Landscaping streets with green walls could help to cut pollution by almost one-third - far more than was previously thought - scientists have suggested.
Trees, bushes and other greenery growing in the "concrete-and-glass urban canyons" would deliver cleaner air at the roadside where most people are exposed to the highest pollution levels, they said.
The researchers, based at the University of Birmingham and Lancaster University, added last week that urban landscaping such as green walls could be created street by street without the need for large-scale and expensive initiatives.
"Because pollution cannot easily escape street canyons, green walls of grass, climbing ivy and other plants have a better opportunity than previously thought to act as a filter," said Birmingham's Professor Rob MacKenzie.
"Instead of reducing pollution by one or two per cent, reductions of more than 10 times this magnitude could be achieved," he added.
Using a computer model that captured the trapping of air in streets helped the research team distinguish the effects of plants in low-lying urban areas from those of plants in parks or on roofs.
"Green walls emerged as clear winners in pollutant removal. Street trees were also effective, but only in streets where the tree crowns did not cause pollution to be trapped at ground level," said MacKenzie.
The Government environmental audit committee estimates that outdoor air pollution causes 35,000 to 50,000 premature deaths a year in the UK, while the World Health Organisation's outdoor-air-quality database puts the global figure at more than one million.
"Up until now, every initiative around reducing pollution has taken a top-down approach - scrapping old cars, adding catalytic converters and bringing in the congestion charge - some of which have not had the desired effect," said MacKenzie.
"The benefit of green walls is that they clean up the air coming into and staying at street level. Planting more of these in a strategic way could be a relatively easy way to take control of our local pollution problems."
Lancaster's Dr Tom Pugh added: "More care needs to be taken how and where we plant vegetation in our towns and cities so that it does not suffer from drought, become heat stressed or vandalised or interact negatively with other aspects of our urban areas."
"The research findings coincide with the completion of Transport for London's (TfL) second green wall at the Mermaid, Blackfriars, installed as part of the Clean Air Fund programme. Our own research shows the ability of different plants to trap particulate matter. Bringing together these strands of research will help to inform planners, designers and green infrastructure professionals." - Nicola Cheetham, head of environment, TfL