"Green Walls" made by trees, bushes and other greenery growing in the concrete and glass "urban canyons" could reduce pollution at roadside much more than previously thought, said British researchers in a paper published on Wednesday.

Researchers at University of Birmingham found that, 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 an air pollution filter.

They reported in the journal Environmental Science and Technology that instead of reducing pollution by 1 or 2 percent, a massive 30-percent reduction in pollution could be achieved.

Plants in cities clean the air by removing nitrogen dioxide and microscopic particulate matter, both of which are usual products of traffic and harmful to human health.

These pollutants are significant problems in cities all over the world, as the World Health Organization's outdoor air quality database estimates that outdoor air pollution cause more than 1 million premature deaths each year.

"Up until now, every initiative around reducing pollution has taken a top-down approach - scrapping old cars, adding catalytic converters to cars, and bringing in the congestion charge - some of which have not had the desired effect," said Professor Rob MacKenzie at Birmingham.

"The benefit of green walls is that they clean up the air coming into and staying in the street canyon - planting more of these in a strategic way, could be a relatively easy way to take control of our local pollution problems."