Furry Brown Dog

Dedicated to the memory of my canine friend…

On foreign standards for air quality reporting

with 11 comments

Update: Re-wrote a bit and added screenshot and a few links for clearer picture

Recently the blog The Heart Truths published a post questioning statements made by minister Vivian Balakrishnan that Singapore is probably the only country which reports 3-hourly average PSI with most other countries going by a much longer 24-hour averaging time was accurate.  Within hours, the blog was rebutted by commenters as well as the Ministry of Environment and Water Resources (MEWR) Facebook page rebutting the blogger’s claim in a note here (also as a press release here). While Roy, the blogger has since updated his post, some issues bear addressing. Here they are in no particular order.

1.  The MEWR Facebook note noted that some of the cited air quality indices for the US, Hong Kong and the UK are based on 24-hour averages, contrary to The Heart Truth’s claim that they provided hourly spot readings.

Having reviewed the links, I believe the verdict on the blogger’s and MEWR’s claim is mixed. Let’s look at some examples I found on real time air quality reporting for other countries.

Hong Kong

In Hong Kong’s case it appears that PM10 (RSP) readings are based on 24-hour averages. But what about PM2.5 readings? Some Facebook commenters have pointed to this press release as evidence to show that Hong Kong reports PM2.5 (FSP) hourly readings. Strangely, the table of API subindex pollutants do not include PM2.5 readings. However, PM2.5 readings appear in the hourly pollutant concentration reports here.

What should one make of this? My own interpretation is that PM2.5 pollutant concentration levels, while not (yet?) incorporated in the overall AQI air quality index have been recognised to be sufficiently important to be reported on a spot hourly basis as opposed to a 24-hour rolling average. MEWR on the other hand appears to be correct that PM10 concentrations are reported as a 24-hour rolling average. Oddly, MEWR made no mention of how PM2.5 pollutant concentration levels are reported for Hong Kong. Why?

United States

MEWR’s note cited a technical report by the US EPA (pg 8 here) that PM2.5 and PM10 concentrations are averaged over 24 hours. On this point, MEWR appears to be solid ground especially since the AQI to pollutant concentration calculator explicitly states that PM2.5 and PM10 readings are based on 24-hour averages.

That should settle it right? Not exactly. Given the size of the United States, I was curious to see if state or local level environmental bodies provided spot hourly concentration readings for PM10 or PM2.5 After all federalism requires that each state and local authority are left in charge of micro-managing their own affairs. EPA as a federal agency may not want to mandate that PM10 and PM2.5 pollutant concentrations are reported in spot hourly readings, but each city’s municipal authority may impose that as an additional reporting requirement to give residents a more timely picture.

Let’s take one of America’s largest states (New York) for example. New York state’s Department of Environmental Conservation provides its residents with a real-time hourly spot readings for PM10 and PM2.5 (where available for each monitoring station):

The NYSDEC Air Quality monitoring website allows a real-time view into the ambient air quality database of the New York State Department of Environmental Conservation. In general, data is polled at the top of each hour from each station. It is immediately displayed as it is collected.

For eg. hourly PM2.5 pollutant concentration readings are available for the city of Newburg here (hope the link works). The data provided is regarded too preliminary which necessitates the addition of a disclaimer at the bottom of every page reading:

The information used is the first available data from our air quality monitoring network. The values have not been verified for accuracy or been through the appropriate quality assurance and control validation procedures.

Of course, it isn’t just New York state of course that provides its residents with hourly readings. Another state, Oregon also provides real-time data which comes with the standard disclaimer that it hasn’t been validated as shown here:

Oregon DEQ  Laboratory RealTime Air Quality Data

What can we make of these couple of examples? Firstly while MEWR is technically and literally correct that the federal environmental agencies do not mandate anything more frequent than a 24-hour averaging time period for PM10 and PM2.5 concentration reporting, states can and do go the extra mile in providing real-time data available on their website. In the case of Singapore, which isn’t exactly a large country like the United States, there is essentially no difference between a federal agency and a state agency. NEA plays both roles. If state-level environmental agencies from Oregon and New York can provide real time spot hourly PM10, PM2.5 pollutant concentration updates, why not NEA?


Just to belabour the above point, let’s throw in another example. This time it’s the Environmental Protection Agency of Victoria state, Australia. The relevant page is here (click data readings to view current pollutant concentration values). Note that it explicitly says that PM10 pollutant concentration values are averaged over 1-hour (yup, not 24-hours or 3-hours) :

This bulletin is updated hourly with information calculated on data readings averaged over 8 hours for carbon monoxide and 1 hour for PM10, ozone, nitrogen dioxide, sulfur dioxide and visibility reduction. Detailed information on the calculation of the index is available below.

And of course, like all instant things it comes with a disclaimer that the data is preliminary and has not been vetted.


One last example. The Canadian province of Ontario also provides real time spot hourly PM2.5 pollutant concentration readings here:

Current Pollutant Concentrations

Beautiful ain’t it? If these state-level agencies can provide real time spot hourly PM2.5 and PM10 pollutant concentration readings why can’t NEA do likewise? Food for thought.

