Urban trees – an air pollution solution?

Central_parkImage: Unsplash, Pixabay.com

One of a series of blogs written by CABI editors for One Health Day – November 3rd 2016

November 3rd 2016 will be host to the first ever One Health Day, an international campaign that aims to bring attention to how planetary health challenges are addressed.  It may not be obvious, but public health and the environment are inextricably interlinked.  The physical environment, which includes housing, sanitation, drinking water and air, has significant effects on human health and well-being.  Therefore, effective management of the environment is important, so that potential health issues can be avoided.  With this in mind, the focus of this blog is urban air pollution, its impact on health, and how trees could help improve the air quality in towns and cities.

According to a press release published by the World Health Organization (WHO) earlier this year, more than 80% of people living in urban areas, where air pollution is monitored, are exposed to air quality that exceeds the WHO limits.  In the past two years, the WHO’s urban air quality database, now covering 3000 cities across 103 countries has nearly doubled in size, with more cities measuring the air pollution levels and recognizing the associated health impacts.  But what are these health impacts?

In most cities, the most damaging air pollutant is particulate matter, emitted from sources such as the combustion of agricultural residues, fossil fuels and fuelwood. When deeply inhaled into the lungs, fine particulate matter (also known as PM10 or PM2.5) consisting of pollutants such as sulfate, nitrates and black carbon, can cause cardiovascular and respiratory diseases, as well as lung cancer. It is estimated to be the cause of 3.2 million deaths per year, as well as causing a range of other non-fatal health problems including asthma and bronchitis.

But urban air quality isn’t the only problem affecting human health in cities.  In many of these urban areas, the air temperatures can increase rapidly during the summer months, resulting in an estimated 12,000 deaths on average each year. A WHO report published in 2012, has predicted that unless cities adapt to the threat, by 2050, deaths from heat waves could rise to 260,000 per year.  

With 70% of the world’s population predicted to live in cities by 2050, heat and air pollution present a major public health issue.  So how can trees help address the problem?  They have already been recognized for their role in monitoring air pollution, so how can they be used to tackle it? A number of studies have shown that tree leaves can filter out particulate matter and many other pollutants from the atmosphere.  Similarly, other scientific studies  show that the shade cast by trees, in addition to the transpiration of water during photosynthesis, can help to reduce air temperatures.

A study published last month by The Nature Conservancy, quantified the effects of trees on air quality in 245 cities worldwide.  The Planting Healthy Air report found that the average reduction of particulate matter near a tree is between 7-24%, while the cooling effect is up to 2°C.  The report also compared the cost effectiveness of trees with other methods of cooling and cleaning the air including the use of industrial scrubbers and limitations on vehicle traffic. 

It concluded that while trees should not replace other strategies to make air healthier, they can be used as part of a suite of interventions that aims to control particulate matter pollution as well as mitigate against rising temperatures in cities.  The report also outlines some principles of tree planting that can be incorporated into a city’s planting plan, from choosing suitable tree species with a large leaf surface area, to considering where the trees should be planted.

The study also found that by investing US$4 per tree per resident in all the cities that were looked at, between 11,000 and 36,000 lives could be saved each year, as a result of having cleaner air.  Research published earlier this year, from the London i-Tree Eco Urban Forest Survey, quantified the benefits of ecosystem services provided by London’s urban forests (including removal of air pollution) at £6 billion, highlighting the economic as well as health and environmental benefits of planting more trees in cities.

While there are many additional benefits of urban trees, including carbon sequestration, stormwater mitigation, aesthetic beauty, noise reduction and erosion prevention, planting trees in cities can have its drawbacks.  For example, thick canopies in heavily polluted streets, especially ones with large volumes of traffic, can limit the circulation of air, trapping the polluted air at low levels, where people breathe.

While it seems clear that trees are not a "one stop solution", they can certainly help in the fight against air pollution in urban areas around the world.  Many believe that we are at the start of the "urban century", with many cities expanding on a colossal scale.  According to the UN, currently over half of the global population live in urban areas and with an estimated 2.5 billion extra by 2050, investment in trees and green infrastructure looks set to become even more important in future.

References

Daniels, M., et al., Estimating particulate matter-mortality dose-response curves and threshold levels: an analysis of daily time-series for the 20 largest US cities. American Journal of Epidemiology, 2000. 152(5): p. 397-40

Lepeule, J., et al., Chronic exposure to fine particles and mortality: An extended follow-up of the Harvard Six Cities study from 1974 to 2009. Environmental Health Perspectives, 2012. 120(7): p. 2012.

Useful resources

Subscribers of the CABI Forest Science and Environmental Impact databases can access further information regarding the impacts of trees on air pollution in urban areas.

CABI blog post: Air pollution, can we reduce the impact of cars on urban air quality?

CABI Forest Science news articles:

i-Tree report values London's urban trees at £6bn

Living near trees is good for your health

Trees improve pollutant dispersal in cities

Trees as bio-indicators of air pollution

Why do urban trees remain undervalued?

External links

Forest Research – Improving air quality

The Economist – Comparing urban air pollution

The Nature Conservancy – “How urban trees can save lives”

UNEP – Urban air pollution

US Environmental Protection Agency – Criteria air pollutants

USDA Forest Service – First national study finds trees saving lives, reducing respiratory problems: Air pollution modeling reveals broad-scale impacts of pollution removal by trees

Valuing London’s Urban Forest report – Results of the London i-Tree Eco Project 

World Health Organization – Ambient (outdoor) air quality guidelines

Related articles

Pollution worsens, particulate matter soars in Delhi
Success! London's Air Pollution Levels are Dropping, Thanks to Smart Policies

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