SINGAPORE - Air quality in Singapore was either "moderate" or "good" on all days in 2020 owing to wetter weather and a reduction in economic and transport activities due to Covid-19, a new report has found.

But the levels of many of the individual pollutants that make up Singapore's air quality index still fall short of updated standards set by the World Health Organisation (WHO).

These were among the key findings of the National Environment Agency's (NEA) inaugural State of the Environment: Air and Water Quality Report published on Thursday (Sept 30).

In Singapore, the 24-hour Pollutant Standards Index (PSI) gives a measure of air quality in the country.

The PSI is calculated based on the average concentration levels of six component pollutants - PM10 and PM2.5, which are particulate matter of different sizes, sulphur dioxide, nitrogen dioxide, ozone and carbon monoxide - over 24 hours.

A spike in concentrations of any one of the six can lead to a deterioration in air quality.

For instance, during bouts of haze, the dominant air pollutant is PM2.5. But wetter weather experienced during the usually dry months of September and October last year kept the haze away.

Air quality is considered "good" when PSI is between 0 and 50 and "moderate" when the readings are between 51 and 100. When PSI is 101 and above, air quality is unhealthy.

NEA said that during the circuit breaker period between April 7 and June 1 last year, there was a 28.8 per cent drop in PM2.5 concentrations, a 38.1 per cent reduction in nitrogen dioxide, a 5.6 per cent reduction in carbon monoxide, and 58.1 per cent reduction in sulphur dioxide, compared to the same time period in the preceeding three years.

The NEA benchmarks Singapore's air quality against the WHO's air quality guidelines and "strives towards achieving the WHO's 2005 air quality guidelines in the long term".

But the WHO on Sept 22 revised its 2005 guidelines, announcing stricter air quality guideline levels for all six pollutants.

The global health authority said it was making the adjustments as there "has been a marked increase of evidence that shows how air pollution affects different aspects of health".

In Singapore, concentrations of four pollutants - ozone, sulphur dioxide, PM2.5 and PM10 - already exceed the WHO's 2005 guidelines.

But the NEA noted in its latest report that 2020 concentrations of sulphur dioxide, PM2.5, PM10, were still the lowest recorded in the past decade.

Apart from haze, industries and vehicles are major sources of particulate matter. Industrial and shipping activities are the main sources of Singapore's sulphur dioxide emissions.

As for ozone, it is a secondary pollutant that is not directly emitted from a source but is instead formed due to chemical reactions in the air. This makes it challenging to attribute ozone level changes to individual sources or causes, said NEA.

If the WHO's stricter standards are factored in, the Republic will also not meet the standards for nitrogen dioxide. This pollutant is mainly produced from burning fuel in factories and vehicles.

As for carbon monoxide, NEA said in its latest report that the eight-hourly mean concentration - referring to the average of hourly readings taken over the time period - of this pollutant has been "well within" the WHO's 2005 air quality guidelines.

WHO's new guidelines are not directly comparable with the figures in the NEA report as the global authority's recommended concentration levels are averaged out over 24 hours instead of eight.

NEA said it is taking action to reduce levels of all six pollutants through various schemes.

This includes tightening standards for industrial plants and providing incentives to owners of older motorcycles, which produce a lot of carbon monoxide.

For water quality, Singapore's inland, coastal, and recreational beach waters, have met their respective water quality guidelines, NEA said.

"Our seven popular beaches achieved a "good" banding on NEA's recreational water quality guidelines, consistent with primary contact recreational activities," said the agency, referring to activities where the whole body is frequently immersed in water, such as swimming, wakeboarding and jet-skiing.

NEA said it has been conducting trials of new monitoring technologies, including the use of low-cost air quality sensors, to provide a higher spatial resolution of the dispersion of air pollutants.

The agency added that it is also collaborating with research institutions to tap additional scientific support.

Mr Ram Bhaskar, deputy chief executive and director-general of environmental protection at NEA, said: "We enjoy clean ambient air and inland and coastal waters throughout most times of the year. This is achieved through a multi-prong pollution management strategy comprising upstream planning controls, surveillance and enforcement, and downstream environmental monitoring.

"The state of our air and water quality serves as a gauge in assessing the effectiveness of the pollution management strategies and drives policy reviews, as we continue to strive towards achieving international environmental standards."