Trees are woven into our urban and rural fabric, and provide so many benefits to our way of life, from urban temperature regulation and noise reduction through to carbon storage and, in an increasingly polluted world, air purification.
So how do trees affect air quality, and should the scientific community be bringing more pressure to bear on landowners and legislators to recognise the role that trees have to play in helping to manage and cleanse our day-to-day living environment?
The air that we breathe
In the UK the NHS has appointed a Director of Forests; perhaps a tacit recognition that trees have a role to play in helping to prevent or cure disease, whether physical or mental. The NHS claims to have planted hundreds of thousands of trees around NHS sites with the intention of providing both visual amenity – and as we shall see, trying to manage and control the incidence of disease.
UK National planning policy has also started to wake up to the role that trees and their environments can play. Development guidelines and process now have to pay some attention to the issue of air quality and link trees into sustainable development policies for green infrastructure provision. By their very nature trees are the largest and most frequent component of green infrastructure and (if well maintained) deliver increasing health benefits over long periods of time.
On a wider global scale the World Health Organisation is clear that air pollution is a major environmental risk to health. Urban outdoor air pollution has been estimated to cause 1.3 million deaths per annum worldwide, mainly through respiratory infections, heart disease and lung cancer.
And we can get more specific. Research in the United States has clearly identified an additional 15,000 deaths due to respiratory tract disease and 6000 deaths due to cardiovascular disease through the loss of one tree species[i]. In this case a pest, the emerald ash borer, has been identified as the culprit. First discovered in 2002 the borer attacks ash trees, killing nearly all of the trees it strikes; in the North Western US states the borer caused the loss of over 100million trees. Researchers from the US Forest Service analysed mortality in these areas and found consistently repeated patterns of increased human mortality related to the regions of pest invasion.
Clean air makes business sense
But why are trees so effective at improving air quality? In essence trees filter the air in two ways. Firstly they can absorb pollutants and trap the molecules within their leaf tissue – sulphur dioxide and carbon monoxide are commonly retained in this way. Secondly, physical particulates such as ash, dust and smoke can be deposited and retained on the surfaces of leaves, branches and twigs. Studies in London have estimated that up to 2,100 tonnes of pollutants are removed by trees from the Greater London Authority area each year[ii]. Because of high population densities in the city, targeted introductions of trees to the most polluted parts of London would have the greatest benefit to future air quality both in terms of reach per head of population and, just as importantly, the commercial return from the investment.
This commercial return is also measurable. In the United States, trees in New York City were identified as removing over 2000 tons of pollutants at a monetary value of $10.6 million per annum[iii]. A similar ecosystem services pilot study (126) in Torbay, West of England, identified 50 tonnes of pollutant removal worth just under £300,000[iv].
But even on a small scale the densities of particulates can be measured. A study in Lancaster showed that just by planting a few small trees between a busy A road and a housing estate, air pollution within the houses diminishes by 50-60%. Again, as those trees grow so their capacity to intercept pollution will increase over time.
In using tree leaves as a passive source of pollution monitoring scientists have discovered there are differences between species of tree[v]. For example, lime and beech remove many more particles from the air than does sycamore. It is estimated that beech trees are capable of extracting up to 5 tons of dust (per hectare) per year, and the larger the tree, the more pollution is removed. Willow, elder, elm and sweet chestnut remove far fewer particulates.
The UN forecasts that 86% of the human population in developed regions will live in urban centres by 2050 so the importance of well established, properly maintained tree stocks in densely populated areas can only become increasingly important. Growing the number of trees will quite simply increase the amount of pollution interception which then has a direct effect on the incidence of a wide range of diseases. There could not be a clearer indicator that we need to retain and manage our urban tree populations as a critical component of our life support systems.
The AA is the UKs leading professional body for arboriculture. It is the voice of arboriculture concerned with the sustainable management of trees where people live, play and work. The Association is currently championing a study of the ecosystem service benefits that trees provide to the city of London
This paper is part of a series of information bulletins prepared by the Arboricultural Association in its 50th year to bring to attention the many benefits of trees to human wellbeing.
Chairman of the Arboricultural Association
[i] Donovan et al., (2013) The Relationship between Trees and Human Health: Evidence from the Spread of the Emerald Ash Borer. American Journal of Preventative Medicine, 44 (2): 139-145.
[ii] Tallis, M., Taylor, G., Sinnett, D., and Freer-Smith, P. (2011) Estimating the removal of atmospheric particulate pollution by the urban tree canopy of London, under current and future environments. Landscape and Urban Planning, (103): 129-138.
[iii] Nowak, D.J., Hoehn, R.E., Crane, D.E., Stevens, J.C., Walton, J.T. (2007) Assessing urban forest effects and values, New York City’s urban forest. Resource Bulletin. NRS-9. Newtown Square, PA: U.S. Department of Agriculture, Forest Service, Northern Research Station.
[iv] Rogers, K., Hansford, D., Sunderland, T., Brunt, A., and Coish, N., (undated) Measuring the ecosystem services of Torbay’s trees: the Torbay i-Tree Eco pilot project. Trees, people and the built environment: Proceedings of the Urban Trees Research Conference 13–14 April 2011. Edinburgh: Forestry Commission.
[v] Mitchell, R., Maher, B.A., and Kinnersley, R. (2010) Rates of particulate pollution deposition onto leaf surfaces: Temporal and inter-species magnetic analyses. Environmental Pollution, 158 (5): 1472-1478.