UK Edition
Trees trap pollution in congested city streets - plant hedges instead, study finds
Hedges, rather than trees, should be planted alongside many of the most congested city centre roads in order to soak up pollution, a new study concludes.
An investigation by an international team of scientists found that in narrow streets with high buildings, an abundance of trees traps toxic fumes at road level.
The new data turns on its head decades of received wisdom that trees necessarily improve the air quality of polluted roads.
Instead, the research team say hedges should be installed on the edge of pavements, as they are closer to the level of most exhaust pipes and can absorb damaging particles before they disperse into the air.
Published in the journal Atmospheric Environment, the study found that, overall, trees play a vital role in battling pollution in towns, but that they can make the problem worse in so-called “street canyons” such as London’s Northumberland Avenue or Euston Road.
Professor Prashant Kumar, who led the research at Surrey University, said: “The emissions from vehicles start to dilute very quickly as you move away from the road, so any hedge that acts as a barrier slowing down the airflow and catching pollutants on the leaves is going to offer people in homes better protection.”
The research was published on the same day as a report from the World Health Organization which found that deaths in the UK attributable to air pollution are worse than comparable European countries such as France and Spain.
A previous study in Guildford found that planting hedges along a busy main road cut toxic fumes by around a third.
Professor Kumar said scientists were currently investigating which species of hedge plant made the best pollution absorbent, but that in principal authorities should plant hedges with the greatest leaf surface area.
Despite their potential to improve air quality in the most polluted parts of towns, hedges are more commonly found in the suburbs, rather than city centres,” he said.
The study showed that the higher foliage provided by trees is often the most efficient way of reducing fumes in broad streets which provide room for clean air to sink to street level.
“In US cities, despite the tall buildings, the trees to a really good job because the roads are wide enough,” said Professor Kumar.
“But in Britain we need to start thinking about planning for hedges to harness their full potential in cities like London.”
Financially straightened councils around the UK are beginning to charge households for collecting green waste, raising fears that people will increasingly remove their hedges to avoid the fee