Singapore’s 3-hourly Pollutant Standards Index (PSI) smashed the previous record of 371 set on Thursday (20 Jun) to hit a new high of 401 yesterday (21 Jun) at 12pm.

On Wednesday, the record was 321, set at 10pm. On 3 consecutive days, the PSI set 3 records which means the haze is not only the worst in Singapore’s history, it is getting worse by the day!

To add insult to injury, the National Environment Agency (NEA) in charge of measuring the amount of pollutants in the air appears to be ’smoking’ (pun intended; Singlish for bluffing, deceiving, misleading) Singaporeans.

Using PM10 instead of PM2.5 to calculate PSI

NEA currently bases its “PSI readings” on PM10 only. As it says on its website, “The 3hr PSI readings are calculated based on PM10 concentrations only” (‘NEA’s PSI webpage‘).

What is PM10? Airborne pollutants come in big sizes and small. The big particles are between 2.5 and 10 micrometres in size i.e. 25 to 100 times thinner than a human hair. These particles are called PM10 (pronounced “P M ten”) which stands for Particulate Matter up to 10 micrometres.

Particulate matter – also known as atmospheric particulate matter or simply particulates – are tiny pieces of solid or liquid matter associated with the earth’s atmosphere.

The symbol for micrometre is µm (a symbol found on NEA’s website). 1 micrometre or 1 micron is one-millionth of a metre.

PM10 is also called “respirable suspended particle” or RSP since RSP are particles with a diameter of up to 10 µm.

What is PM2.5? You guessed it, PM2.5 (pronounced “P M two point five”) as in Particulate Matter up to 2.5 µm in size are the relatively small airborne pollutants.

PM2.5 is also known as fine particles since fine particles have a diameter of 2.5 µm or less.

Difference between PM10 and PM2.5

Fine particles i.e. PM2.5 are lighter and stay in the air longer and travel further than PM10. PM10 can stay in the air for minutes or hours while PM2.5 can stay in the air for days or weeks. PM10 can travel as little as 1 km or as much as 50 km. PM2.5 can travel much further – many hundreds of kilometers.

Here is the important question. Which is more dangerous: PM2.5 or PM10?

When you inhale, you breathe in air along with any particles that are in the air. The air and the particles travel into your respiratory system i.e. your lungs and airway. Along the way, the particles can stick to the sides of your airway or travel deeper into your lungs.

The further the particles go, the worse the effect.

Bigger particles are more likely to stick to the sides or get wedged into one of the narrow passages deep in the lung. Smaller particles can pass through the smaller airways. Because PM2.5 travels deeper into the lungs and is made up of substances that are more toxic e.g. heavy metals and cancer causing organic compounds, it is far more dangerous than PM10. PM2.5 particles are so fine they can even enter the bloodstream.

So, because PM2.5 can penetrate deep into the lungs and is more toxic, measuring it is considered a more accurate reflection of air quality than simply measuring PM10.

The PSI readings that are updated every hour, on the hour, on NEA’s website do not take into account PM2.5 but are based solely on PM10. The hourly PSI readings that appear in the media, such as the screaming headline in Thursday’s (20 Jun) edition of The Straits Times: “RECORD PSI AT 10 PM 321 Plans in place if haze worsens” are, make no mistake, PM10 readings.

Is PSI obsolete?

PSI is just a name for an air quality index. The name is not as important as the computation behind it. In Singapore’s case, the computation is obsolete. Singapore’s PSI is based on an American model developed in the late 60’s. In 1999, with advances in technology and knowledge, the Americans replaced the PSI with the Air Quality Index (AQI) to incorporate new PM2.5 and ozone standards.

The Chinese experience

To illustrate how unpopular and antiquated the PSI is, consider that even China has switched to providing PM2.5 AQI readings [Link].

In the past, China gave only PM10 PSI readings, which the Chinese citizens rejected because they knew from the air they were breathing that the reading had to be much higher. So the Chinese people turned to the website of the American Embassy in Beijing for both hourly PM2.5 readings and hourly AIQ numbers based on those readings. This caused the Chinese government no end of embarrassment, so finally they too issued PM2.5 readings and AQI numbers according to their interpretation. Beginning early 2012, hourly air quality updates are now available online for more than 70 Chinese cities.

To be sure, the Chinese AQI interprets data somewhat less stringently than the American AQI, but it is still way better than the relic which is the PSI for the simple reason that the AQI incorporates PM2.5 whereas the PSI does not.

The Chinese experience mirrors what Singaporeans felt on Thursday morning (22 Jun) when they knew instinctively that the PSI could not be so low (‘Netizens accuse NEA of grossly understating PSI readings‘).

So this is the first way NEA ’smokes’ Singaporeans, by using an antiquated and inferior air quality indicator.

Giving 3-hour average numbers instead of 1-hour spot numbers

The second way NEA ’smokes’ Singaporeans is by giving out 3-hour average numbers instead of 1-hour spot numbers. This has the effect of smoothening out PSI numbers so that the real high is never known to the public. When the PSI hit a supposed high of 401 at 12pm yesterday (21 Jun), you can bet your last N95 mask that the real number was even higher.

How much higher you might ask? For the answer, we have to look at the actual 3-hour PSI posted on NEA’s website and study the “math” behind it.

