The three-hour Pollutant Standards Index (PSI) climbed to 401 at noon on Friday, firmly into the hazardous range. This was a slight increase from 400 at 11am. Anything above 300 is hazardous.

The 24-hour average PSI was 181-232, up from 180-227 an hour before. The 24-hour average concentration of PM2.5, which is very fine particulate matter, was 232-292.

Queues were spotted at pharmacies across the island in the morning before they had even opened, as Singaporeans tried to stock up on respiratory masks. Many ran out of masks within minutes, while others did not have any to sell at all to begin with.

Some pharmacies, such as the Guardian outlet at Paragon, are allowing customers to reserve N95 masks, and said they would contact them once stock becomes available. Other stores said fresh stock of masks will be coming soon.

This comes after the Health Ministry said that it has sufficient stock of N95 masks, and will be placing more orders for the masks. Nationally, Singapore has close to 9 million N95 masks in its armoury, said director of medical services K. Satku on Thursday.