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]


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