Children living in areas of New York City with more street trees have a lower prevalence of asthma, as indicated by a study of children aged 4-5 years in 42 health service catchment areas. The rate of early childhood asthma was estimated as 29% lower for every 343 trees per square kilometer for the study group.
Lovasi, Gina S., James W. Quinn, Kathryn M. Neckerman, Matthew S. Perzanowski, and Andrew Rundle. (July 2008). Children living in areas with more street trees have lower prevalence of asthma. Journal of Epidemiology and Community Health 62(7): 647-649. http://dx.doi.org/10.1136/jech.2007.071894