Why Singapore’s air quality index won’t tell you what the haze is like right now

Singapore uses its own Pollutant Standards Index (PSI), which tells you what the air quality has been like, on average, over the past 24 hours. It also differs from the more stringent U.S. Air Quality Index; at times, Singapore’s PSI will say the air is “moderate” while the U.S. AQI deems it unhealthy for sensitive groups like children and the elderly

September 23, 2019 Benjamin
Why Singapore’s air quality index won’t tell you what the haze is like right now

The spreading of haze across South-East Asia from ongoing wildfires in Indonesia has fired off a controversy over the different ways air quality is reported.

Most polluted cities ranking
On Sunday, Sept. 22, Singapore topped IQAir AirVisual’s most polluted major cities rankings

In Singapore, the National Environment Agency uses the Pollutant Standards Index (PSI). It has five categories from “Good” to “Hazardous.”

Singapore Pollutant Standards Index
Singapore’s Pollutant Standards Index

In the early hours of the morning of Sept. 18, when the haze started to thicken, the PSI reported that most Singapore regions were in the “Moderate” category, with an overall value at 1 a.m. of 90-102. At the same time, AirVisual reported an Air Quality Index value (more on that later) of 134, or “Unhealthy for Sensitive Groups.”

There are two main reasons for the difference. The first is the timeframe.

Singapore’s National Environment Agency updates the PSI on its website every hour, but each new reading is actually an average of the previous 24 hours. This means that the PSI figure of 90 (Moderate) at 1 a.m. was an average of the previous 24 hours. It doesn’t tell you what the air was like at 1 a.m.

In contrast, IQAir AirVisual reports Singapore air quality data every hour, or in real-time. In this way, if there is a sudden dangerous spike in air pollution, the value will reflect this. This information is then available to you immediately, and you can act to protect yourself, for example, by wearing a mask, limiting outdoor activities or running an air purifier.

The second difference has to do with the index, or scale, used.

IQAir AirVisual uses the U.S. Air Quality Index, which is one of the world’s most stringent. When Singapore’s PSI says the air is “moderate,” the U.S. AQI can deem it “unhealthy for sensitive groups,” including children and the elderly.

The US AQI and the Singaporean PSI are both calculated using the same six pollutants, and both use the dominant pollutant – the pollutant with the highest Index value – to determine the overall Index value. During haze, particulate matter 2.5 (PM2.5) is usually the highest and therefore determines the overall AQI or PSI value most of the time.

Singapore Pollutant Standards Index versus US Air Quality Index
Singapore and the U.S. calculate Index categories according to different breakpoints

When PM2.5 density is between 13-55 micrograms per cubic meter (µg/m³), the Singapore PSI says the air quality is “Moderate.” But the US AQI considers roughly half of the Singapore “Moderate” category (35.5-55.4 µg/m³) to be “Unhealthy for Sensitive Groups,” offering a warning to children, the elderly and people with heart and lung disease.

Using the U.S. AQI, IQAir AirVisual reports data from air monitoring stations operated by the Singapore National Environment Agency, the National University of Singapore and its own air quality monitors.

IQAir AirVisual validates all data before publishing it on its platform, so no anomalous readings get through. That means that if a faulty government air monitor anywhere in the world suddenly reports unusually high readings, AirVisual’s unique A.I. system would spot the fault and it would not affect that city’s overall air quality reading.

To have access to the fastest, most up-to-date information, follow real-time Singapore haze info on IQAir AirVisual’s website or air quality app.


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