Azara Blog: An alleged link between trees and (lack of) asthma

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Date published: 2008/05/01

The BBC says:

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 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.

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 children whose asthma was so severe that they required hospital treatment.

At least these researchers tried to account for some of the obvious factors (in particular wealth). But they still have not demonstrated that they have found any causation rather than just a correlation. And the last paragraph is an indication that whatever effect there might be could prove not to be significant in the cases that most matter.

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