The haze crept up on Singapore yesterday and pushed the air quality readings in the western parts of the island into the "unhealthy" band. But unlike June last year, when protective face masks were flying off the shelves, the reaction yesterday was more measured.

At no point during the day was there a rush for masks, checks at six pharmacies in the west showed. These included stores at popular malls such as JEM and Jurong Point.

Residents said they were not worried yet.

"You can see a slight haze and the air smells a bit funny, but I barely noticed it," retiree Chooi Saek Choong, 75, who lives in Buona Vista, told The Straits Times.

Clementi resident and game designer Janus Ong, 31, echoed the sentiment. "It's not very bad, nothing like last year," he said.

On June 21 last year, the three-hour Pollutant Standards Index (PSI) reading hit a historic high of 401, making the air "hazardous" and sparking a surge in demand for protective N95 masks.

Yesterday, the 24-hour PSI crept past 100 into the "unhealthy" range at 6am in the west - the first time the 24-hour index had crossed 100 anywhere on the island since its revision in April to account more directly for small, toxic particles called PM2.5.

While air in the west remained unhealthy for the rest of the day, readings for the rest of Singapore were in the 80s, at the higher end of the "moderate", 51-100 range.

FENG ZENGKUN