SINGAPORE, Oct 5 — Air quality in parts of Singapore returned to the unhealthy range early today, with the 24-hour Pollutant Standards Index (PSI) reaching 110 in the central region and 102 in the west at 6am, according to The Straits Times.

The hourly PM2.5 reading, which indicates immediate concentrations of fine particles, was also elevated in the two regions, at 93 in the central region and 81 in the west.

According to the report, readings in the north, south and east remained within the normal range, between 49 and 52.

Air quality is classified as unhealthy when the 24-hour PSI is between 101 and 200, while an hourly PM2.5 reading of 56 to 150 indicates an elevated concentration of fine particles, The Straits Times reported.

The PSI, which measures a 24-hour rolling average of six key air pollutants, had fluctuated between moderate and unhealthy levels yesterday.

It peaked at 103 at about 3am before falling into the 90s for most of the day, the report said. The reading briefly returned to the unhealthy range at 101 at about 9pm before falling to 99 by 11pm.