2. Commenter cheesemacoroni quoted a report commissioned by HK’s EPD as follows to support MEWR’s claim that foreign standards rely on 24-hour averaging times:

It is worth noting that while the concentrations of most air pollutants are measured by a shorter averaging time (like the 1-hour average) for AQI / API calculations, particulate matter (PM) is averaged over a 24-hour period. This is due to the lack of scientific evidence with respect to the exposure-response relationship for PM over a one-hour period (Cairncross et al, 2007).

However, immediately following the quoted text is the following:

As a result, when PM is the dominant pollutant, the AQI / API system is not responsive enough to reflect a sudden surge in the level of PM, because the index is based on its concentrations averaged over the past 24 hours. There is inevitably a time lag between the rise in concentration recorded at the monitoring stations and the rise in AQI / API readings; this time lag will delay the issuance of health advisories for impending air pollution episodes. An example that highlights this problem is presented in Appendix 1.

What does this tell us? Does it not tell us that the study’s expert authors recognised that a 24-hour averaging time would cause the air quality index to lag behind actual pollutant concentrations which is similar to the critics’ argument that a 24-hour averaging period is too slow? It’s amazing how a little context easily debunks the commenter’s argument. Furthermore, Appendix 1 of the report goes on to illustrate how the organisers of a 10 km race had in Nov 2006 decided to let the race go ahead despite hazy skies on account of the API reading in the locality being a mere 52. As a result, 43 people suffered from discomfort with 5 being admitted to hospital for treatment. The study’s authors go on to note that, in complete opposite to the commenter’s position that:

It is logical to assume that the organizers would postpone or cancel the race if they are aware of the rapid rise of the RSP concentrations. However, the use of the hourly API at Yuen Long (which was 52 and representative of the past 24-hour average concentration of RSP) as evidence that the air quality of Yuen Long was acceptable at the time of the race was a mistake and probably contributed to this incident. Instead, the hourly RSP concentration reported at 8am would much better information on the air quality to the organizers.

Which is precisely what supporters of an hourly spot PSI reading are saying. The study as quoted actually supports their position.


An NUS PhD student, Jeremy Chen has done a bit of mathematical modelling and determined that NEA’s 3-hour PSI averaging cannot possibly be based on a simple moving average model because in mathspeak, the error rate (the estimated spot PSI values tend to oscillate) grows with time. He proposes a weighted average model which has been implemented on this website which appears, prima facie to work quite well.


Written by defennder

June 24, 2013 at 12:58 AM

11 Responses

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  1. Hi,

    Thanks for this article! It is really good!


    My Right to Love

    June 25, 2013 at 12:48 AM

  2. Minor comment on your addendum: Have seen Jeremy Chen’s article, and have also tried weighted moving average myself before that for my own hourly PSI spot charts. While it gives a smooth curve that does not oscillate as much, it is unlikely to be the reality on the ground, where fluctuations in values are expected to be the norm. Besides, I have it from the horse’s mouth (NEA) that their averaging methodology is not weighted.


    June 25, 2013 at 3:27 PM

  3. Under haze conditions, NEA’s data shows that PM2.5 is consistently higher than PSI. Official health advisories take into account both the 24-hour PSI and the 24-hour PM2.5, *whichever is worse*. If that is the case, why isn’t the higher level advisory “Everyone should avoid all physical activity outdoors” issued when PM2.5 exceeded 150? (http://childrenofthehaze.appspot.com/)


    June 25, 2013 at 11:55 PM

  4. Please see http://www.diy.sg/compare-PSI-and-AQI-pm10-pm2-5.html
    for comparisons between PSI (PM10), AQI (PM10), AQI (PM2.5)

    Based on 24-hr PM10 data extracted from NEA website.

    AQI (PM10), and AQI (PM2.5) are calculated.

    Lim Wee Cheong

    June 26, 2013 at 2:12 AM

  5. Harin, I think http://www.quantumblah.org/?p=1414 got the correct solution.

    Instead of using the 3-hr PSI figures directly, they have to be first converted to
    PM10 (microg/m3). After the averaging calculations are done, convert back to PSI.

    Lim Wee Cheong

    June 26, 2013 at 2:20 AM

  6. FBD

    If the NEA decides to issue such stats with a big disclaimer.
    Guess what the average reaction will be, “how come cannot double confirm”


    June 26, 2013 at 10:06 AM

  7. NEA might be publishing hourly reading after forceful nudging from Netizens. Looks like NEA is not so irresponsible by preempting its backwardness and unwillingness to upgrade with excuses such as if there’s a big disclaimer, response will be “how come cannot double confirm”.


    June 26, 2013 at 11:01 AM

  8. Wee Cheong, thanks for the link to Quantum Blah. His reasoning is correct, and his suggested numbers for 21 June are consistent with my own hourly calculations.


    June 26, 2013 at 3:26 PM

  9. […] blog, Furry Brown Dog further opined in an article (link) that, “Having reviewed the links, I believe the verdict on the blogger’s and MEWR’s […]

  10. Hi Furry Brown Dog,

    I just want to really thank you for writing this article – it has provided me with greater insights and have allowed me to as well develop a stronger article, from your understanding.

    Thank you for this.

    I have written a rebuttal and response to the MEWR and have incorporated parts of your article into the response:


    Thank you very much. 🙂


    My Right to Love

    June 27, 2013 at 12:15 AM

  11. model model tas wanita

    September 26, 2013 at 7:43 AM

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