24h PSI level hits 'Unhealthy' level in S'pore on Feb. 27, 2021

No spike in levels of PM 2.5 and PM 10.

Zhangxin Zheng | February 27, 2021, 11:22 PM

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Some Singaporeans might have noticed the poorer air quality in the past two days.

On Feb. 27 evening, the National Environment Agency also reported that the 24-hour Pollutant Standards Index (PSI) was at the "Unhealthy" level.

In particular, the 24-hour PSI level was above 100 in the north at 7pm.

Spike in ozone

The 24-hour PSI is computed based on these six air pollutants: PM2.5, PM10, ozone, sulphur dioxide, nitrogen dioxide and carbon monoxide.

PM2.5 is the dominant pollutant during haze episodes.

According to NEA's website, the reason behind the spike in the PSI level is likely due to an increase in ozone during that period.

There is no anomaly in the readings of other pollutants including PM 2.5 and PM10 and therefore the drop in air quality is unlikely due to the forest fires in the region.

What is ozone?

Ozone at ground level is formed when pollutants (hydrocarbons as well as nitric oxide and nitrogen dioxide) emitted by cars, refineries, chemical plants, and other sources react chemically in the presence of sunlight.

Ozone can reach unhealthy levels on hot days in urban environments.

Mothership has reached out to NEA for comments.

Top image via Unsplash by Christoph Theisinger