National institutions are closed and a number of businesses are making decisions on Monday morning about whether to push on with a return to work.

The YWCA decided on Sunday evening that its five childcare centres, catering to up to 600 children, will close on Monday, along with its four school holiday programs at Condor, Narrabundah, Majura and Kingsford Smith. Earlier the group had planned to keep its childcare centres open, but conditions deteriorated to such an extent on Sunday the decision was changed.

The YMCA also closed its five childcare centres and three holiday care programs until further notice. It also closed its sailing club, Paddle Hub paddle board club, and Chifley gym.

The Canberra Business Chamber described the crisis that has gripped the capital as unchartered territory.

"This is not an issue that Canberra or Australia more widely has dealt with," Canberra Business Chamber chief Michael Schaper said. "It's unchartered territory, Just how do you deal with this sort of situation?"

Canberra on Sunday once again has the worst air quality index of any major city in the world, coming in worse than Sarajevo in Bosnia Herzegovina, Lahore in Pakistan and Delhi in India.

The orange-tinged smoke has made breathing outside very difficult. It has entered homes and offices buildings across the capital, setting off hundreds of automatic fire alarms.

The Bureau of Meteorology says the smoke is likely to be at its thickest in the morning and evening, and it's forecast to remain for coming days.

On Sunday, the National Gallery, the National Portrait Gallery and Old Parliament House were closed. The galleries were worried not only about staff, but also about their artworks.

"We ... must take this action to protect our portrait collection and our staff," the portrait gallery said on its website.

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Annika Scott at Old Parliament House said the concern was not the air-conditioning but the smoke let in when the doors were regularly opened and the impact it could have on heritage collections. Old Parliament House would make a decision each day this week on whether to open. The National Library expects to be open on Monday.

Many flights into and out of Canberra were cancelled on Sunday.

Dr Schaper said some shops and other businesses had an obligation in their leases to remain open - and they should talk to their landlords. They should talk to their staff about safety and take stock of whether there was sufficient trade to be worth opening, he said.

"It's a bit of a whammy for local business community," he said, with businesses still having to pay staff wages but little trade.

The Australian Public Service Commission hasn't made a decision on whether public servants should be at work, as it is up to each agency.

"It's up to each agency to monitor conditions on their premises, employ appropriate mitigation strategies to minimise the effects of smoke and communicate any special arrangements with their staff," a spokeswoman said.

"The APSC has encouraged their staff in Canberra and in Sydney to monitor air quality advice and alerts through ACT and NSW Health."

The Australian National University remains closed on Monday when it will make decision on whether to reopen Tuesday or shut its doors for longer. New chancellor Julie Bishop began her contract on January 1 and was to be at the university this week. The Mount Stromlo Observatory also remains closed, as does the university's Kioloa campus on the South Coast.

One of the city's biggest building firms, Geocon, returns to work on Monday. A spokesman said P2 masks arrived from Brisbane on Sunday, but the company would a make a call on Monday morning about whether some work was unsafe given the air quality.

Gecoon has about 350 construction staff plus contractors.

Construction Forestry Mining and Energy Union acting secretary Zac Smith said no one should be expected to work outside if conditions were anything like as bad as Sunday.

The union had worked with companies on some closures before Christmas and would continue to assess air quality and safety each day. The union had used 400 and above as a rough cut-off before Christmas.

Masks were not practical or useful in building, where workers were doing strenuous labour. They were also less effective for people with beards.

"If you can remove someone from the risk, that's always preferable to personal protective equipment," he said.

On Sunday, the ACT government said it had accepted an offer from the Commonwealth of a stock of P2 masks, which it said would be made available to people "particularly sensitive to smoky conditions and those who are unable to avoid prolonged outdoor exposure". Details are yet to be released.

The government said new stocks of masks would also be in chemists and hardware stores by Sunday afternoon or Monday. The government has published information on how to fit them.

"While our advice to the community remains that the best way to avoid exposure to the smoke is by staying indoors (with windows and doors closed) we understand that many people in our community also have a preference to use P2 masks," the Emeergency Services Agency said.

Business chamber chairman Archie Tsirimokos said businesses and organisations must do what was best for their staff and clients.

"People's safety's going to have to be paramount, that's got to be the first consideration," he said. "Some businesses are simply not going to operate."

Businesses would have to make that decision based on air quality as early as possible on Monday.

"There's a work, health and safety consideration and obviously there's a moral consideration," he said. "Most businesses will want to do the right thing and make sure they don't expose their staff and clients to any risk."

Peter Strong from the Council of Small Business Organisations Australia called for WorkSafe and the Fair Work Ombudsman to advise businesses on the smoke and its health hazards.

"At the moment no one has real answers," Mr Strong said.

Mr Strong said the impact of the smoke on small businesses and their employees could be severe if they are unable to work and bring in revenue to pay their employees.

"If a business closes, that's people's jobs lost."

Mr Strong said it was clear climate change was already having an economic impact.

"This is something that has to be confronted in a professional way," he said.

The ACT publishes a 24-hour average of air quality, rather than point in time readings. At 2pm on Sunday, the index showed a 24-hour average reading of 3404 in Florey, 3076 in the city, and 1954 in Monash. Readings above 200 are considered hazardous.