His warning came as ACT Health authorities said they would begin publishing hourly air-quality results rather than a 24-hour rolling average, which masks sudden changes. Hourly readings would be published for the very small particulate matter (PM2.5). The ACT has three air-monitoring stations. On Wednesday, the city station was down.

Greg Jones said all employers should do risk assessments. Only essential outdoor work should be done, and for essential work employees should have regular breaks in air-conditioning and regular rotations - which could mean hiring extra staff.

"I would expect that any worker that was working today [Wednesday] would be on regular breaks, would not be undertaking particularly heavy or strenuous activity," he said.

"If P2 masks are able to be appropriately fitted and stay dry then P2 masks can assist. But a P2 mask is not the be all and end all in terms of people working outside."

Inside, businesses should open buildings to "flush" or purge the air on better days such as Tuesday this week, and move staff to parts of buildings with better air quality. They should also clean, change or upgrade air filters and run air-conditioning on recycling on poor days, and consider air-purifiers. If a building had a strong smell, looked smoky or people felt uncomfortable, employers should consider stopping work. Occupational hygienists could install air quality monitoring equipment.

"These are unprecedented conditions," Mr Jones said. "It would be safe to say that most buildings around Canberra are affected by smoke internally in varying degrees and it's a matter of each building owner or employer making an assessment on a daily basis, on an ongoing basis, whether that building is safe to occupy."

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With the official return to work this week, businesses have made different decisions on whether to open or close. Some construction sites were closed on Monday and Tuesday, but Geocon, the city's biggest builder, is operating normally. About 300 construction staff are on sites around Canberra and have been offered P2 masks. Geocon says it is monitoring air quality.

The Canberra Centre remains open, a spokesman saying it was a place where people could get respite. But access would be limited to some doors on smoky days to limit smoke getting into the building. Up to five shops have been closed on the poor days this week, concerned about the impact on staff and stock.

Canberra Business Chamber chief executive Michael Schaper said there was no precise cut-off between safe and unsafe working conditions and individual businesses were having to make their own decisions. He called on regulators to recognise this uncertainty if complaints were made.

"There is a degree of we just don't quite know the threshold point, no-one's got an absolutely clear answer," he said.

The chamber has advised businesses that they can stand down workers when there is no work and "generally, there is no legal obligation to pay staff during periods where they cannot be gainfully employed and the circumstances are outside the employer's control, such as a natural disaster".

But Mr Schaper said employers should continue paying staff if they could and should also allow staff to use leave.

"While the reasonable thing and the right thing to do is to pay people and that's what most businesses will do and should do, under the law where there's no work to be done and where it's outside the control of the employer are some cases where you don't have to pay them, we don't want to see that happen," he said.

Construction union acting ACT secretary Zac Smith said it would be "morally reprehensible" for any employers to make workers bear the cost.

The construction award required workers to be paid when "atmospheric conditions" prevented work, which is how the smoke should be treated, he said.

Mr Jones said he was minimising exposure of his own Worksafe staff through having them do essential work only. This included responding to concerning reports of poor air quality in workplaces.

Employers who failed to do risk assessments, monitor air quality and put mitigation measures in place risked breaching work health and safety legislation.

"An activity that contributes to unsafe an workplace, whether a fall from heights or working in adverse environmental conditions, is a breach of their duty under work safety laws," he said.

Professor of global environmental health at the Australian National University Sotiris Vardoulakis said the city was in uncharted territory.

"This is a very different situation to what we've seen in the past when we have had air pollution from bush fires but it didn't last very long," he said.

Studies on firefighters had shown to have reduced lung function immediately after bushfire seasons and recovery to normal within a few months, but bushfire seasons were now longer and more intense.

Professor Vardoulakis said it was "a fine balance to encourage people to reduce their exposure without being alarmist and without panicking".

People with no underlying health issues could be reassured that long-term health impacts were unlikely to experience long-term health impacts. The concern was for people with cardio vascular illness and respiratory issues such as chronic bronchitis, asthma and emphysema. Young children were also at higher risk because their lungs were still developing, along with pregnant women and people over 65.

Masks were impractical for long periods and not designed for children. The best option was to monitor air pollution, stay inside, close doors and windows, use air conditioning on recirculation mode, use an air purifier with HEPA filter and ventilate the house on good days.

Canberra's chief psychiatrist Dr Denise Riordan urged people to look after their mental health by sticking to routines, eating and sleeping well, and not constantly watching and listening to smoke and bushfire related information.

"A lot of people are going to feel quite distressed, quite anxious and overwhelmed," she said. While for most, the anxiety would pass, for others including children and young people and people with current or previous mental illness, and people who lived with previous stressors, such as people who had experienced the 2003 bush fires, the impact could be worse.

Authorities expect smoke to linger for some weeks.

Mr Jones also called on employers to be ready to support workers who had been impacted by the fires directly, or had their holiday plans disrupted.