SINGAPORE: As hazy conditions persist in Singapore, monitoring air quality has likely become more routine for residents, organisations and institutions.
The country announced on Friday (Oct 9) several measures it would take and consider, such as distributing N95 masks and closing schools, if the 24-hour Pollutant Standards Index (PSI) hits certain levels.
The National Environment Agency (NEA) provides two key indicators for Singapore’s air quality - the 24-hour PSI and the 1-hour PM 2.5 reading.
But a quick search online will turn up other figures as well.
Last week, several international news outlets reported that Singapore topped IQAir's live ranking of the world's most polluted major cities.
The Swiss air quality monitoring platform tracks pollution levels using a modified version of the US Air Quality Index (AQI), known as US AQI+, rather than Singapore’s PSI or PM2.5 readings.
It applies US AQI breakpoints, which mark the boundaries between air quality categories, to hourly pollutant measurements.
So why do these readings differ, and which ones should people in Singapore pay attention to?
What do the different air quality indicators mean?
The AQI is a general system that considers up to six main pollutants: PM2.5, PM10, carbon monoxide, sulfur dioxide, nitrogen dioxide and ground-level ozone.
According to IQAir’s website, AQI is calculated differently around the world, with China and the US having the two most widely used systems.
The AQI and Singapore’s PSI follow a similar calculation process, said Professor Steve Yim, director of the Centre for Climate Change and Environmental Health at Nanyang Technological University.
Pollutant concentrations are averaged over specified running periods and converted into sub-index values. The highest sub-index among the pollutants considered then determines the overall index value.
Singapore’s PSI, for example, takes into account six pollutants: PM2.5, PM10, sulphur dioxide, carbon monoxide, ozone and nitrogen dioxide.
If PM2.5 has a sub-index of 150 and that is the highest value among the pollutants, the overall PSI reading will be 150.
During haze episodes in Singapore, the dominant pollutant is usually PM2.5, which refers to fine particulate matter measuring 2.5 micrometres or less in diameter.
The main difference between the indexes lies in the thresholds used for their various air quality categories, said Prof Yim.
For example, he noted that under AQI, air quality is considered unhealthy when PM2.5, as the dominant pollutant, falls between 55.5 µg/m3 and 125.4 µg/m3. Under the PSI, the unhealthy range is 56µg/m3 to 150 µg/m3.
“These ranges for determining AQI categories are set by governments according to their situation,” said Prof Yim.
“So, it should be noted that the index from other countries may not be applicable directly to a country.”
Countries also split their index bands differently, said Associate Professor Liya E Yu from the National University of Singapore’s (NUS) Department of Civil and Environmental Engineering.
For example, she noted that the US AQI is a six-band system as it devotes a specific band to "Unhealthy for Sensitive Groups".
There is no equivalent band in Singapore’s PSI system or Malaysia’s Air Pollutant Index.
Singapore's PSI, however, provides separate health advisories for different population groups, including children, older people and those with chronic heart or lung conditions.
Singapore's PSI, the AQI and Malaysia's API serve the same general purpose - to communicate air pollution levels and their potential health implications to the public, said Provost’s Chair Professor Rajasekhar Balasubramanian from NUS’ Department of Civil and Environmental Engineering.
But since they differ in calculation methods, concentration breakpoints for each air pollutant, and health categories, their values should not be compared directly without accounting for these methodological differences, he added.
Dr Yu similarly advised against comparing figures from different systems as it would lead to confusion.
Instead, people should focus on the health advisories or air quality descriptions that accompany each system’s grading framework to get guidance on activities and plans amid air quality conditions, he said.
Should Singapore residents pay attention to the AQI on other sites?
IQAir, which uses US AQI categories and breakpoints, told CNA that it provides air quality data across more than 140 countries to give users a consistent index for comparing conditions across cities and nations.
The platform said its approach was not intended to replace national standards or suggest that every country should use the same regulatory system.
“Rather, it offers a common, health-focused reference for a world in which air pollution is measured and communicated in many different ways,” said the platform.
According to IQAir, it derives its Singapore readings by combining measurements from government monitoring stations with validated non-government sensors operated by private citizens, businesses, nonprofits and schools.
