In this presentation, I will review the results of a global study that reviews the literature on vegetation and human health, and then maps the potential for this strategy to help more than 200 major cities globally. Urban vegetation already plays a small but important role in reducing particulate matter concentrations and ambient air temperatures, but the majority of cities globally are losing vegetative cover over time, so the ability of nature to ensure healthy air is declining over time precisely when billions more people are moving to cities. Looking at city-level averages, however, obscures the power of nature. Nature will not be the solution for air pollution or climate change, but it can serve as a secondary screen to protect vulnerable populations, with measurable health benefits. We document how targeted tree planting can be a cost-effective strategy for ensuring public health in particular locations, and discuss the characteristics of ideal locations for tree planting. We estimate that for the global cities we studied, trees and other vegetation could provide meaningful health benefits to millions of people.

We synthesize the empirical and modeling literatures on the effect of trees on ambient PM concentrations and average urban summer daytime maximum temperatures. We construct estimates of both the average (city-wide) and local (street- or neighborhood level) PM (particulate matter) and heat mitigation from increased urban tree cover. Our city-wide PM mitigation analysis updates the results of earlier Urban Forest Effects (UFORE) modeling studies with more recent data on urban PM deposition velocities, tree cover and PM concentrations. Our city-wide heat mitigation analysis builds on recent NASA studies that examine surface heat mitigation by vegetation and biome-specific surface-air temperature relationships. Our street and neighborhood-level analysis draws on the growing empirical literature on the PM capture efficiency of urban trees and the local cooling intensity and distance of street trees and parks. Using tree cover, street and population data we identify potential street tree and tree patch planting locations in more than 200 cities globally. We estimate the PM removal and heat mitigation potential of each potential location for surrounding residential populations. Using tree planting and maintenance cost data for each city, we rank potential planting sites in each city according to their cost-effectiveness in PM and heat mitigation using size of mitigation impact and number of people affected as key metrics. We estimate the local and city-wide mitigation large-scale tree planting could achieve in each city and the annual tree planting and maintenance budgets that would be needed to achieve mitigation impacts of various sizes for different numbers of people.

The Nature Conservancy, the University of Louisville’s Department of Environmental Cardiology, and other key partners are launching a first of its kind study to determine whether “greening” a residential neighborhood (planting trees and other vegetation) can produce measurable benefits to human health. Despite the demonstrated link between air pollution and health problems, and the link between trees/vegetation and air quality, there is no longitudinal study in the scientific literature that demonstrates that more trees and other vegetation lead directly to better health outcomes. This talk will describe the study design, methodology, and goals. Medium-sized neighborhoods in Louisville Metro with low existing vegetation will be randomized and selected as either “control” or “test” areas and surveyed for land use, built-environment, and vegetation cover. A cohort of 500 participants will be recruited, divided evenly between control and test neighborhoods. From these participants, data on residential environment, health behavior, social environment, and the perception of well-being will be acquired using validated questionnaires. Blood, urine, and hair samples will be collected and cardiovascular disease (CVD) risk will be computed for each participant based on measurements of blood pressure, cholesterol, diabetes, and smoking status. Novel biomarkers of subclinical cardiovascular injury such as circulating stem cells, plasma thrombotic response, inflammation, and physiological stress will be measured to evaluate the mechanisms underlying CVD risk. The goal of the study is to quantify the association between neighborhood vegetation, pollutant exposure, and CVD risk.

Member cities of C40’s Cool Cities Network (CCN) have been working to mitigate extreme urban heat. The CCN is focused on sharing best practices and research to better understand the urban heat island effect (UHI) and develop tools and resources that help cities advance UHI mitigation and adaptation techniques such as cool roofs, green roofs, cool pavements and others. In this presentation, the City of Toronto, a member of the Cool Cities Network will talk about the measures they have undertaken to address the extreme heat issues. These measures include actions to protect vulnerable people from health impacts of extreme heat through the City's Heat Alert Response program; heat vulnerability mapping research; strategies to cool buildings such as Toronto's Green Roof bylaw and research related to the health benefits of greenspace, indoor and outdoor cooling areas, and indoor maximum temperature standard.