SINGAPORE - Air quality hit unhealthy levels again in the northern parts of Singapore on Sunday (March 28) evening, because of high concentrations of ozone, according to the National Environment Agency's (NEA) website.
At 8pm, the Pollutant Standards Index (PSI) reading stood at 114, in the unhealthy range, though it returned to the moderate range at 9pm, with the PSI dropping to 97.
Air quality is considered unhealthy when the PSI is in the range of 101 to 200. Under these conditions, healthy people are urged to reduce prolonged or strenuous outdoor physical exertion, while vulnerable groups, including the elderly, should minimise prolonged physical activity.
When the PSI is above 200, it is in the very unhealthy range.
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.
As at 9pm on Sunday, the eight-hour average ozone concentration, was above 100 in the north, central and south regions of Singapore, according to the NEA website.
During periods of haze caused by forest fires in the region, the dominant pollutant is PM2.5.
On Sunday, the hourly concentration of fine particulate matter remained in the normal range. The 24-hour average of the PM2.5 readings was within the moderate range.
Similar conditions had been reported on Feb 27, when PSI levels had gone into the unhealthy range.
An NEA spokesman told The Straits Times on March 2 that the pollutants which contribute to the formation of ozone, such as nitrogen dioxide and volatile organic compounds, were within the normal levels.
However, 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 on Feb 27 was 35.3 deg C. This was the highest recorded in 2021 for the northern region, the spokesman said.
Coupled with high ultraviolet levels, this could have contributed to the elevated ozone levels, reaching the unhealthy range, she added.
ST has contacted NEA for comment.