Quantifying Air Quality – The Pollutant Standards Index (PSI)


Most Developed Countries (DCs) regulate environmental standards by putting caps on the emissions of criteria pollutants that empirical evidence has shown to be harmful to human health1. The PSI is one way to quantify air quality and has been adopted by Singapore to be the authoritative indicator of air quality in the country. Once quantified, it also aids in informing the public about the risks posed by different levels of air quality by affixing health advisories to each level.

Since 2013, every incident of transboundary haze brings with it a period during which the PSI becomes an oft-cited figure amongst ordinary Singaporeans in our daily lives. It seems as if when communicating about air quality with one another, merely saying that air quality is unfavourable would be unsatisfactory. A means by which we can put a number to air quality gives people a sense of scale as well as benchmarks for comparison as air quality usually varies as often as throughout the course of a day.

How is the PSI calculated?

The concentration of each criteria pollutant is compared to a standard index which is used to determine the sub-index assigned to the most recent recording of the concentration of the pollutant in question. The highest sub-index, whichever criteria pollutant that may represent, is taken as the PSI level2.

The PSI, as it was calculated before April 2014, took into account five criteria air pollutants. These were SO2, nitrous oxides, CO, PM10, and O3. The inclusion of PM2.5 occurred in 20142 when the National Environment Agency was of the opinion that a greater focus on PM2.5 was necessary to better reflect reality on the ground in the air as PM2.5 are finer particles that pose a higher threat to human respiratory health as compared to the more generalized pollutant known to us as PM10.

Was the inclusion of PM2.5 as a criteria pollutant in April 2014 necessary?

Velasco and Rastan (2015)3 of the Singapore-MIT Alliance for Research and Technology and the National University of Singapore respectively, certainly seem to think so. A paper published by the pair in 2015 noted that “the real hourly pollution levels” during the haze incident that year had still been unrecorded. This was in no small part attributable to the fact that before 2014, the only indication of PM2.5 in the atmosphere was the PM10 reading. This was in spite of the fact that during the episode, it was shown that the reported PM10 levels had been an under-representation of the significantly poor quality of air that existed in reality at the time. The methodology undertaken by Velasco and Rastan was to use newly collected information on PM2.5 concentrations since April 2014 and derive statistical models by which they could predict with an insignificant degree of uncertainty the actual hourly PM2.5 during the episode in mid-2013. Their findings indicate that these values could very well have been “twice the maximum 24-hour moving average reported by the authorities.”3

The findings of Velasco and Rastan seem to provide credence to the notion that incorporating PM2.5 as a new criteria pollutant measured by the PSI was a good move on the part of the local authorities. While it seems intuitive that more needs to be done to overcome the problem of the lag time in the official PSI readings published by NEA, refining the methods of measurement of the PSI seems to be a step in the right direction.


[1] Hamilton, S.F. & Requate, T. (2012). Emissions standards and ambient environmental quality standards with stochastic environmental services. Journal of Environmental Economics and Management, 64, 377-389. Retrieved from http://ac.els-cdn.com/S0095069612000538/1-s2.0-S0095069612000538-main.pdf?_tid=694d5626-7df9-11e5-ae0e-00000aacb35f&acdnat=1446094767_6ae8a0a14e35f289d579711c6397edb6.

[2] National Environment Agency. (2015). PSI. [Online]. Retrieved from http://www.nea.gov.sg/anti-pollution-radiation-protection/air-pollution-control/psi/psi.

[3] Velasco, E. & Rastan, S. (2015). Air quality in Singapore during the 2013 smoke-haze episode over the Strait of Malacca: Lessons learned. Sustainable Cities and Society, 17, 122-131. Retrieved from http://ac.els-cdn.com/S2210670715000463/1-s2.0-S2210670715000463-main.pdf?_tid=fd72d6f8-7dfb-11e5-b248-00000aab0f26&acdnat=1446095874_7c02d98301a4465fb7cb71472bc5fa0a.


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