Scientists have published new evidence showing that selective planting of vegetation between roads and playgrounds can substantially cut toxic traffic-derived air pollution reaching school children.

The new findings, published this week in the journal Scientific Reports, demonstrate that roadside vegetation can be designed, installed and maintained to achieve rapid, significant and cost-effective improvement of air quality.

Exposure to traffic-related air pollution has been linked with a range of health risks including cardiovascular, respiratory and neurological health. These risks are likely to be exacerbated in young children attending primary schools next to busy roads as their major organs are still developing and children have a higher breathing rate than adults.

Exposure to fine particulate matter in air pollution is reportedly the largest environmental risk factor contributing to cardiovascular deaths and disease globally, and is linked to around six to nine million premature deaths each year.

A team of researchers led by Barbara Maher, Emeritus Professor at Lancaster University, and supported by Groundwork Greater Manchester, installed ‘tredges’ (trees managed as a head-high hedge) at three Manchester primary schools during the summer school holidays of 2019.

One school had an ivy screen installed, another had western red cedar and the third school had a mixture of western red cedar, Swedish birch and an inner juniper hedge. A fourth school, with no planting, was used as a control.

The school with the ivy screen saw a substantial reduction in playground particulate matter concentrations, but an increase in black carbon. The playground with the mixture of planting saw lower reductions in air pollution to that of the western red cedar.

The biggest overall reductions in particulate matter and black carbon were shown at the school with western red cedar planted. The results showed almost half (49%) of black carbon and around 46% and 26% of the fine particulates, PM_2.5 and PM₁ emitted by passing traffic were captured by the western red cedar tredges.

The tredges also significantly reduced the magnitude and frequency of acute ‘spikes’ in air pollution reaching the playgrounds.

Professor Maher said: “Our findings show that we can protect school playgrounds, with carefully chosen and managed tredges, which capture air pollution particulates on their leaves. This helps to prevent at least some of the health hazards imposed on young children at schools next to busy roads where the localised air quality is damagingly poor, and it can be done quickly and cost-effectively.”

The scientists believe that western red cedar performed best at preventing the particulate air pollution from reaching the playground because its prolific, small, rough, evergreen leaves act like a filter, capturing particulate pollution and stopping it circulating in the atmosphere. When it rains, the particulates wash off – ending up in the soil or drains – enabling the leaves to then capture more particulate pollution.

Professor Maher said: “Western red cedar tredges work well because this species’ leaves form millions of tiny rough corrugated projections, each of which can bump into the particulates suspended in the air and ‘capture’ them in their ridges, furrows and pores.

“This takes them out of the local atmosphere and therefore reduces the exposure to these traffic-sourced air pollution particulates of the children and staff in the playground.”

The researchers believe species like ivy were not as effective at capturing particulate pollution as the western red cedar because of the smooth, waxy surface of its leaves. It therefore acts more akin to a fence where it blocks the transport of some particulate matter but is not as effective at capturing and thus removing it from the air.

The researchers suggest these benefits highlighted by the study are not just limited to schools and that carefully selected and managed tredges could be used in other parts of urban areas to reduce the damaging health impacts of exposure to traffic pollution.

The study was supported with funding from Manchester City Council and Transport for Greater Manchester, and Groundwork Greater Manchester which installed the tredges and ran ‘citizen science’ workshops with classes from the schools to highlight issues around air quality and steps young people and their families can take to make a difference.

Councillor Tracey Rawlins, Executive Member for Environment for Manchester City Council, said: "We were keen to be part of this study as Manchester seeks to embrace innovation in our efforts to become a greener city with cleaner air and tackle climate change. We note these positive findings with interest and will consider how we can use the lessons from this project to make further targeted use of green infrastructure in the city."

The study’s findings are detailed in the paper ‘Protecting playgrounds: local-scale reduction of airborne particulate matter concentrations through particulate deposition on roadside ‘tredges’ (green infrastructure)’.

Researchers on the paper include: Barbara Maher and Vassil Karloukovski of Lancaster University; Tomasz Gonet, formerly of Lancaster University and now Jaguar Land Rover; Huixia Wang of Xi’an University of Architecture and Technology, China; and Thomas Bannan, University of Manchester.

DOI: 10.1038/s41598-022-18509-w