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Reading the PSI and PM2.5 data on Singapore haze


  1. PSI measures overall air quality and PM2.5 measures the concentration of smaller particles that can settle in the lungs.
  2. The published PSI and PM2.5 values are 24hr averages and cannot accurately predict the level of air pollution in relation to the haze for our decision making when it comes to our immediate activities.
  3. We can perhaps use the 3hr PSI averages to draw a closer inference on the extent of PM2.5 in the air.
  4. The 24hr PM2.5 report by regions can then help us identify if we are in a “safer” or less “safe” zone for outdoor activities, but should be read together with the 3hr PSI average. The PSI 3hr average (not the 24 hour PSI) gives us a closer assessment of how bad the haze is across the country.
  5. During the haze period, pets may be susceptible to respiratory problems, environment allergies, etc. Be especially careful to monitor senior pets, and pets with known heart or respiratory problems.

Updated 26 Aug 2015:
You can now find the hourly PM2.5 numbers on NEA website here and also on aqicn.org here.
You can find the 24 and 3-hr PSI charts here. You can find historical PSI readings here.

The following article explains the above points in greater detail.

Based on prior experiences with the haze, we know that there are two readings we look at when deciding when it is safe for dog to go out for a walk. These are the PSI index and the PM2.5 index.

Both are important. PSI measures overall air quality and PM2.5 measures the concentration of smaller particles that can settle in the lungs.

PM2.5 measures particles smaller than 2.5 micrometres, about 25 to 100 times thinner than human hair…. Because the PM2.5 travels deeper into the lungs AND because the PM2.5 is made up of things that are more toxic (like heavy metals and cancer causing organic compounds), PM2.5 can have worse health effects.

The NEA (National Environment Agency) has updated their webpages. What they had last year has been changed to reflect their new way of measurement this year.

With effect from 1 April 2014, Singapore will move to an integrated air quality reporting index, where PM2.5 will be incorporated into the current Pollutant Standards Index (PSI) as its sixth pollutant parameter.

PSI is computed based on 24-hour average of PM2.5 concentration levels, among other pollutants. PM2.5 is the dominant pollutant during haze episodes.

The PSI will therefore reflect a total of six pollutants – sulphur dioxide (SO2), particulate matter (PM10) and fine particulate matter (PM2.5), nitrogen dioxide (NO2), carbon monoxide (CO) and ozone (O3). – NEA

The thing is, based on an old channelnewsasia news report last year, we learnt that a PM2.5 value of 40 and above is not safe for dog to venture out. However, in the new computation for the PSI index that incorporates PM2.5, the break point set for the moderate range is 13-55. I suppose the range is expected to average out in the end, but I am rather uncomfortable with the idea that there is always the possibility that I might be out walking the dog in PM2.5 – 50 air. Paranoid much? :P

Now I’m no expert in the area, but since I am uncomfortable with the information I have. I thought it would be better to continue to look at PM2.5 numbers by themselves, despite their inclusion into the PSI measurement. At the same time I decided I should further verify if an average PM2.5 value of 40 is still a good benchmark, since the breakpoint for that average to occur appears to be set at 55.

So exactly what PM2.5 levels are considered safe?

Imagine my surprise when I read a 2013 Sydney Morning Herald report that says there is no such thing.

The evidence comes from 17 high-quality investigations carried out among 312,000 people in nine European countries, according to the paper in The Lancet Oncology.

Unexpectedly, the new study found a cancer risk at every level, and confirmed that the higher the level, the greater the risk.

Every increase of five microgrammes per cubic metre of PM2.5 drove the risk of lung cancer up by 18 per cent.

No safe level of air pollution, smh.com.su

But before we start to freak out, let’s remind ourselves that we are in the short term exposure category (hours and days) where haze is concerned.

Healthy children and adults have not been reported to suffer serious effects from short-term exposures, although they may experience temporary minor irritation when particle levels are elevated. – epa.gov


Air quality standards

Of course life has to gone on, so standards have been implemented as a guide as to what can be considered clean air quality for longer term exposures (years).

EU air quality standards limit … PM2.5 exposure to (a yearly average of) 25 microgrammes per cubic metre.

The UN’s World Health Organisation has guidelines recommending annual exposure limits at 10 microgrammes per cubic metre for PM2.5

Singapore‘s annual target for PM2.5 is also 10 microgrammes per cubic metre

The U.S. Environmental Protection Agency‘s standards for average annual PM2.5 level is 10 to 15 microgrammes per cubic metre.


