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]


Traffic Pollution Modelling — Article Summary

After several posts about pollution sources and effects of street pollution, I thought that it would be interesting to look at how researchers observe and track pollution. One way that they do so is by modelling; modelling is a useful tool to have for long term predictions, but as this paper by Berkowicz et al (2006)  will show, there is still room for improvement for these models.

The paper summaries comparisons between data obtained from traffic pollution modelling with the COPERT model and OSPM mdoel (Operational Street Pollution Model) and that of actual street level measurements. They found that there are significant underestimations present in the modeled data, and proposed a new set of parameters (traffic emission factors, to be precise) which appears to give more accurate results.

Why is modelling prone to inaccurate data? This is because of what modelling actually is. For instance, modelling looks at data on source emissions, which is often calculated based on 1) traffic data and 2) vehicle specific emission factors. These are sometimes not actual measurements; for instance, the authors note that vehicle specific emission factors are often estimated with different methods and are found not to just vary according to vehicle type, but also on driving conditions — this is something that had not been taken into account.

Parameters assigned to different models may also be problematic. For instance, the COPERT modelling in European context aims to be a simple method for estimation of national emissions of traffic related pollutants. It uses vehicle emission factor, which is a function of vehicle speed; differentiation between vehicle types, fuel used, engine capacity or weight, emission legislation category. It also includes correction for cold starts and degradation of emission reduction equipment with mileage. This sounds good, and simplified, but unfortunately the authors note that the parameters used may be erroneous.

For example, they thought that emissions predictions on the national is not quite possible because it is difficult to isolate source pollutions on the national level — there are many variables that complicates pollution levels, as compared to merely street measurements.

Meanwhile, the Danish OPSM model (Berkowicz, 2000; cited in Berkowicz et al, 2006), is a simple parametrized model (parametrization of flow and dispersion conditions — how air pollutants are dispersed in the atmosphere —  in street canyons). The good thing about this model is that it requires little CPU time so that the model can be modeled for longer time periods — this is useful because this is precisely what models try to do (predicting what will happen in the long term given a certain set of conditions)

However, an important result from this paper is that while models provide a good estimation for street level pollution, they are rather unreliable when it comes to providing data urban /regional studies. This is because it is easier to isolate pollution sources on the streets, as compared to that of the whole region. Despite so, traffic pollution modelling is a tricky process.  Factors involved in modeling may be inaccurate, and may result in underestimation of data (for instance, too low emissions attributed to heavy diesel traffic (5% of traffic) resulted in  underestimations of NOx on the street of Jagtvej, Copenhagen, by almost 30%, and an underestimation of CO2 by 60%.

The authors concluded the paper by warning readers that it is essential to note any potential biases from inaccurate emission values attributed to different vehicles, and calls for more comparison between models and actual street level measurements. These information will better improve existing models.


Berkowicz, R., Winther, M. and Ketzel, M. (2006). Traffic pollution modelling and emission data. Environmental Modelling and Software 21: 454-460

Gadgets to monitor air quality at Edmonton

I came across this interesting initiative to help raise awareness on air pollution in the city of Edmonton, Alberta.

In response to the poor air quality (in particular, during the 2009/10 and 2010/11 winters when calm wins and a temperature inversion trapped air pollutants in the city) in Edmonton, Alberta Capital Airshed (a provincial industry and environmental advisory group) are partnering with city authorities to make portable and personal air quality monitors for citizens in the city by making use of crowd-mapping software with the help of HabitatMap. This is done with the aim of raising awareness about the dangers of poor air quality which residents may not be aware of, such as that of PM2.5 which are too small to be seen with the naked eye.

How the monitors work is that participants will bring their monitors along, and the monitors will take in data which will be uploaded real-time to an online map that is available to the public. These monitors measures the amount of PM2.5 in the air by running a small stream of air past an infrared LED light. A small sensor located in the device measures how much light is scattered by the PM2.5 particles in order to detect the PM2.5 concentrations in the air.

