Green spaces have a positive impact on our health and well-being. As Professor Prashant Kumar of the Institute for Sustainability explains, maximising those benefits is a science…
As a nerdy child, I was constantly told to turn off the TV or computer and go outside, as the fresh air would be good for me. I’d trudge out into the garden or the park on some freezing cold, grey afternoon thinking, ‘Is it really?’
Professor Prashant Kumar, Co-Director of the Institute for Sustainability at the University of Surrey, photo by Paul Stead Photography.
‘Oh yes,’ enthuses Professor Prashant Kumar, Co-Director of the Institute for Sustainability at the University of Surrey. ‘There’s lots of evidence that people who have easy access to green spaces are more likely to be happier and have better mental health and well-being than those who don’t.’
Why is that? ‘Nature has an impact on people. You see the greenery and the open space, rather the crowded urban environment. Your brain has less to process than being in front of a computer. It gives you a chance to recharge.’
We tend to think of such green space in terms of parks and gardens, woodlands and fields, but Professor Kumar is an expert on the wider ‘green infrastructure’ where we live. That includes everything from roadside verges and hedges, to urban trees and street planters. He’s particularly interested in studying the most effective ways to implement such ‘greening’ to improve air quality and health.
‘We’ve done some extensive studies on this,’ he tells me. ‘You put a hedge between a main road and a school or hospital and you can quickly see the reduction in black carbon — it can be by as much as between a third and a half. But it also depends on the type of hedge, how wide and high it grows, and what kind of state it’s in.’
By closely studying the effects of these different factors, Professor Kumar and his team have developed the free online resource HedgeDATE. You simply fill in a few details about the street in question and the system calculates the reduction in pollutant concentrations that could be achieved by planting a hedge. It also recommends the best species of hedge and how best to manage it once planted.
Another project has been led by the Global Centre for Clean Air Research (GCARE), of which Professor Kumar is founding director. ‘We installed the first ever “living gate” at a school,’ he says, at Sandfield Primary School in Guildford, in an effort to combat high levels of pollution from busy nearby roads.
‘The gate into the school now has some 140 plants on it, with an automatic irrigation system. The beauty of it is that it’s all part of the existing structure. The school children also took ownership of the project. They helped place the plants in the gate and then you’d hear them say things like, “That’s my plant and I’m going to look after it.”’
The result was extraordinary. Together, the living gate and a ‘green screen’ of ivy reduced pollution levels inside by between 42% and 44% — the variation caused by the direction of wind — and reduced traffic-related noise by 5 decibels.
‘This kind of green screening creates a physical barrier to the emissions,’ explains Professor Kumar. ‘But there’s a lot more going on. We saw the effect of the wind. We found that particles of pollutant get deposited on green leaves and are then washed away when it rains. Then there is the effect of turbulence and other factors which provides time for the dilution of pollutants. By understanding these complex interactions, we can better target our work.’
Professor Kumar is keen to point out that green infrastructure of this sort ‘doesn’t eat the pollutants as such — it blocks them from getting to particular sites, or at least slows their progress. It’s important to understand that you don’t solve the problem of pollution by planting lots of trees and hedges. For that, you need to tackle the source. But at the same time, there are multiple benefits from increasing green infrastructure. As well as air pollution, its good for the mitigation of flooding, overheating and noise. It has a role in carbon sequestration. There are the benefits for mental health and well-being.’
This range of impacts is important to avoid the unintended consequences of any intervention. ‘This is something that doesn’t get talked about enough,’ he continues, ‘but things done for the right reasons can have the wrong effects. Often that’s because people are focused on a single issue. My main interest is air pollution but I’ve learned I must consider everything else.’
He gives a particular example. ‘What’s called a “street canyon” is where you have high buildings on both sides of a relatively narrow road. This means limited airflow and high concentrations of pollution. So, you apply a green solution and put in a tree. That makes the street look nicer and improves the ecosystem in some ways, so you see some benefits. But when you study the data you find that the tree obstructs wind flow through the street canyon and so concentrations of air pollutants don’t reduce. They might even get worse.
‘Then the species of tree might produce a lot of pollen, a different kind of air pollutant with effects on health. Or there are species such as eucalyptus that produce biogenic volatile organic compounds — BVOCs. Those, in the presence of sunlight and the nitrous oxides from road traffic create another pollutant: ozone.’
Other factors to consider are the size — in height and width — that a chosen species of tree (or any other kind of greening) is likely to grow and whether it is easy to maintain, so as not to be a burden on the council. The choice also needs to be non-toxic and non-invasive, so that it doesn’t negatively impact local habitats. Is there one species that meets all these criteria?
‘Well, you don’t want the same, single species everywhere,’ says Professor Kumar. ‘You need to consider diversity.’ What’s more, all these factors need to be considered before an intervention. ‘It’s very difficult to change things once a tree is planted. There are strict regulations about cutting down trees and you often face local resistance.’
All these factors need to be considered as part of the planning process — a point Professor Kumar made to a House of Commons select committee on urban green spaces last December. At local government level, he is director and principal investigator of the UKRI-funded RECLAIM Network Plus which provides support to those interested in installing green (and blue) infrastructure and boasts a wealth of research and other resource.
Another project he’s involved in, GP4Streets is backed by a £2m grant from the Natural Environment Research Council. ‘There are lots of partners involved in this do-it-yourself initiative, a green prescription for streets,’ he explains. ‘The problem is that very built-up urban areas tend to have the most people living there, the highest energy consumption, the most exposure to pollution and the highest risk of overheating. But they also have the least free space available for greening interventions, so we need to find innovative ways of working.’
These innovations include, ‘developing tools to help councils in decision-making, so that they factor in the multiple benefits from greening. We can also help residents to be more strategic about what they do with what space they have, such as if they’ve got a garden, to maximise the impact.’ That suggests lots of small, very localised interventions that all add up to something bigger. ‘Yes, as well as carefully monitoring results so we can better understand the impact of interventions at this scale.’
‘It’s true of all this work,’ he says: ‘The more we understand, the more effective we can be.’
This interview was first published in Air Quality News Magazine issue 28 in March 2025.
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