askST: Why are Singapore's PSI readings so different from those used elsewhere, and which are correct?

The skyline at around 12pm on Sept 19, when the 24-hour PSI reading in the southern part of Singapore was 128. The 1-hour PM2.5 reading then was 23.
The skyline at around 12pm on Sept 19, when the 24-hour PSI reading in the southern part of Singapore was 128. The 1-hour PM2.5 reading then was 23.ST PHOTO: LIM YAOHUI

SINGAPORE - After three years of clear skies, the haze is back - and the National Environment Agency's (NEA) air quality monitoring network is in full gear.

As air pollution descended on Singapore over the past week, many have questioned whether the figures on the NEA's website have downplayed the severity of the haze.

Instead of relying on readings provided by the NEA, some turned to other air quality readings, such as the World Air Quality Index (AQI), which is compiled by a non-profit organisation headquartered in Beijing based on measurements provided by affected countries.

During hazy episodes over the past week, people have pointed out that while the AQI reflected unhealthy levels, the NEA's 24-hour PSI showed "moderate" air quality.

Neither is incorrect.

The discrepancy in both readings, the NEA told The Straits Times on Thursday (Sept 19), boils down to the way the readings are calculated.

"There are no international guidelines on how air quality indices should be computed," said an NEA spokesman. "Countries adopt different index systems based on their local needs and circumstances."

Singapore perspective

The NEA provides two air quality readings - the 24-hour Pollutant Standards Index (PSI), and the 1-hour PM2.5 readings. Both serve different functions.

Forecasts of the 24-hour PSI should be relied on when planning future activities, such as whether one should go for a picnic the next day.

The 1-hour PM2.5, on the other hand, is meant to gauge immediate activities, such as whether going for a jog is the best option now.

This hinges on the time period over which pollutants are measured.

The 24-hour PSI measures the average concentration levels of six component pollutants over the given period.


This includes PM10 and PM2.5 - which are particulate matter of different sizes - sulphur dioxide, nitrogen dioxide, ozone and carbon monoxide.

As it is an average measured over 24 hours, the reading could remain low even if the air currently appears milky.

The NEA also provides forecasts of what the PSI would be like in the day ahead.

These forecasts communicate what the air quality is expected to be in the next 24 hours and can be used for planning activities ahead of time.

The 1-hour PM2.5 reading, however, measures the average hourly concentration of only one of the six pollutants: PM2.5. These refer to tiny particles that are 30 times smaller than the diameter of a strand of human hair - small enough to be breathed deep into the lungs.

As PM2.5 is the dominant pollutant during periods of transboundary haze, and has the most influence over the 24-hour PSI readings, it provides a good indicator of the current air quality, said the NEA.

However, as Singapore has experienced over the past week, weather conditions can be fickle.

Changing wind patterns, for instance, could make skies hazy one moment and clear the next, which happened over the weekend.

After air quality entered unhealthy levels last Saturday, Singapore enjoyed a brief reprieve from the pollution caused by the fires in Sumatra on Sunday, when southerly winds switched to blow from the south-east instead.

As a result, the 1-hour PM2.5 concentration levels can also fluctuate over the course of a day, the NEA noted.

This volatility is accounted for in the 24-hour PSI reading, since it measures the average concentrations of pollutants over 24 hours.

Different methodologies

In calculating Singapore's 24-hour PSI, the NEA uses data from a network of air-monitoring sensors islandwide. These measure concentration levels of the six pollutant particles that Singapore uses to calculate its PSI.

For each pollutant, a sub-index is calculated on a scale from 0 to 500 based on concentration levels.

The 24-hour PSI is the average of the highest sub-indices in each of the past 24 hours.

In comparison, the website computes its AQI as an adaptation of the United States Environmental Protection Agency's (EPA's) reporting system. The website uses the latest particulate matter data instead of the 24-hour average data.


The US EPA calculates its AQI from five major air pollutants regulated by the US government's Clean Air Act. These are ozone, particle pollution, carbon monoxide, sulphur dioxide and nitrogen dioxide.

Similar to the PSI, the AQI is tabulated by first converting raw measurements into a separate AQI value for each pollutant. The highest of these AQI values is then reported as the AQI value for that day, said the EPA.

The NEA clarified that the website relies on the average PM2.5 concentration data that is posted every hour by the agency.

"However, does not follow Singapore's system of computing PSI. Instead, it computes an air quality index using the United States Environmental Protection Agency Air Quality Index (AQI) reporting system."

This difference in methodologies is why the final PSI or AQI readings can be different.


The NEA rolled out the 1-hour PM2.5 reading in 2016, in the aftermath of the severe haze crisis the year before and in response to confusion over 24-hour and 3-hour PSI readings then.

The 3-hour PSI has since been scrapped in favour of the 1-hour PM2.5 readings.

PM2.5 particles are harmful to human health.

They can become trapped deep in the lungs and are tiny enough to pass through linings into the bloodstream. Long-term exposure on a regular basis has been linked to increased risk of death from heart and lung complications such as lung cancer or heart disease.

Based on the NEA's classification of 1-hour PM2.5 readings, concentrations range from "normal" to "very high".

The range of 0-55 micrograms (mcg) per cubic m, for instance, will be described as "normal", while anything above 251 mcg per cubic m will be described as "very high".

On a regular, non-hazy day, the maximum concentration of PM2.5 is usually between 20 and 35 mcg per cubic m. It starts becoming a serious problem when the numbers hit 100, and dangerous when they exceed 200.

To put things in perspective, on a day when PM2.5 levels hit 100 mcg per cubic m, a person will take in around 1,100 mcg of these pollutants if he stays outdoors throughout the day. This is much less than the amount of PM2.5 pollutants a smoker will inhale for every cigarette consumed, which is in the range of 10,000 mcg and 40,000 mcg.

However, the NEA said it is not pegging 1-hour PM2.5 readings to health advisories of "good", "moderate" or "unhealthy" levels because the health effects of exposure to particulate matter have been based on 24-hour measurements.

The spokesman added that existing studies demonstrate good correlation of health effects with 24-hour PM averages.

"While some organisations and websites compute indices for hourly air quality levels, such figures are not supported by any studies or health findings."

•Not sure if something is fake news? Readers can send an e-mail with their questions and a link to the suspect article to

•Reports published can be found on the ST website under a special "fake news debunked" section at