[Tree and forest effects on air quality and human health in and around urban areas]

The problems associated with air pollution and higher air temperatures in cities have been known for over a century, but so have the impacts of trees and forests on improving air quality and regulating air temperatures. Science has advanced our understanding on the various ways that trees affect air quality and temperatures. Trees affect air quality and consequently human health in both positive and negative ways by regulating air temperatures, altering the local microclimates, altering building energy use, removing air pollutants and emitting various chemicals. While the overall effect of trees and forests is an improvement in air quality, local scale forest designs near pollutant sources need to be considered as trees alter wind flow and can affect pollutant dispersion and local concentrations. Forests can limit pollution dispersion and increase local pollutant concentrations (e.g., along streets), but can also protect sites from pollutant emissions and have substantial impacts on lowering local pollution concentrations (e.g., in forest stands). By understanding how trees affect air quality and air temperatures, better landscape designs can be implemented to use trees and forests to improve human health.