Here is a picture of the monitor!

Picture of the air quality monitor which will be provided to some residents of the city of Edmonton. Source:


Slote, E. (2015) New air quality gadgets demystify pollution. Edmonton Journal [Online] Retrieved from: [Last accessed: 21 March 2015]

Pittsburgh’s Air Pollution (1940s vs Today)

I saw this BuzzFeed article contrasting the difference in air pollution (through photographs) in Pittsburgh’s streets almost 70 years ago. (Do take a look!) The contrast is quite startling; seeing photographs of the hazy air (the outlines of buildings nearby are obscured) gives a better picture of just how severe pollution issues are in the past too.

While looking through the pictures, I felt quite happy that there is much progress for the better, and this made me wonder if it is possible for many highly polluted streets and regions to improve their air quality too. What is important to note is that measures take time — perhaps, we cannot expect air quality to improve overnight. But we can be hopeful that many decades later, there is a possibility of cleaner air.

Indeed, the Atlantic had used these pictures to tell netizens who are criticising Beijing’s air quality that it takes time, and Pittsburgh took close to a century to reduce the amount of smoke and soot particles in the air to the level it is at today. Contrast Pittsburgh’s development in the 19th century as the city, powered by coal, emerged as one of the main metalworks industries in the country with Beijing’s excessive coal powered power plants today.

This is Pittsburgh back in the 1940s. Source: [Last accessed: 1 March 2015]

After looking at these pictures, I have some thoughts (these are not necessarily referring to Pittsburgh and Beijing; I am thinking about many cities around the world in general):

1. The political will of the city/country’s authorities

After looking through the pictures, it might be tempting to say that severe air pollution around the world is acceptable for developing cities, and that they just need to be given a longer time to solve their air pollution issues because they are inevitable with industrial development. I think that it is unacceptable to say that just because other countries were able to pollute the air without much dire consequences (laws were not as strict as those today), one should be permitted to pollute the air as much as they would want to. It may be unfair for countries who are at the other end of the developmental ladder, but I feel that there is a responsibility towards not just a country’s own citizens, but citizens of the world once there is ample evidence and knowledge on tackling these issues.

That said, we should be aware of the unequal power relations on the world stage — would developed countries push the developmental ladder away from developing countries? Ideally, developed countries with the expertise and experience should provide their technological known-how for other countries to tackle the same problem they faced many years ago, but which country is entirely selfless to do that without any reservations or motives (settling a deal, etc)? I think not.

2. The situation may not have improved as much as we thought it had

For instance, back to Pittsburgh: it is reported in 2014 that the air quality is still bad despite improvements, due to shifts towards natural gas from coal-powered plants, with increases in unhealthy ground-level ozone. Soot and smoke particles may have declined, but many cities around the world are facing a new set of pollution problems which are less visible. PM2.5, carbon dioxide and nitrogen dioxide are just a few examples.


Hopey, D. (2014). Pittsburgh region still gets poor marks for air pollution. Pittsburgh Post-Gazette. [Online] Retrieved from: [Last accessed: 1 March 2015]

Madrigal, A. (2013). Aghast over Beijing’s Air Pollution? This was Pittsburgh not that long ago. The Atlantic. [Online] Retrieved from: [Last accessed: 1 March 2015]

Diesel traffic — respiratory effects on asthmatic persons

After posting about a research on diesel pollution exposure on school buses previously, I thought it will be good to look deeper into the effects of such exposure. The journal article that I looked at did not provide any particularly surprising information to me, since I already had a rough idea about how polluted diesel exhaust is, and how they pose various problems to our respiratory tracts. What this article taught me more about is some specific respiratory effects (it is from a health journal, the New England Journal of Medicine!) and some considerations when it comes to possible experimental design.

