Street Canyon Effect — a useful model?

I mentioned briefly about the Street Canyon Effect in my previous posts, and I thought that I should be elaborating on what it is about in more detail.

In summary, the street canyon model is a model of air pollution on the streets, flanked by columns of high rise buildings (what we increasingly see in urban areas). Important geometrical information for this model includes the aspect ratio (aspect ratio = height of buildings/width of street), which is used to classify the street canyons into different categories.

Because the street canyon affects the temperature, wind speed, and wind direction within the canyon, it consequently affects the air quality within the street canyon. For instance, through fluid mechanics, researchers found that street level pollution through contributions by vehicle exhaust is higher at the leeward side as compared to the windward side. Neighbouring street canyons may also affect the type of wind flow and hence air quality.

Why is this important?

  1. Implications of Traffic-related Air Pollution (TRAP) — small particulate pollutants such as PM2.5 and other traffic pollutants are harmful to health. In fact, the Global Burden of Disease 2010 estimated that 3.1 million deaths are due to exposure to ambient particulate matter.
  2. Increasing urbanisation trend in many regions around the world meant that more people are exposed to heavy traffic associated with the urban environment.

Researchers have applied this model to many different cities. One example is the research by Zhou and Levy (2008) on the impact of street canyon effects on population exposure to traffic pollutants in mid-town Manhattan, New York City, using the Operational Street Pollution Model (mentioned 2 posts previously!)  which is based on the Street Canyon Model. Their research found that there is a pressing need to control mobile emission sources on the streets to reduce population exposure to traffic air pollutants.

Also, Hong Kong is notorious for its street canyons; perhaps the air purifier (that I posted about yesterday) will definitely be a step to targeting the city’s pollution issues.

All in all, there are practical usage of such models, especially since it is hard to obtain data from every single region. However, we must always be aware of any potential limitations — for instance, the exclusion of certain air pollutants, simplification of reality to models (because that will happen to make models work!).


Cheung, C. and Kao, E. (2015). Scientists examine the health risks of Hong Kong’s notorious “Street Canyons”. South China Morning Post. [Online] Retrieved from: [last accessed: 9 April 2015]

Wang, A. and Ho, B. (2013). Characterising Urban Street Canyons in Downtown Vancouver. [Online] Retrieved from: [last accessed: 9 April 2015]

Zhou, Y. and Levy, J. I. (2008). The Impact of Urban Street Canyons on population exposure to traffic-related primary pollutants. Atmospheric Environment 42(13): 3087-3098.


Air Purifier as a Bus Stop?

Hong Kong’s air pollution is extremely bad. And a recent article shows how one company, Sino Group teamed up with another, Arup, to develop a roadside air purifier. The air purifier, while being unable to solve the problem directly (point and diffuse sources of air pollution such as industries, mainly from mainland China, and vehicles on the streets), at the very least emphasises how bad the air is for residents.

This purifier can remove pollutants such as PM2.5 too, and pumps out the clean air at the top so that the device forms an air curtain, and may also cut down the street canyon effect caused by buildings (the street canyon model is also used for the OSPM model mentioned in the previous blog posts!)

I have to replicate the photo from the article here, just to show how it actually looks like!

Clifford (2015) however notes that if the government were serious about minimizing street level pollutants, they should be actively trying to reduce the number of pollution sources too. I do agree with that but I suppose a stark reminder about the poor air quality seems to be a good campaign method — it has at the very least, caught my attention even though i am viewing it on a computer screen. Reducing pollutants just not involves active measures, but also about being sufficiently aware and sustaining the reduction through an understanding and awareness of the present day scenario.


Clifford, M. (2015) Beat the Hong Kong Smog: Outdoor Air Purifier Doubles As Bus Stop. Forbes. [Online] Retrieved from: [last accessed: 8 April 2015]

Six Cities Study (the US) — Air Pollution and Mortality

When we meet with the smog (or hazy air), our first reaction tends to be because it is unsightly and it is foul-smelling, and not because it is harmful to our health. While we have much more knowledge about how harmful air pollutants are (especially small particulates such as PM2.5), back in the 1970s this was not always the general knowledge.

Let us not forget that it took a serious hazy event in 2013 for Singapore to include PM2.5 in its Pollutant Standard Index (PSI) too.

A guide to reading the PSI released by the NEA in 2014 (with PM2.5 included within the PSI readings). Retrieved from:

As such, the “Harvard Six Cities Study” is famous for being one of the first to look at the correlation between air pollution and mortality over a long time period, and which lead to substantive action taken by the US government to regulate fine air particulates. The study reported the mortality of 8,111 randomly selected residents from six different U.S cities — Harriman (Tennesse), Portage (Wisconsin), St. Louis (Missouri), Steubenville (Ohio), Topeka (Kansas) and Watertown (Massachusetts) — with the aim of estimating the effects of air pollution on mortality while keeping other risk factors under control. These randomly selected residents were recruited between 1974 and 1977, and had their medical and occupational history recorded. These participants were also subjected to lung function tests. Air quality from their surroundings is taken by both ambient air monitoring stations in their towns, and also from air sampling devices worn by some participants or such devices from their homes. These residents were contacted all the way till 1991 to determine their health status, while death certificates are collected for those who have passed away in order to find out what is their cause of death.

What the study found is that mortality due to lung cancer and cardiopulmonary disease is most strongly associated with levels of fine air pollutants. Many other researchers have published follow-up articles based on this research (which is neatly summarised by the article from The Pump Handle by Monforton (2012).

Just last year, 20 years after the follow-up research for the study ended (in 1994), the lead author of the original paper, D. Dockery, gave a brief overview on what changes there have been since the study was published. These include new standards put in place by the Governmental Protection Agency (EPA), and an improvement in both health and air quality in the six cities from the original study. Another important point he made is that the federal Office of Management and Budget found that the largest estimated benefit from all federal regulations in 2007, is the reduction of just one air pollutant — fine particulate matter. The impacts of the study not just includes new standards in place, but has also laid “a firm scientific foundation for regulatory policies” .

This “six cities study” and it’s subsequent follow-ups definitely paint a clear picture of what is at stake for air pollution, and the benefits that cities may reap (not just in terms of health) if they take active steps to decrease the amount of fine particulate matter in the air.


Dockery, D., Pope, C. A., Xu, X., Spengler, J. D., Ware, J. H., Fay, M. E., Ferris Jr, B. G. and Speizer, F. E. (1993). An Association between Air Pollution and Mortality in Six U.S. Cities. The New England Journal of Medicine. 329:1753-1759

Feldscher, K. (2014) Landmark air pollution study turns 20. Harvard School of Public Health — Featured News Stories. [Online] Retrieved from: [last accessed 3 April 2015)

Lauerman, J. F. (n.d). A Tale of Six Cities. Flue Cube. [online] Retrieved from: [last accessed: 4 April 2015]

Monforton, C. (2012) Public Health Classics: Assessing air pollution and health in six U.S. cities, researchers’ findings changed the air we breathe. The Pump Handle [Online]. Retrieved from: [last accessed: 4 April 2015]

National Environment Agency (2014) PSI (with effect from 1st April 2014). PSI. [Online]. Retrieved from: [last accessed: 3 April 2015]