By Mikaël J. A. Maes
City dwellers are continuously subjected to air pollution levels above what is considered healthy, putting people more at risk for respiratory diseases, strokes, heart diseases and lung cancers. Leaving behind the busy day-to-day life and escaping to urban green spaces is becoming common practice for people living in cities, who go to green spaces like parks to relax and enjoy the greenery. They walk their dog and let the children play wild and free. And while the city folk are enjoying the park, hardly any of them are aware of the many other benefits these green spaces are quietly providing them.
Water quality and runoff
Urban green spaces such as green roofs and walls, street trees and gardens capture and absorb rainfall, reducing the amount of storm water runoff and prevent flooding. Green infrastructure also reduces the amount of nitrate leaching from the soil into the water supply. Nitrates are naturally occurring molecules that can badly affect ecological communities when their quantities exceed levels deemed acceptable in the natural environment. By reducing nitrate leaching into the water supply and keeping pollutants out of the rivers, urban green spaces can significantly improve the health of surrounding waterways1.
The urban heat island effect
Urban areas are usually warmer than surrounding areas. As more and more vegetation is lost and the amount of paved surfaces increases, it means less shade and moisture to keep urban areas cool.
According to the U.S. Environmental Protection Agency (EPA), for every 0.6°C; increase in air temperature, electricity demand for air conditioning increases by 1.5%-2.0%. But trees shading homes have been shown to reduce attic temperatures by as much as 22°C.
One of the most important roles of plants and trees is to hold soil in place, keeping it from being washed away, and reducing the chance for flooding, mudslides and dust storms. Not only can soil absorb rainfall to reduce flooding, it can also trap and remove pollutants in water, which are broken down by the root systems of trees and soil microbes.
Urban green spaces also remove air pollutants and dust. According to a report published by Virginia Polytechnic Institute and State University, one tree can remove up to 26 pounds of carbon dioxide annually, which equals 11,000 miles of car emissions2. A 2006 study estimated the total annual air pollution removed by urban vegetation in the US was 711,000 metric tons – a natural service valued at around $3.8 billion3. On top of directly storing carbon in tree biomass, trees also reduce carbon dioxide (CO2) emissions by cooling the air, allowing residents to minimize their electricity usage.
Although trees are generally regarded to improve air quality, numerous studies have indicated that trees can occasionally worsen air pollution, through ozone and pollen production4,5 and aerodynamic effects6.
Human health and wellbeing
People experience both immediate and long-term health benefits by interacting with urban green spaces. One hour of gardening has been shown to burn up to 300 calories, equivalent to walking or cycling at a normal pace for the same amount of time7. Freshly grown vegetables from your garden can have up to three times more vitamins than frozen or canned food.
Besides the obvious physical effects on your health, a study from the University of Exeter has shown that people who move to greener areas experience an immediate and continuous improvement of their mental health, lasting at least 3 years8. Green spaces offer urban dwellers a place to recover from stress and regain confidence in themselves.
One study by Professor Roger Ulrich from Chalmers University of Technology in Sweden showed that certain nature and garden scenes significantly ameliorate stress levels, within just five minutes. Even for longer periods, patients in a room with a window facing a natural scene have shortened post-operative hospital stays, compared to patients in a similar room but with a window facing a brick wall9.
The United Nations projected that 66% of the world population will live in urban areas by 205011. Any delay in tackling urban environmental challenges could result in significant loss of life, given the high population densities found in urban areas.
Even now, for the billions of people already living in urban environments, it is of vital importance that we take steps to mitigate and adapt to climate change – this will become an increasingly important strategy as the detrimental effects of climate change worsen.
1. Gaston, K. J., Davies, Z. G. and Edmondson, J. L. 2010. Urban environments and ecosystem functions. In: Gaston, K. J. ed. Urban ecology. Cambridge: Cambridge University Press, pp. 35-52.
2. Bennett, R., Codiga, D., Cohen, N., De Sousa, C., Fink, J., Gardner, A. & McPhearson, P. T. 2013. Sustainability in America’s cities: creating the green metropolis. Island Press.
3. Nowak, D. J. 2006. Institutionalizing urban forestry as a “biotechnology” to improve environmental quality. Urban Forestry & Urban Greening, 5(2), 93-100.
4. Benjamin, M. T., & Winer, A. M. 1998. Estimating the ozone-forming potential of urban trees and shrubs. Atmospheric Environment, 32(1), 53-68.
5. Lovasi, G. S., O’Neil-Dunne, J. P., Lu, J. W., Sheehan, D., Perzanowski, M. S., MacFaden, S. W., … & Perera, F. P. 2013. Urban tree canopy and asthma, wheeze, rhinitis, and allergic sensitization to tree pollen in a New York City birth cohort. Environmental Health Perspectives (Online), 121(4), 494.
6. Vos, P. E., Maiheu, B., Vankerkom, J., & Janssen, S. 2013. Improving local air quality in cities: to tree or not to tree?. Environmental pollution, 183, 113-122.
7. Phelps, J., Hermann, J. R., Parker, S. P., & Denney, B. 2010. Advantages of gardening as a form of physical activity in an after-school program. Journal of Extension, 48(6), 1-7.
8. Alcock, I., White, M. P., Wheeler, B. W., Fleming, L. E., & Depledge, M. H. (2014). Longitudinal Effects on Mental Health of Moving to Greener and Less Green Urban Areas.
9. Ulrich, R. 1984. View through a window may influence recovery from surgery. Science, 224(4647), 224-225.
10. UN, 2015. World Urbanization Prospects – Revision 2014. New York: United Nations.[Accessed 4 May 2017]