SINGAPORE – Air quality readings in Singapore reached the “unhealthy” range on Sunday (Sept 22), leading to hazy skies.

At 9pm, the 24-hour Pollutant Standards Index (PSI) for all regions was over 100, with the south recording the highest figure at 124. It was 111 in the west, 112 in the central and north and 118 in the east.

In the morning, at 8am, readings for the four regions were under 100, except the south, which was at 105.

A PSI reading of zero to 50 indicates good air quality while a reading of 51 to 100 is in the moderate range. A reading of 101 to 200 is considered unhealthy.

The air quality is considered “very unhealthy” when the PSI ranges from 201 to 300, and “hazardous” when the PSI reading is more than 300.

The worsening air quality is a concern for many of those watching the final day of Formula 1 races on Sunday.

In the south, where the F1 race is being held, the one-hour PM2.5 concentration reading was in the “elevated” range of 65 at 9pm compared to 101 at 8am. The PM2.5 was 55 in the west, 54 in the north, 57 in central and 59 in the east.

The one-hour PM2.5 concentration, which the National Environment Agency said is a better indicator of current air quality, measures the average hourly concentration of PM2.5 particles – the dominant pollutant during haze episodes – and is meant to help people gauge immediate activities, such as whether they should go for a jog.

There are four bands on the PM2.5 concentration scale: 0 to 55 for normal, 56 to 150 for elevated, 151 to 250 for high, and very high for any higher readings.

Online chatter compared the situation on Sunday to previous days.

Twitter user @_JLMH_ said the haze is “pretty bad today” while user @xyrael felt “today’s haze is the worst since Thursday”.

User @yoonsy asked what the contingency plan for F1 was, given that the “haze is back”.

User @aerdnachen took a picture of her Apple watch, which showed the day’s weather as “Haze”.