Close up picture of diesel exhaust fumes. Source: [Last accessed: 26 February 2015]

McCreanor et al (2007) did a research on the short-term respiratory effects of exposure to diesel traffic in asthmatic persons in an urban, roadside environment. What this research aims to do is to build on research from previous studies that were conducted under laboratory conditions — previous studies have not been entirely consistent and exposure to those pollutants may not reflect that of real conditions. McCreanor et al (2007) suggests that these discrepancies may be due to interactions between different pollutants, or that pollutants under laboratory conditions may have removed. Instead of conducting the research indoors, they got participants to expose themselves to diesel traffic by the streets.

They got 60 adults with mild or moderate asthma to walk for 2 hours along Oxford Street, London, and Hyde Park on a separate occasion (I briefly mentioned in my first post that Oxford Street was raised in the spotlight just recently in 2015 for extremely high nitrogen dioxide levels!). The study was limited to winter months to avoid other variables such as exposure to pollen. As expected, participants had significantly higher exposure to fine particles, in particular, PM2.5 when walking along Oxford Street. There were also higher exposure to other ultrafine particles, elemental carbon, and nitrogen dioxide. As a result, participants with prolonged exposure to air pollution by the streets recorded consistent respiratory effects such as a reduction in the forced expiratory volume in 1 second and forced vital capacity. Respiratory tracts were also inflamed after prolonged exposure, and these effects are more severe for those with moderate asthma.

Of course, having an experimental design as such will lead to a lot more confounding variables — the authors mentioned that the respondents produced symptomatic responses, but other variables such as traffic noise may have played a part in increasing stress levels, which may trigger the onset of asthmatic symptoms too.

Diesel exhaust is also cancerous — according to NHS Choices, WHO’s International Agency for Research on Cancer (IARC) decided to classify diesel exhaust as carcinogenic instead of “probably carcinogenic” in 2012 after there is sufficient evidence that diesel exhaust is a cause of lung cancer.

How bad are diesel exhaust fumes? As a reference, The Daily Mail quoted Kurt Straif, director of the IARC department that evaluates cancer risks, who mentioned that the effects of breathing diesel exhaust is of the same magnitude as that of passive smoking. (So, try not to stay indoors too when there are people smoking in the room!)

Picture of a cigarette. Source: [Last accessed: 26 February 2015]

(And since there are 2 pictures showing a lot of smoke, Diesel Exhaust + Cigarette smoke = Smoky Streets indeed! Can’t resist not mentioning the blog name, haha. Even though the contents of this blog should be much more than just this two topics!)


McCreanor, J., Cullinan, P. Nieuwenhuijsen, M.J., Stewart-Evans, J., Malliarou, E., Jarup, L. Harrington, R. Svartengren, M., Han, I., Ohman-Strickland, P., Chung, K.F., Zhang, J. (2007). ‘Respiratory Effects of Exposure to Diesel Traffic in Persons with Asthma’. The New England Journal of Medicine 357: 2348-2358.

NHS Choices (2012). ‘WHO: ‘Diesel Exhaust Fumes Cancerous”. [Online] Retrieved from: [Last Accessed: 25 February 2015]

The Daily Mail (2012). ‘Diesel exhaust fumes are ‘major cancer risk’ and as deadly as asbestos and mustard gas, says World Health Organisation’. [Online] Retrieved from: [Last Accessed: 26 February 2015]

Some updates on pollution by firecrackers

I posted 3 days ago about the likelihood of air pollution due to fireworks in Beijing during the upcoming Chinese New Year celebrations. Over the next few days, I managed to read some news updates regarding the situation. Indeed, what authorities predicted came true: real time air-quality data from Beijing recorded a sharp jump in PM2.5 levels immediately after midnight. The article on IBT Pulse tells you the whole story with a few pictures that helps illustrate the point, do take a look at the article here!

The South China Morning Post also reports that “the air quality index rose from around 50 to 456 within a few hours, with average PM2.5 levels hitting 413 micrograms per cubic metre – more than five times the national safe standard – at about 1am” (As a comparison, at 20th Feb, 9pm, Singapore’s hourly PM2.5 level is at 57 at the central area, and about the 25 to 40 range for other areas (north, south, east west); the bigger numbers are the overall PSI readings, not the PM2.5 readings. The PM2.5 readings are in a smaller print.)

