Riyasat Ali

SINGAPORE:- Air quality in the central, southern, western and eastern parts of Singapore remained in the unhealthy range on Tuesday (Sep 29) morning, with the northern region crossing the threshold at 7am. The 24-hour Pollutant Standards Index (PSI) in the northern region hit 101 at 7am, while the central region's reading was the highest at 155. The hourly PM2.5 reading, which reflects air quality at a single point in time, was in the elevated band across all regions as of 9am, ranging from 61 to 112.    The 24-hour PSI readings for the central and southern regions first hit 104 and 103, respectively, at 8pm on Monday. The air quality in the western region entered the unhealthy range at 9pm, with a PSI reading of 101. Air quality in the eastern region subsequently dipped into the unhealthy band at midnight on Tuesday. PSI readings have dipped in and out of the band over the past weeks. The last time the air quality in all five regions was in the unhealthy range was on Sep 15.   In its latest media advisory on Monday evening, the National Environment Agency (NEA) said that the 24-hour PSI was forecast to be in the high moderate to mid-unhealthy range for the next 24 hours. Moderate to dense smoke haze from fires in southern Sumatra and Kalimantan has drifted to Singapore, said NEA. While some showers are forecast in Singapore on Tuesday, there remains a risk of smoke haze affecting Singapore, with the prevailing winds expected to blow from the east or southeast.    A runner doing stretches in central Bishan at the Kallang Park Connector amid hazy conditions on the morning of Sep 29, 2026. The 24-hour PSI is Singapore's indicator of daily air quality and forms the basis of NEA's daily health advisories. It is calculated using six air pollutants - PM2.5, PM10, ozone, sulphur dioxide, nitrogen dioxide and carbon monoxide - and reflects the average air quality of the past 24 hours.  The 1-hour PM2.5 reading, on the other hand, is the hourly average of the PM2.5 pollutant levels. PM2.5 is fine particulate matter with a diameter of 2.5 micrometres or shorter, and is the dominant pollutant during haze that causes harm to health. A 24-hour PSI reading is considered "unhealthy" when it is in the 101 to 200 range, "very unhealthy" when it is within the 201-300 range, and "hazardous" when it goes over 300. The hourly PM2.5 reading is classified in four bands: normal, elevated, high and very high. It is considered elevated when it ranges from 56 to 150. The "high" band ranges from 151 to 250, and readings above that are classified as "very high".