Air quality improves on Friday morning as haze thins overnight

A light haze over Upper Bukit Timah Road on the morning of Sept 20, 2019. PSI in the west at 7am was 66 with the hourly PM2.5 at 17. ST PHOTO: NG SOR LUAN

SINGAPORE - Workers heading to work and Primary 6 pupils taking their first Primary School Leaving Examination paper on Friday (Sept 20) morning were able to breathe a little easier.

Air quality improved overnight, with 24-hour Pollutant Standards Index (PSI) readings and 1-hour PM2.5 concentrations decreasing earlier on Friday.

The 24-hour PSI reading was 59 to 64, within the moderate range, at 9am on Friday. This was an improvement from a reading of 91 to 103, in the moderate to unhealthy band, at 8pm on Thursday.

By 4pm on Friday, the reading was 63 to 64.

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

The air quality is considered "very unhealthy" when the PSI ranges from 201 to 300.

Meanwhile, the 1-hour PM2.5 concentration, which the National Environment Agency (NEA) said is a better indicator of current air quality, was 15 to 29 micrograms per cubic m at 9am.

This range, which is within the normal band, was similar to the PM2.5 reading of 20 to 26 at 8pm on Thursday.

At 4pm on Friday, the 1-hour PM2.5 concentration had crept up again and was 43 to 63 micrograms per cubic m, in the normal to elevated bands.

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.

For the rest of Friday, until about 7pm, the air quality is expected to be largely similar to that on Thursday.

Going by forecasts the NEA issued on Thursday evening, the 1-hour PM2.5 concentration readings are expected to remain in the normal band, while the 24-hour PSI is forecast to be between the high end of the moderate range and low end of the unhealthy range.

But if haze from the surrounding region is blown in, the 1-hour PM2.5 reading could enter the elevated band.

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