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.


Hazy Days

hdb clouds


Geography has been in the news again. Singapore has been, in recent days, experiencing its worst haze in two decades due to forest fires in Sumatra, Indonesia. The haze is a prime example of how Geography has an impact on public health that is not always easy to alleviate quickly, especially when not many contingency plans exist on the part of the Government. This, in turn, has led people to call for policy planners to place greater emphasis on preparing for foreseeable environmental problems such as the annual transboundary haze.

The haze, itself, is a result of a combination of atmospheric conditions involving the South Westerlies (surface winds caused by the deflective effect of the Coriolis force and other forces during the South West Monsoon season). Moreover, the orographic effect provided by the Sumatran highlands has brought about anomalous dry conditions in Singapore, permitting the haze to persist.

Much has been said about who is to blame and the public seems to be obsessed with being aware of the Pollutant Standards Index (PSI) levels on an hourly basis. While the government has pledged to find long-term solutions to the issue of the haze, not many concrete policies have been announced yet.

The following is a letter that I wrote last year in JC2 with two of my classmates in Temasek Junior College as part of a General Paper assignment on the transboundary haze. We were tasked to write a letter to a minister to suggest and evaluate solutions to the haze. The letter, of course, was never sent as that was not a requirement of the assignment. Here it goes:


Tan Si En

Yap Zhi Jiun

Yudhishthra Nathan

Temasek Junior College

22 Bedok South Road

Singapore 469278


16th July 2012


Dr Vivian Balakrishnan

Ministry of the Environment and Water Resources
40 Scotts Road
#24-00 Environment Building
Singapore 228231


RE: Mitigating Indonesian Forest Fires

Dear Sir,

We refer to your reply in Parliament to a question tabled by Nominated Member of Parliament Mr Nicholas Fang on the 14th of May 2012 regarding the outlook for the haze situation in Singapore and Southeast Asia, caused by the forest fires in Indonesia. While we commend the Government’s existing efforts in tackling the problem, we would like to suggest other ways in which the Ministry could go about addressing the haze issue arising from the annual Indonesian forest fires. It is our belief that a two-pronged approach would be the best way in which problems such as the haze can be nipped in the bud. On the one hand, prevention of the forest fires altogether would likely prove to be a worthwhile cause in the long-term as this would eliminate the source of the haze. This may be done by discouraging slash-and-burn techniques employed by farmers and timber companies. On the other hand, however, we recognise taking measures to contain the fires would additionally reduce our susceptibility to being affected by the haze. Hence, all stakeholders, from the companies to the individuals that cause the fires, need to be engaged in our efforts to improve environmental conditions while maintaining a good diplomatic relationship with Indonesia.

Companies carrying out their operations in Indonesia have a significant role in contributing to the clearing of the Indonesian forests through the forest fires. Since Singapore is Indonesia’s third largest trading partner and many of those companies have their headquarters based in Singapore, Singapore could review trading regulations such that the timber companies are discouraged to carry out their activities through ecologically unfriendly methods, especially the clearing of forests by burning. Singapore could reduce corporation and import taxes for timber companies which are able to provide evidence of programmes that they have implemented to prevent the burning of the forests. Through such incentives, companies such as Cargill with palm operations in Indonesia would make an effort to use methods other than burning to clear the forests. These new regulations could reduce the occurrence of forest fires set by companies, ultimately reducing possibilities of haze affecting Singapore, hence reducing tension between Singapore and Indonesia due to its non-confrontational approach. The potential loss in revenue from the taxes is a worthwhile sacrifice in exchange for the improvement of environmental conditions.

In the case where preventive measures are not as successful as expected and the conflict between Singapore and Indonesia worsens, mediation should be one of the approaches to relieve the tension between the two countries. A mediation programme should be set up by ASEAN whereby all ASEAN member countries will be required to attend mediation sessions when conflicts arise due to differing opinions on matters regarding haze, forest fires and other environment-related issues. This mediation programme would be complementary to the ASEAN Transboundary Haze Pollution Agreement where all nations are required to reduce haze pollution as well as the ASEAN Vision 2020 towards a cleaner and greener ASEAN. Mediation is important in maintaining the relationships of countries within ASEAN since it is also a non-confrontational approach and may not significantly affect or worsen relationships between the countries. Furthermore, ASEAN can be resourceful in gathering solutions from all member countries so that the problem of haze and forest fires within the region may be effectively resolved. However, mediation does not solve the haze problems in the short term as it is a slow and bureaucratic process, especially if some nations are uncooperative. As such, even though mediation can be used to solve conflicts that arise due to haze problems, other more direct approaches on handling the matter should be implemented as well.

Besides mitigating forest fires through the use of bilateral and regional platforms, international Non-Government Organizations such as the World Wide Fund for Nature (WWF) could be an excellent platform in raising awareness on the effect of forest fires among the poor subsistence farmers in Indonesia. The Ministry of Environment and Water Resources in Singapore could collaborate with the Indonesian Ministry of Environment to initiate programmes to train volunteers from these NGOs in educating farmers. Such an education programme would entail teaching the farmers about the control and management of forest fires to prevent their spread, the suitable period of burning if they have no choice but to continue burning as well as the dangers of dumping flammable materials in the forests. NGOs could also designate a week in the year to be an Anti-Forest Fire Week – which can they can slowly expand over months, perhaps – to curb forest fires. Besides this, NGOs could join hands with the local farmers in an effort to carry out afforestation. Since these subsistence farmers have been carrying out slash-and-burn when their previous farmland had been deemed infertile after prolong cultivation of crops, NGOs could encourage farmers to fallow their land by ploughing and leaving the land unseeded and uncultivated for a season or more. During the period of fallowing, farmers could take up other jobs such as helping in afforestation efforts. This will not only reduce the occurrence of forest fires but also inculcate responsibility and good environmental values in the farmers. Even though the actions of poor subsistence farmers account for only 5% of the forest fires annually, it is of utmost importance to raise their awareness of the negative effects that forest fires can have regionally.

In conclusion, it would be beneficial to recognise that there are several ways in which the issue of forest fires can be dealt with in a sustainable manner that will not cause conflict between Singapore and Indonesia. By assuring Indonesia that the Government of Singapore is prepared to render assistance to them in various ways, our strategic partnership with Indonesia can only be strengthened. We hope that you will seriously consider our proposals and we look forward to receiving a favourable reply.

Thank you!


Yours sincerely,

Tan Si En

Yap Zhi Jiun

Yudhishthra Nathan

(Civics Group 25/11 of Temasek Junior College)