Source: Screenshot by me!

Source: Screenshot by me!

Hopefully, these updates will provide you with a better picture of the pollution caused by firecrackers!


FlorCruz, M. and Poladian, C. (2015). Chinese New Year 2015 Fireworks: Lunar New Year Celebrations Spur Air Pollution Debate. International Buisness Times. Pulse. [Online] Retrieved from: [Last accessed: 20 February 2015]

Li, J. (2015). Beijing Welcomes Auspicious Snowfall amid heavy pollution over Lunar New Year. South China Morning Post — China. [Online] Retrieved from: [Last accessed: 20 February 2015]

Happy Chinese New Year! (And let’s not forget about pollution)

It’s Chinese New Year soon, and I thought I could have a post about firecrackers. In particular, firecrackers causing pollution (after all, this is a pollution blog!)

Firecrackers are banned in Singapore, but they are still rampant in cities such as Beijing. According to the International Business Times, Beijing’s city government has attempting to cut down firecrackers in the city. While this may not be such a big deal in Singapore (Firecrackers were completely banned since 1 August 1972 when the Dangerous Fireworks Act came into operation), in many other places firecrackers are still seen as part of the Chinese New Year tradition — the loud sounds and lights are believed to ward away evil spirits.

For Beijing’s case now, other than firecrackers being a fire hazard, it is a major source of street pollution with red confetti expected to be scattered across the streets, and with the weather this coming Friday (the 2nd day of the Chinese New Year) expected to be windless, city authorities have anticipated that smoke from firecrackers are likely to persist in the air for a long period of time. These smoke, a result from combustion, contains pollutants such as PM2.5, which is harmful to health. As cited by Wang et al (2007), these fine aerosols which consist of sulfur dioxide, carbon dioxide, suspended particles, and traces of heavy metal poses threats to human health. This study also highlights the problem of air pollution due to these fireworks, by drawing on the chemistry of these fireworks, and found ions, metals and black carbon in aerosol samples. Pollutants were also found to be much higher during the festive season than from other days.

Results from this report tallied with observations over other years; for instance, Channel News Asia reported that on Lunar New Year’s Eve, 2013, the concentration of PM2.5 grew from 150 micrograms per cubic metre to 346 micrograms per cubic metre within an hour due to the fireworks.

While some residents are grateful for the authorities’ decision to cut down on firecrackers, some remained disgruntled. One resident said

“If the authorities are serious about tackling pollution, they should tackle industrial pollution. Setting off fireworks is just a once-a-year event. It is not as bad as industrial pollution.” (Source: CNA)

I felt that this line of reasoning, is unfortunately, problematic. If we have to tackle pollution issues, it takes more than just managing large pollution sources, and it definitely takes more than the authorities to do their job. As citizens, we do have responsibility towards our environment — small actions such as cleaning up after our trash, using environmentally friendly cars if we can afford them, minimising our use of plastic goods, and in this case, reducing the number of firecrackers may not seem to matter, but the effects add up over the long run. Admittedly, cultural practices may be hard to change…

In any case, happy Chinese new year to all and may you all have an auspicious year ahead! 🙂

Happy Chinese New Year!


Channel News Asia (2015). Beijing Air Quality set to worsen as Lunar New Year Approaches. [Online] Retrieved from: [Last accessed: 17 February 2015]

Sim, S. (2015). Beijing Chinese New Year Fireworks spark crackdown over air pollution concerns. International Business Times. [Online] Retrieved from: [Last accessed: 17 February 2015]

Singapore Infopedia (2013).Regulating the use of Fireworks. [Online] Retrieved from: [Last accessed: 17 February 2015]

Wang, Y., Zhuang, G., Xu, C., and An, Z. (2007) The air pollution caused by the burning of fireworks during the lantern festival in Beijing. Atmospheric Environments. 41(2). pp. 417- 431.