SINGAPORE – The air quality in Singapore deteriorated on Oct 3, with the 24-hour Pollutant Standards Index (PSI) reading at 11am dipping into the unhealthy range in the central region (102).

Although they remained in the moderate range, the National Environment Agency’s (NEA) PSI readings for all other regions also increased.

PM2.5 readings, which are used as a gauge for immediate activities, were in the elevated range in three of the five regions.

The three regions were east, west and central, with a PM2.5 reading of 66, 57 and 63 respectively.

Air quality is considered unhealthy when the 24-hour PSI is in the range of 101 to 200, while an hourly PM2.5 reading between 56 and 150 indicates an elevated concentration of fine particles.

The PSI – a 24-hour rolling average of six key air pollutants and used in planning for outdoor activities the next day – last crossed into the unhealthy range on Sept 30. The day saw the entire island swathed in a fine milky haze, with the reading hitting a peak of 134 in central Singapore as at 1am.

The high for the year was 167, recorded on Sept 29, also in the central region.

In its daily haze advisory issued in the evening of Oct 2, the NEA said the PSI is forecast to be in the moderate to low unhealthy range.

It added that moderate to dense smoke plumes from southern Sumatra in neighbouring Indonesia were observed to be drifting towards Singapore. Smoke haze was also observed from fires in Kalimantan and the smoke haze could be transported towards Singapore under the prevailing south-easterly or southerly winds.

The PSI crossed into the unhealthy range four weeks ago, hitting 101 in central Singapore on the morning of Sept 4. In the days that followed, readings in the central, ea...