SINGAPORE — Haze conditions worsened across parts of South-east Asia since Friday, with Singapore’s air quality slipping back into the unhealthy range as smoke from forest fires in Indonesia drifted towards the city-state.

The 24-hour Pollutant Standards Index (PSI) in Singapore’s central region reached 107 at 3pm today, up from 102 at 1pm, according to the National Environment Agency (NEA), The Straits Times reported today.

The west region recorded the next-highest reading at 95, while the south registered 79, with both remaining in the moderate range.

The deterioration was more pronounced in the central region, where the hourly PM2.5 reading – a measure of fine particulate matter – jumped from 67 at 11am to 117 at noon before easing to 93 at 3pm.

The NEA advises the public to use hourly PM2.5 readings when deciding on immediate outdoor activities and the 24-hour PSI to plan activities for the day ahead.

The latest deterioration follows an earlier spell of unhealthy air in Singapore this month, with the central-region PSI reaching 101 on September 4.

In its latest advisory, the NEA said smoke plumes from forest fires in Kalimantan had been observed drifting towards Singapore, while some plumes were also seen over southern Sumatra.