An order to widely restrict work on account of the haze would severely impact the Singapore economy and society, said Dr Ng Eng Hen, who is heading the new inter-ministerial committee set up in response to the worsening smog problem.

It is not sensible for now to have fixed thresholds, given that a person could just as easily be exposed to pollutants at home as he would be at school or at work, said Dr Ng at a press conference.

Speaking after chairing the first meeting of the committee on haze yesterday, he said: "(To stop work) is not sustainable because it means a mass closure of outdoor businesses, airports, ports and other sectors."

Dr Ng, who is also Defence Minister, said that the impact on society would be "very great" and that there would be knock-on effects if places such as petrol kiosks or supermarkets were forced to shut.

"It's not sensible or sustainable for us to do that. We need to keep Singapore going," he said as he unveiled a set of national guidelines on what the public should do if the 24-hour Pollutant Standards Index (PSI) hits certain levels.

The guidelines state that if the 24-hour PSI goes above 300, which is classified as "hazardous", even healthy people should stay indoors to reduce their exposure. Those who need to work outdoors should protect themselves by wearing a N95 face mask.

Dr Ng said that the 24-hour PSI is a better indicator of the health impact of the haze than the three-hour reading because the effects of exposure to pollutants are based on a "continuum" rather than a "threshold".

Yesterday, the National Environment Agency (NEA) began issuing a rolling 24-hour PSI reading every hour on its website, in addition to the three-hourly updates.

Addressing public fears of a shortage of the N95 masks - given that pharmacies across the island have quickly sold out their stocks - Dr Ng said that such masks were required only for outdoor work when the 24-hour PSI breached the 300-mark.

The government will ensure, however, that some 200,000 poorer households here will soon get free N95 masks. The Singapore Armed Forces will be mobilised to transport a million masks from warehouses to grassroots organisations in the coming week; the masks will then be sent to needy families. The Health Ministry said on Thursday that it has a ready supply of nine million masks on hand.

Meanwhile, supermarket chain NTUC FairPrice said that it will soon cap the price of N95 masks and limit the number that each individual can buy in a bid to prevent panic buying.

Queues were seen at pharmacies and hospitals throughout the day yesterday as people cleared the shelves of available masks; some were seen reselling the masks at three or four times the original price.

Dr Ng, who noted these "supply chain bottle-necks", assured the country that there were enough masks to go around.

Even without a stop-work order from the Singapore government, many businesses and event organisers here have taken matters into their own hands by suspending operations and cancelling outdoor activities until the situation improves.

The Education Ministry (MOE) announced yesterday that all school activities will be cancelled until June 30. Schools are now on their month-long June vacation, but students typically return for co-curricular activities and other events. The new school term begins on July 1.

MOE said on its website: "Should the 24-hour PSI level continue in the 'very unhealthy' range over the next few days, MOE will take guidance from the Inter-Agency Haze Task Force on the need for further action."

The Mizuno Passion Mount Faber Run, which was to be flagged off tomorrow, has been postponed until Oct 13. The run's organisers and event sponsor VGO Corporation made the call to push it back by four months after considering the health, safety and well-being of the 2,500 participants who had registered for the 10 km run.

The recreational water theme park, Wild Wild Wet at Pasir Ris, was shut to visitors for a second day yesterday. The Singapore Flyer observation wheel was among the other attractions that remained closed.