Can roadside trees screen houses from pollution?

For two weeks in May, researchers installed a row of 30 silver birch trees alongside a busy city street in Lancaster, England. The effect on indoor pollution in nearby houses was dramatic: Levels of small particles dropped by more than half. The results suggest that planting roadside trees could be a big health boon to residents, the study authors say.

Pollution has been linked to illnesses such as heart disease and respiratory problems. Other research has shown that reducing levels of small airborne particles can increase the life expectancy of people living in the area. And the issue doesn’t just apply to people walking outside — particles can infiltrate houses as well. For example, indoor pollutant levels tend to rise during rush hour.

Trees can act as a barrier against pollution because they catch the particles on their leaves. But how effective are they? Previous estimates of pollutant reduction by trees have ranged from 60 percent to less than 1 percent. But these studies were based on models, so the authors of the current study wanted to take actual measurements.

Their study site was a road in Lancaster where more than 12,000 vehicles drive per day. The researchers measured real-time particle levels in two houses on that street before and after the trees were installed along the curb. They also measured particle deposition in eight houses by swiping TV screens or computer monitors in those homes. Some of the sampled houses were in front of the tree line, and some were not.

In the houses screened by the trees, pollutant levels dropped by more than 50 percent, the team reports in Environmental Science & Technology. The researchers also examined the trees’ leaves and found that they had captured clumps of particles, as expected. The particles were lodged near leaf hairs and inside dents on the leaf. Instead of simply planting trees all over the city, urban planners could take a more directed approach and plant pollutant-screening trees along roads, the authors suggest.Roberta Kwok | 21 November 2013

Source: Maher, B.A. et al. 2013. Impact of roadside tree lines on indoor concentrations of traffic-derived particulate matter. Environmental Science & Technology doi: 10.1021/es404363m.

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