At 6am, the 24-hour PSI readings ranged from 105 to 142, while hourly the PM2.5 readings ranged from 58 to 78. SINGAPORE – The air quality across the island deteriorated overnight, with the 24-hour PSI recording unhealthy levels of air pollution early on Oct 24. SINGAPORE; Hazy conditions persisted into the afternoon of Oct 17, even as the 24-hour Pollutant Standards Index (PSI) in central Singapore dipped back into the "unhealthy" band.