A study published in the July issue of the Journal of Epidemiology and Community Health1 documents the relationship between the density of street trees and the prevalence of childhood asthma in New York City. Findings suggest that street trees are associated with a lower prevalence, although no causality was inferred. I would like to point out a number of methodological issues which should benefit future studies on this subject.

The prevalence of asthma was determined for 4-year-old and 5-year-old children using data from school screenings in 1999. Street tree density was derived from the 1995 street tree census completed by the Parks and Recreation Department of the City of New York and expressed as the total number of trees on street segments divided by the land area. Data were aggregated at the level of United Hospital Fund (UHF) areas. Additional variables used in the analysis were population density, racial/ethnic composition and a measure of proximity to pollution sources. The initial correlation analysis suggested a negative association between street tree density and prevalence of asthma. However, one of the strongest positive associations was between street tree density and population density. This initially appears somewhat counterintuitive, until it is …