The harmful impact of urban air pollution could be combated by strategically placing low hedges along roads in a built-up environment of cities instead of taller trees, research has found.
Higher trees only have more of an impact in reducing air pollution in areas which are more open and are less densely populated by taller buildings.
In some environments, trees actually make the pollution more concentrated depending on prevailing wind conditions and built-up configurations, according to the study published in the journal Atmospheric Environment.
Future urban planning needs to consider designing and implementing more "green infrastructure", such as trees or hedges in the built environment to create a more healthy urban lifestyle, said Professor Prashant Kumar from University of Surrey in Britain.
Green infrastructure in cities is an urban planning solution for improving air quality as well as enhancing the sustainability of cities for growing urban populations.
These green solutions include street trees, vegetation barriers (including hedges), green (or living) walls, and green roofs.
They act as porous bodies which influence local dispersion of pollution and aid the deposition and removal of airborne pollutants, making the air cleaner.
"We all know air pollution is a major factor of everyday urban life. This comprehensive review highlights that trees and hedges, as well as other green infrastructure, must be used strategically to help create healthier, less polluted cities that are also more pleasant for everyone to live and work in," Kumar said.
The study also highlighted that green infrastructure has both positive and negative impacts on air quality at street levels, depending on the urban location it is in as well as its vegetation characteristics.
In a "street canyon" environment, where buildings like skyscrapers are close together on either side of the street, high-level green infrastructure (such as trees) generally have a negative impact on air quality.
Instead, low-level hedges reduce air pollution exposure in such places. In a similar way, green walls and roofs act as a sink to effectively reduce pollution, the study said.
In open road conditions, thick, dense and tall vegetation barriers restrict the freshly emitted vehicle emissions from reaching roadsides in high concentrations where people walk, cycle or live nearby, the study said.
The study was part of iSCAPE: Improving Smart Control of Air Pollution in Europe.
"Under the framework of the iSCAPE project, we are currently performing targeted field investigations to quantify the effects of different types of green barriers along the busy open-road sides," Kumar said.
