Roadside Shrubs Buffer Against Traffic Air Pollution

Nov. 10, 2021 — Air pollution from traffic can accumulate where buildings line city roads, creating the effect of a polluted urban canyon. But new research suggests that shrubs along streets could buffer against exposure for pedestrians and cyclists. But the trick is determining the best place to put the hedge.

Researchers in the United Kingdom used instruments to measure particulate matter in the air at 13 areas around a hedge along Du Cane Road, near White City in West London. The hedge runs along the sidewalk between the road and residential apartment buildings.

After collecting daily readings for almost 7 weeks in late summer of 2020, the investigators mapped concentrations of particulate matter relative to the hedge. They found that the concentration of polluting particles was lower at a height of 1 to 1.7 meters (39.4 to about 67 inches) in front of the hedge, where many pedestrians and cyclists would be breathing.

But wind direction affected where pollution accumulated around the hedge, with higher levels in some areas behind the roadside greenery. The researchers, whose work was published in Environment International, speculate that other nonhedge vegetation nearby might trap pollution.

For city planners, the implication may be that planting greenery at random even with the best of intentions, risks worsening air pollution in some areas. Local weather patterns, street and building layout, and surrounding areas are all likely factors in determining best hedge placement for air pollution mitigation.