For the mathematically inclined


A = (a + b + c)/3 where A = 3-hr PSI at 12pm, a = 1-hr PSI at 12pm, b = 1-hr PSI at 11am, c = 1-hr PSI at 10am

B = (b + c + d)/3 where B = 3-hr PSI at 11am, b = 1-hr PSI at 11am, c = 1-hr PSI at 10am, d = 1-hr PSI at 9am

C = (c + d + e)/3 where C = 3-hr PSI at 10am, c = 1-hr PSI at 10am, d = 1-hr PSI at 9am, e = 1-hr PSI at 8am

D = (d + e + f)/3 where D = 3-hr PSI at 9am, d = 1-hr PSI at 9am, e = 1-hr PSI at 8am, f = 1-hr PSI at 7am

E = (e + f + g)/3 where E = 3-hr PSI at 8am, e = 1-hr PSI at 8am, f = 1-hr PSI at 7am, g = 1-hr PSI at 6am

NEA is very sneaky because if you want to calculate the 1-hour PSI at any time (what every Singaporean wants to know) from the above, you will find it is mathematically impossible.

A, B, C, D, E are known values, being “PSI readings” published by NEA every hour on the hour. However, a, b, c, d, e, f, g – the 1-hour PSI values – are all unknowns. To calculate exactly the 1-hour PSI values i.e. a, b, c, d, e, f, g, there must be at least as many simultaneous equations as there are unknowns. Unfortunately, there are too few simultaneous equations and too many unknowns. For example, if you take the first 3 equations above involving A, B and C, you get 3 equations and 5 unknowns; if you take all 5 equations involving A, B, C, D and E, you get 5 equations and 7 unknowns. So, what Singaporeans call “the real PSI” remains a secret.

To prove “the real PSI” remains a secret, let us assume that the 3-hour PSI is a constant 180 throughout the day. It is tempting to think that the 1-hour PSI must also be constant i.e. 180 throughout, but no, it could be that as well as a recurring 170, 190, 180, 170, 190, 180, 170, 190, 180, etc.

Nevertheless there is useful information to glean from all the 3-hour PSI readings, such as:

A – B = (a-d)/3

B – C = (b-e)/3

C – D = (c-f)/3, etc

Now C – D = (c-f)/3 implies c – f = 3(C – D). This is where it gets interesting.

Consider the 3-hour PSI readings for yesterday morning (21 Jun) from 4am to 12pm. They are 104 (4am), 96 (5am), 94 (6am), 111 (7am), 158 (8am), 256 (9am), 367 (10am), 400 (11am), 401 (12pm).

Substituting C = 367 and D = 256 into c – f = 3(C – D), we get c – f = 3(367 – 256) = 3(111) = 333. This means there was a big jump in the 1-hour PSI of exactly 333 from 7am to 10am. Although we do not know the exact 1-hour PSI value for 7am (which we have seen is impossible to calculate), since everyone knows the sky was somewhat hazy, we can safely assume it to have been about 138 which means the 1-hour PSI for 11am was 138 + 333 = 471!

So, the 1-hour PSI readings for yesterday morning were likely in the region of 87 (4am), 90 (5am), 105 (6am), 138 (7am), 231 (8am), 399 (9am), 471 (10am), 330 (11am), 402 (12pm). This means the “real PSI” peaked around 471 around 10am!

Data Removal

The third way in which NEA ’smokes’ Singaporeans is by denying them information.

If you visit NEA’s website just after midnight, you will be shocked to find just one 3-hour PSI reading staring you in the face. If you want to know what the PSI reading was an hour ago at 11pm, or 2 hours ago at 10pm, sorry hor, who ask you to click so late, why you never click earlier, you die your business.

You search in vain for the missing data but alas, it has left the public domain and is safely in the pocket of the PAP. Old PSI readings are not archived or cached. This is to emasculate Singaporeans, for knowledge is power.


The fourth way NEA ’smokes’ Singaporeans is by confusing them.

Look at NEA’s PSI webpage. See how the large blue words “PM2.5 Readings” take almost centrestage?

You would think from a quick glance that the numbers beneath “PM2.5 readings” are… PM2.5 readings! How deceptive!

It is only upon reading the “fine print” after the numbers that you realise they are “PSI readings based on PM10 concentrations only”.

Another way NEA sows confusion is by calling PSI numbers “PSI readings”. Does NEA even know what it is talking about, one wonders.

Do PM Lee and Ministers Shanmugam and Vivian know what they are talking about? Or are they all charlatans?

PSI is not a reading. PSI is a number, a value, a calculation derived from a formula and based on measurements or readings.

The amount of PM2.5 or PM10 in the air is a reading. The amount of sulphur dioxide, carbon monoxide, nitrogen dioxide and ozone in the air, they are readings. These five readings or measurements are periodically fed into the AQI or PSI formula to obtain the AQI or PSI number, value or figure.


The American AQI is the gold standard of air quality indicators. It takes into account hazardous PM2.5 particulates on an hourly basis and interprets the data stringently. Singapore should junk the outdated PSI asap and adopt the latest American model.

For now though, as long as the PAP is sleeping, Singaporeans will have to be content with the PM2.5 values that NEA publishes 3 times a day – at 8am, 12pm and 4pm – and figure out how dangerous or safe the air is using this excellent AQI Calculator.