Furry Brown Dog

Dedicated to the memory of my canine friend…

Why 1-hour and spot PSI/PM10/PM2.5 readings matter

with 2 comments

I have seen a number of arguments against mandating the reporting of hourly spot PSI or PM10/PM2.5 concentration levels. One popular argument  typically goes that spot readings fluctuate wildly and can thus cause panic amongst the general population when readings spike. A 3-hour PSI reading, the argument goes would smoothen out fluctuations and allow the authorities to issue recommendations which remain valid over the course of a day instead of over-reacting to spikes and fluctuations in PSI levels.

However, let’s take a step back and ask yourselves. How many Singaporeans have looked at the 3-hour PSI reading in the past week and then outside the window and doubted the official NEA numbers? How many of us have woken up to a PSI reading of just over 100 then looked out the window to realise the outline of the neighbouring block remains barely visible? Or when the smog suddenly cleared the last few days but the 3-hour PSI reading still hovered above 200?

If I wanted to head outside to the market or hawker centre these few days, the first thing I would do is to look outside the window. Then I would check the PSI levels. If they agreed, I would have little difficulty deciding between putting on the tight-fitting and uncomfortable N95 mask before heading downstairs or simply going there. But what if what I saw outside conflicts with reported PSI values? How would I then decide? My previous post (point 2 here) also highlighted a real-world example of how delayed air quality reporting in Hong Kong resulted in a 10-km race organisers deciding to go ahead with the race simply because the AQI was merely 52 even though skies were clearly hazy. As a result, dozens reported discomfort with 5 runners admitted to hospital for treatment.

Let’s put it bluntly. NEA’s 3-hour PSI reporting methodology results in a 2-hour delay in reporting air quality figures. This 2-hour lag is precisely why so many people are so distrustful of the PSI numbers; it doesn’t reconcile with what they see outside their windows. While there is little reason to believe NEA is fudging PSI numbers, their reporting methodology is basically the source of much public distrust. Digressing a little, recall that not too long ago HDB was defending Singapore public housing prices saying that flat prices are still very much affordable because:

As a result, an average of 15,000 young couples become owners of HDB flats every year. Most of these couples who buy new flats use 20 to 25 per cent of their monthly income to pay for their flats.

Over 80 per cent of new flat buyers pay their housing loans entirely out of CPF, without using cash. While the maximum possible loan tenure for an HDB loan is 30 years, flat buyers can, and many do, choose shorter loan tenures.

Not surprisingly, the MND minister was gone after the next election. And let’s just apply sensible reasoning here. If the air quality in Singapore is such that it improves and degrades very quickly over the span of a few hours does it make sense to try to issue health recommendations over an entire day? Some people on Facebook joked that we should lock the ministers up in a room with PSI 500 for an hour daily and ask them why they are coughing and complaining so much when the average 24-hour PSI is only 50.

Public mistrust in turn breeds suspicion that official figures are made up, incrementally and cumulatively it’s not surprising that more and more people turn to alternative unofficial sources, distrustful of official statistics. Does the Singapore government really want to encourage growing cynicism? Additionally it’s not that NEA would have to do extra work to measure and compute the hourly PM10 figures; as the Straits Times shows here, in order to calculate the 3-hour reading you would need the hourly spot readings.

Lastly, let’s note that health experts have pointed out that Singapore’s haze situation is very different from other countries:

However, doctors pointed out that Singapore’s situation is different from that of, say, Hong Kong or Mexico, where the level of pollution is constant throughout the day.

Associate Professor Philip Eng, a senior consultant in respiratory medicine, said: “In our situation, it is different because the winds and the PSI levels change so drastically. The studies might be looking at health impact based on pollution levels that are more constant … here the PSI in one day can swing from 100 to 400. So it is hard to assess the health impact like this.”

Dr Lim Ing Ruen, an ear, nose and throat surgeon at Mount Elizabeth Medical Centre, felt that neither a 24-hour nor a three-hour measurement will give a very good picture of the health impact.

“People should be assessing the PSI level right before they go out, to see if the level at that particular hour poses a threat. An average figure for the whole day may not be that reflective, as the figures can fluctuate wildly throughout the day. The key is spot PSI.”

Written by defennder

June 24, 2013 at 11:29 PM

Posted in Singapore affairs

2 Responses

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  1. […] Phneah: Not all online criticism are intended to politicize the haze – Furry Brown Dog: Why 1-hour and spot PSI/PM10/PM2.5 readings matter – The Asian Parent: SARS in Singapore: Is it back? – BBC: South East Asia haze: What is […]

  2. サンダル レディース

    kate spade 財布

    September 7, 2013 at 5:50 PM


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