Air quality in Singapore hit unhealthy levels in the north over the weekend because of higher levels of a pollutant known as ozone, and not transboundary haze.

"At 7pm on Saturday, the Pollutant Standards Index (PSI) in the north region entered the unhealthy range due to heightened levels of ozone," the National Environment Agency (NEA) said in response to queries from The Straits Times.

At that point, the PSI reading - a measure of air quality here - in the north was 102. It climbed to a peak of 108 at 8pm, before dropping to 90 at 10pm.

Air quality is considered unhealthy when PSI is in the range of 101 to 200. During such periods, healthy people are urged to reduce prolonged or strenuous outdoor physical exertion, while vulnerable groups, including pregnant women, children and the elderly, should minimise such exertion.

When air quality is moderate - with PSI readings between 51 and 100 - normal activities can go on.

The PSI is calculated based on six pollutants - particulate matter, fine particulate matter (PM2.5), sulphur dioxide, carbon monoxide, ozone and nitrogen dioxide.

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

During periods of haze caused by forest fires in the region, the dominant pollutant is PM2.5.

An NEA spokesman said the PSI last entered the unhealthy range for eight hours on Nov 13 and 14 in 2019, in the southern region here, due to elevated PM2.5 levels.

Family physician Leong Choon Kit from Mission Medical Clinic said: "In theory, higher ozone levels will irritate the respiratory system. Vulnerable people may get breathlessness, wheezing, cough, or chest pain."

Ozone is a gas that is made up of three oxygen atoms. Whether it is beneficial or harmful to humans depends on where it is found.

"Good ozone" occurs naturally in the upper parts of the atmosphere, where it forms a protective layer that shields life on earth from the sun's harmful ultraviolet rays, according to the United States Environmental Protection Agency's website. This is why a hole in the ozone layer is of concern.

But when ozone is found at ground level, it is considered a pollutant, and is one of the main components of smog.

Unlike other forms of pollution caused by direct emissions, ozone is not directly emitted into the air.

Instead, it is formed when other components of the air - nitrogen oxides and volatile organic compounds - react under specific environmental conditions, including the presence of sunlight.

"The pollutants which contribute to the formation of ozone - nitrogen dioxide and volatile organic compounds - were within normal levels," said the NEA spokesman.

"But weather conditions such as ambient temperature, ultraviolet levels, wind speed, wind direction and rainfall can also influence the formation of ground-level ozone."

For example, the maximum temperature last Saturday was 35.3 deg C - the highest recorded this year for the northern region.

"This, coupled with the high ultraviolet levels, could have contributed to the elevated ozone levels, reaching the unhealthy range," the spokesman said.

Associate Professor Koh Tieh Yong, a weather and climate scientist at the Singapore University of Social Sciences, told ST that ground-level ozone can be destroyed by hydroxyl radicals - a "cleaning agent" formed naturally in the atmosphere from water vapour.

But as the northern regions of Singapore are located farthest from the large expanse of sea to the south, the concentration of hydroxyl radicals would have been lowest there, Prof Koh explained.

Moreover, the ongoing dry conditions due to the current phase of the north-east monsoon season are bringing in drier winds.

"This makes it harder to remove ozone once it is formed," he said.

UPDATE ON TRANSBOUNDARY HAZE

Last month, Indonesian President Joko Widodo urged the authorities there to prepare for forest fires later this year as hot spots had been detected in Sumatra.

It is unlikely that smoke from hot spots in Sumatra contributed to the bad air quality in Singapore this time. This is because during the north-east monsoon season, winds blow mainly from the north or north-east.

A spokesman for NEA's Meteorological Service Singapore told ST that as the wind is likely to continue blowing from the north or north-east over the next few days, "any smoke haze from the fires in Sumatra is not likely to be blown towards us".

"However, we may be affected by the haze should more hot spots develop in Sumatra and the winds shift to blow from the west or north-west," he said.