childhood asthma has increased by up to 50 per cent in the past 25 years throughout the world.
The rise is believed to be linked to reduced exposure to bacteria in modern homes, which leaves the immune system underdeveloped and prone to over-react.
Researchers from Columbia University in New York who surveyed rates of asthma in children aged 4 and 5 found it was highest in parts of the city where tree density was lowest.
The rate of asthma fell by a quarter for every extra 340 trees per square kilometre, a pattern that held true even after taking account of differing sources of pollution, levels of affluence and population density.
It is the first time that tree density has been linked with asthma. Trees produce pollen which may exacerbate asthma in children who already have the condition, but is not thought to cause it to develop.
The researchers found that areas with a high number of trees not only had lower levels of asthma but also a lower rate of hospital admissions among children already diagnosed with the condition, though this was statistically non-significant.
Any effect of the tree pollen appeared to be counteracted by the beneficial effect of the trees in other ways.
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