The smoke, which is expected to descend on the city about 4pm, has blown in from bushfires in NSW, and not from the small blaze that broke out in Namadgi National Park on Monday, according to the ACT Emergency Services Agency.

The ESA originally stated the smoke would return on Tuesday, before updating the forecast.

The Hospital Hill fire in Namadgi was contained late on Monday, with fire crews continuing to patrol and monitor the situation.

News of the smoke's imminent return will have Canberrans again reaching for their P2 smoke masks.

When Robert Sanchez learned that Canberra chemists had be restocked with P2 face masks on Tuesday, he did not hesitate to take his elderly father to one of the nearest outlets.

For days his 90-year-old father, Nino, has battled with a sore throat and breathing difficulties as thick bushfire smoke has blanketed the nation's capital, making conditions particularly hazardous for the elderly, the pregnant and those with chronic respiratory conditions

"Dad has been spending all his time indoors, everyone has," Mr Sanchez said outside the Capital Chemist in Mawson, where he collected two masks. "When we heard that [masks] were available we came to get some before it gets worse again."

While air quality was much improved on Tuesday, with pollution levels considered fair in much of the capital in the morning, Mr Sanchez is not alone in fearing that conditions will deteriorate later in the week.

Already, there were signs of worsening air quality as Tuesday progressed. By lunchtime, readings of dangerous particulate matter in the atmosphere had risen in Civic, Monash and Florey, and air quality was rated as poor.

Across the ACT pharmacies have been handing out P2 masks supplied by the the federal government free to customers considered particularly vulnerable to the effects of smoke, including the chronically ill, the elderly, children and pregnant women.

Colbee Court Chemist pharmacist Christine Uy said demand for face masks had eased Tuesday morning after an improvement in air quality in the previous 24 hours.

"In the past few days we have sold 600 [masks]," Ms Uy said. "On Sunday we sold 200 in one afternoon."

By Tuesday morning demand has eased, and she had handed out just eight of the free masks by late morning.

As at 1pm Tuesday, air quality in Civic, Monash and Florey was rated as poor, with air quality index readings between 106 and 125 points, though this was far better than on the weekend when conditions were considered extremely hazardous.

Despite the improved conditions, the Heart Foundation has urged people, particularly those older than 65 or with conditions such as heart failure, to take steps to protect their health.

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The foundation's chief medical adviser, cardiologist Professor Garry Jennings said that, in addition to tiny particles that could penetrate deep into the lungs, bushfire smoke contained harmful gases such as carbon monoxide.

Professor Jennings warned the small particles could cross into the bloodstream, helping inflame and narrow blood vessels, worsening pre-existing hear conditions and increasing the risk of heart attack.

The improved conditions encouraged several businesses and institutions including the National Gallery of Australia and Questacon to reopen.

But a number of workplaces remain closed to all but essential staff, including the Department of Home Affairs, the Australian National University and the University of Canberra.

The ANU reaffirmed its decision to remain closed on Tuesday, but both it and the University of Canberra announced they would reopen Wednesday.

It has urged staff who can work from home to do so in order to reduce the power load and demand on campus.

WorkSafe ACT said it had received "a small number of notifications" about workplace air quality in recent days, and the ACT Government said the Access Canberra call centre and WorkSafe inspectors had received calls from several workers and employers for advice about operating in smoky environments.

"Due to the heavy smoke conditions impacting the ACT, WorkSafe is encouraging employers to look at mitigation strategies to manage the current heat and smoke conditions, particularly by stopping strenuous or outdoor work where possible," the government said.

The government said employers should conduct a risk assessment and revise it as conditions change.

It said employers could provide P2 masks "if there must be prolonged work outside or for those workers who are sensitive to smoke" but advised that the most effective way to avoid the smoke haze was to stay indoors because "masks do not guarantee protection".

It said most employers were heeding the advice.

The Canberra Business Chamber has advised its members that they must, "as far as reasonably practicable", provide a safe and healthy workplace for their employees.

"We recommend that strenuous outside work should be temporarily suspended until the air quality improves," the Chamber said in an advisory issued late on Monday.

Unions ACT called on employers not to force employees to work in hazardous conditions.

It said it had been contacted by numerous workers concerned that they had been required to work without protective equipment such as P2 dust masks.

"The bushfire smoke that has blanketed Canberra is a serious health hazard. There is no safe exposure level for air pollution like bushfire smoke," Unions ACT said.

A Health ACT spokesperson said 100,000 P2 masks released by the federal government from its pandemic stockpile would be available from pharmacies across Canberra this morning, reiterating that priority would be given to those who health was most at risk from the smoke, including those with chronic health conditions, the elderly, children and pregnant women.