How does our air compare with the air quality standards?

Since our PSI reports are based on 24hr and 3 hr averages, let’s look at the standards for these.

Singapore follow’s WHO’s interim target for 24hr PM2.5: 25 microgrammes per cubic metre 

The U.S Environmental Protection Agency‘s standards for 24hr PM2.5 is 35  microgrammes per cubic metre 

And if we were to compare these standards to the trends monitoring the slightly hazy conditions we have been experiencing in the past week:-Graph extracted from NEA website last night and this morning – 30 Sep

24hr PM2.5 averages appear to still fall within the limits (based on the standards established by Singapore and US EPA) for all the zones except for the West regions. The air quality is described as moderate, not unhealthy, so depending on the individual you could choose to worry more or worry less.

If it were me, I would be more careful about taking the dog out for long walks during this periods, especially if I live in the West. I probably would be reluctant to take the dog out for the day if the PM2.5 goes beyond 40, and only for short walks when the PM2.5 hovers between 30 to 40 consistently like we are seeing in the west.

West regions for air quality reporting

Lim Chu Kang, Choa Chu Kang, Bukit Panjang, Tuas, Jurong East, Jurong West, Jurong Industrial Estate, Bukit Batok, Hillview, West Coast, Clementi

NEA FAQ page


The difference between the 3hr-average and the 24hr-average PSI

Bearing in mind that the PM2.5 numbers are averages of hourly readings taken over 24 hours, these are only indicative and may not be the best numbers to refer to for up to the hour decisions like “Should I take dog out to run for an hour now?

If we were to look at the 3hr PSI readings, we can see that the average hourly readings do show a greater fluctuation over time.

 And here I’ve overlapped the 24hr readings with the 3hr readings, so it’s easier to see what I am talking about. The 24 hr PSI shows stable moderate levels around the range of 70 and above for the period being monitored across the 5 zones, but the 3hr averages show that nationally there are bigger fluctuations across the day.

Particularly around the 5pm-8pm hours where dogs get their evening walks, the 3hr PSI can actually be pretty high compared to the 24hr PSI being reported by zones.


What is the implication on the 24hr PM2.5?

Extrapolating this to PM2.5 levels, which are the dominant pollutant during haze episodes, is it logical for us to infer that the PM2.5 levels could be similarly inflated if we have the corresponding 3hr readings, instead of the more stable 24hr readings?


So what can we do?

In that sense, the average PSI reading over 3 hrs would give us a more realistic assessment of air quality for decision making over our immediate activities. This is particularly in the case of senior dogs or dogs with known heart or respiratory problems like pugs and bulldogs. They have higher risks and can experience difficulties and require vet attention, even with short term exposures.

Unfortunately, the NEA haze website does not give us the 3hr view for PM2.5 broken down into different zones. So for the truly concerned, it probably makes sense to refer to both graphs now and then and make your own assumptions and conclusions from there.

The two graphs discussed are updated on this NEA page – 24hr PSI, 24hr PM2.5 readings, 3hr PSI readings Trend Graphs. This page is no longer available on NEA website.  Updated 26 Aug 2015.

Updated 26 Aug 2015:
You can now find the hourly PM2.5 numbers on NEA website here and also on aqicn.org here.
You can find the 24 and 3-hr PSI charts here. You can find historical PSI readings here.


Continue reading: Three ways to keep dog safe from haze

stay safe stay healthy


Disclaimer: I am not an expert. And I am just sharing my thought processes here as part of my trying to understand the information on the haze. I will happily be corrected if you are an expert in the area and have relevant advise for us pet owners to understand how better to react to responsible pet ownership during the haze. :)



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  1. savedbydogs

    Sounds like it’s time for doggy air-masks with a filtration system; Donna would rock that look!

  2. We’re in the country and don’t have a clue about that.

  3. We don’t get much haze around here. I think big cities like LA might.

  4. We don’t get haze here really. Maybe on a super hot, super humid day, but even then it isn’t bad. I guess that is a benefit to not living in a large city.

  5. Henry

    I go for 4 km run when psi was 104

    • Yah I see people running still at high PSIs. It’s like some people smoke even when they know it’s not good for them, I guess. :P

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