IQAir said this “community-powered network” allows it to provide more geographically detailed observations, although experts caution that readings from non-government sources could use lower-cost sensors whose readings may not always be directly comparable with those from government monitoring stations.
IQAir said Singapore’s PSI framework was last substantially revised in April 2014, when PM2.5 was incorporated into the index.
The US Environmental Protection Agency system, which IQAir uses, updated its AQI thresholds for PM2.5 in May 2024 to reflect updated health evidence and strengthen public-health protection, the platform said.
“This means Singapore’s core PSI framework is more than a decade old, while the US AQI has undergone a more recent update,” said IQAir.
Besides the 24-hour PSI, Singapore also publishes 1-hour PM2.5 readings to reflect current air quality.
Experts told CNA that NEA’s readings were a reliable and accurate source of air quality measurements.
Prof Balasubramanian advised Singapore residents to use NEA’s figures as the primary reference for official local information.
“An online site may display a precise-looking AQI number without necessarily providing a more accurate picture of local air quality,” he explained.
“Reliability depends on where its measurements come from, how frequently they are updated, which pollutants and averaging periods it uses, and whether it reports actual measurements or model-based estimates.”
Prof Yim also cautioned that the AQI readings on some websites could be based on data from low-cost and community air quality sensors.
He advised readers to be careful when considering AQI readings on websites which showed air pollutant concentration levels that differed significantly from those listed on NEA’s website.
Even after validation, such data might not be comparable to those from government networks due to possible inconsistencies, including data accuracy, sensor reliability and measurement locations, he said.
The professor acknowledged that there was room for improvement in the PSI values and how the data was reported for better communication.
But the current NEA readings are “relatively reliable and accurate” references, he said.
NEA told CNA that it uses highly specialised analysers to monitor Singapore’s air quality and its equipment follows “internationally recognised measurement methods for reliable and scientifically robust measurements”.
The 24-hour PSI reflects air quality in line with 24-hour air quality indices in the US, EU and the United Kingdom, it said.
The agency advised users to refer to its official readings online and on the myENV app for accurate information on Singapore’s air quality.
Have any websites been flagged for inaccurate data before?
Third-party platforms may use different data sources, averaging periods, index categories and calculation methods to derive their AQI, NEA told CNA.
In 2019, the agency warned against comparing its 24-hour PSI readings with AQI readings on air quality website Aqicn.
This came after a growing number of online users referred to alternative air quality readings. In some cases, the websites reported readings that seemed to indicate that Singapore’s air quality was poorer than what NEA suggested for the same period.
NEA said at the time that Aqicn used an incorrect PM2.5 reading to derive its final AQI figures, as its calculations were based on the 1-hour PM2.5 reading instead of the 24-hour PM2.5 reading, which the US EPA recommends.
Asked for an update on the reliability of the website, which is still operating, NEA said Aqicn applies US AQI thresholds, meant for 24-hour data, to 1-hour PM2.5 levels.
The website also relies on a combination of data measured from low-cost, less robust sensors and published NEA data for its calculations, it added.
“Readings by such third-party platforms are therefore not directly comparable with NEA’s PSI.”
The website did not respond to CNA’s questions about whether it has updated its methodology.
According to a notice on the site, its air quality data is unvalidated at the time of publication and may be amended, without notice, at any time.
How should readers use NEA’s readings?
NEA’s 24-hour PSI and 1-hour PM 2.5 readings serve complementary purposes.
“The 24-hour PSI provides a stable indicator of overall pollution exposure, while the 1-hour PM2.5 concentration reflects a more responsive indication of short-term changes,” said Prof Balasubramanian.
Because it reflects pollution over a longer period, the 24-hour PSI may not immediately reflect short-lived pollution peaks that could be important for certain individuals.
This is where the hourly PM2.5 readings can help.
“The 1-hour PM2.5 concentration responds rapidly to changes in smoke haze plume transport, wind direction, atmospheric mixing and emissions along the smoke plume trajectory,” said Prof Balasubramanian.
It can therefore help people make immediate decisions, such as whether to exercise outdoors during a haze episode, he said.
But people should be mindful that a single hourly reading may not represent the pollution exposure accumulated over an entire day, he added.
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Source: CNA/rl