Children living in areas with more street trees have lower prevalence of asthma

An increase of tree density (343 trees/km^2) was associated with a lower incidence of asthma.

"Street trees were associated with a lower prevalence of early childhood asthma. This study does not permit inference that trees are causally related to asthma at the individual level. The PlaNYC sustainability initiative, which includes a commitment to plant one million trees by the year 2017, offers an opportunity for a large prospective evaluation." [Conclusions]

Also see comments by Zandbergen (22 August 2008).