Doctors are warning even healthy people could develop serious illnesses, because of the smoke haze that's blanketed parts of the country, including some of our major cities, for weeks on end.

The Australian Medical Association (AMA) has warned prolonged exposure to toxic smoke could affect the health of many Australians.

In a statement released late last week, AMA president Tony Bartone described the length and density of smoke exposure as "a new, and possibly fatal, health risk" most of us have never faced before.

"With denser smoke haze and longer periods that people endure smoke inhalation, there is a much higher risk that previously healthy people will face developing serious illness," Dr Bartone said.

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Australia's bushfire crisis has killed at least 20 people, destroyed or damaged at least 2,000 homes, and led to emergency declarations in two states.

These bushfires are responsible for smoke haze that has been affecting the air quality for millions of people, including those living in some of our largest cities, including Sydney, Canberra, and Melbourne.

Air quality in Canberra recently reached more than 22 times the hazardous rating. While in December, some Sydney suburbs had air quality more than 10 times the hazardous amount.

And this bushfire season is far from over.

AMA vice president and respiratory physician Chris Zappala said the longer people were exposed to air pollution, the more likely they were to develop respiratory problems.

"Ordinarily, patients can handle a few days or a week of smoke particulates in the air ... but this exposure is going on at reasonably high levels for a lot longer," Dr Zappala said.

"We don't know whether that is going to cause an increased amount of respiratory disease, but we would presume a greater number of people are likely to get affected over time."

Concern for those with undiagnosed asthma

Bushfire smoke irritates the respiratory system, and contains fine particles that can travel deep into the lungs, causing damage.

While most healthy people can handle short-term exposure to air pollution (though it may cause itchy eyes and throat irritation), smoke particles can aggravate respiratory conditions (such as asthma), and even trigger a heart attack or stroke.

Some of us are more vulnerable — namely children under 14, older people, pregnant women, and those with pre-existing heart or lung problems.

But Dr Zappala said there was also a small group of people in the community who have respiratory disease they are not aware of.

"Being undiagnosed and unmedicated is a new risk for some people who, but for fire and smoke exposure, appear otherwise healthy," he said.

"The concern is that because this exposure is entering a duration that we don't really have any experience with, those patients that might have scooted under the radar ... are actually going to manifest disease."

He said anyone who develops breathlessness, wheeziness, chest tightness, or a persistent cough should see their GP. Borrowing someone else's inhaler or using over-the-counter Ventolin is not recommended.

Respiratory physician Matthew Peters said he was more worried about people who have asthma but think it's relatively mild.

"My greater concern is people who think it's mild, who take their reliever puffer ... and think everything is hunky-dory," said Professor Peters, head of respiratory medicine at Concord Hospital in Sydney.

"They are the ones who are probably at greatest risk of getting caught out the longer this goes on."

He said if people were in the habit of using a reliver (for example Ventolin) frequently, it was worth considering preventer medication while air quality remained poor.

Hazardous smoke has been shrouding Canberra for days. ( Supplied: James Dyer )

While it is too early to say whether the bushfires have increased the incidence of respiratory illnesses, Professor Peters expects data to show an upswing in cases of asthma and pneumonia.

"The historical pattern of bushfires is two or three potentially horrific days and tragic consequences to people and property — but then it's all gone," he said.

"But now we've had six weeks ... this is unprecedented."

P2 masks and other ways to reduce your risk

The best way to reduce your risk of breathing in polluted air is to limit your exposure.

Where possible, try to stay indoors with the windows and doors shut, and avoid vigorous exercise outdoors when air quality is low.

When it comes to face masks, P2 masks can help protect against smoke, but will only do so if used properly.

People who are sensitive to smoke are advised to take extra care when air quality is poor.

That means if you have a heart or lung condition, it's important to follow your medical plan, and have access to your medication or inhaler.

Precautions for pregnant women

The Royal Australian and New Zealand College of Obstetricians and Gynaecologists (RANZCOG) has also raised concern about the impacts of prolonged bushfire smoke on pregnant women and their unborn children.

"Exposure to air pollution in pregnancy has been linked to increased rates of preterm birth, decreased birth weight, hypertensive disorders of pregnancy and gestational diabetes," said RANZCOG president Vijay Roach.

He said it was important to note these risks increased with long-term exposure.

But Dr Roach said as cities and communities were enveloped in smoke, pregnant women were advised to take extra precautions and limit outdoor activities.

"For those unable to avoid prolonged exposure to inhaled air pollution, [face] masks may have a role," he said.

Long term impacts unclear

The health impacts of the current bushfires are not yet clear, and it will be a while before we know what they are.

Part of the challenge is that little research has been done on the health implications of bushfire smoke exposure over weeks or months, according to senior research fellow Christine Cowie.

"It is uncertain how medium-term exposure to these sporadic bushfire pollutions events impact on long-term health," Dr Cowie from the University of New South Wales told the ABC.

In addition to smoke, Dr Bartone said the ongoing nature of the bushfires also brought health risks associated with heat, fatigue and stress.

"The mental health burden of this disaster on our communities will be considerable," he said.