against asthma (Donovan et al. 2018). We're not exactly
sure how greenspace protects children, but we suspect
that the microbial communities on the leaves of trees
and plants helps children's immune systems develop
properly, so that they are less likely to develop immune
diseases like asthma. This finding is consistent with
research showing that other sources of microbial expo-
sure help protect children against asthma. For example,
having older siblings, going to day care, and growing up
on a farm are lifestyle traits that may reduce the likeli-
hood of childhood asthma (Douwes et al. 2008; Strachan
1989). Although these findings help us understand
the potential causes of the disease, it's unreasonable
to expect related public-health interventions – these
lifestyle traits are outside the realm of a physician's
medical guidance. In contrast, it may be relatively easy
to increase a child's exposure to diverse greenspace.
Greening, greenspace,
morbidity, and mortality
Cities across the world are undertaking ambitious proj-
ects to expand tree canopy by increasing the number
of trees planted throughout public and private spaces.
Evidence suggests that an increase in tree canopy or
green space could decrease sickness and death in urban
populations (Rojas-Rueda et al., 2019). In the case of
Philadelphia, for example, Forest Service scientists, with
collaborators, conducted a Greenspace-Health Impact
Assessment to estimate the annual premature mortality
burden for adult residents associated with projected
changes in tree canopy cover under the city's 2009 plan
called Greenworks Philadelphia (Kondo et al., 2020a).
Between the years 2016-2025, using up-to-date expo-
sure-response functions in the tool, the number of >>
The Pennsylvania Horticultural Society (PHS) LandCare program, in collaboration with community members, turned a vacant
lot into the Ed Bradley Community Oasis in West Philadelphia. Photo Courtesy of Pennsylvania Horticultural Society
urban-forestry.com 17
