SINGAPORE, Oct 7 — Barely a day after air quality improved slightly in northern and southern Singapore, the 24-hour Pollutant Standards Index (PSI) in both regions returned to the unhealthy range.

The 24-hour PSI in northern and southern Singapore crossed into the unhealthy range at 11am.

By 1pm, all five regions recorded unhealthy PSI levels, ranging from 101 to 132.

Meanwhile, one-hour PM2.5 concentrations ranged from 65 to 126, with western Singapore recording the highest concentration reading of 126.

Concentration readings between 56 to 150µg/m3 dall within the elevated band.