"I'm afraid the chickens have come home to roost," said Devra Davis, a public health advocate and author based in the US.

Dr Davis, a scientist in the Clinton White House, shared the 2007 Nobel Peace Prize for co-authoring the first United Nations Intergovernmental Panel on Climate Change report.

She said the bushfires in NSW looked "dreadful".

"I've been looking at the images that NASA has provided of the smoke slowly swirling from NSW across the Pacific Ocean. I know it's sickening your cities," said Dr Davis, who is president of the non-profit Environmental Health Trust.

Dr Davis has studied the health effects of smoke.

"Whenever you burn hydrocarbons, you get byproducts - these include things like benzene and ultra-fine particulates," she said.

For seven weeks, the Hunter has been experiencing long periods of hazardous, very poor and poor air quality, as reported hourly on the NSW government's air quality index.

Some days the air has seemed clear, but this does not mean it's safe.

"The smoke you can see is not necessarily as dangerous as what you cannot see. The ultra-fine particulates are the ones you cannot see.

"Ultra-fine particulates are released by tobacco smoke and from burning wood. These ultra-fine particles are capable of remaining suspended in the air for months. They can be transmitted across large distances, as they are now."

Ultra-fine particles are smaller than 2.5 microns in diameter. They're listed on the government's air quality index as "PM2.5".

The Hunter has been experiencing hazardous levels of these particles in the last couple of weeks.

"They can be a hundred times smaller than a human hair. Because they're so small they can be deeply inhaled. They get into the nose and all the way into the lungs.

"When they're really small - less than one micron - they can get through the lungs into the bloodstream. When that happens you can literally get serum viscosity - thickening blood."

Breathing this type of pollutant puts vulnerable people at a higher rate of stroke and heart attack.

"People vary a great deal in their ability to tolerate pollution. The very young to very sick, those who have asthma or pre-existing conditions, they're more vulnerable. This is not a one-size-fits-all thing," she said.

"What we generally need to do is try to reduce exposure to toxic agents whenever possible."

Ultimately, the particle pollution gets deposited somewhere.

"We know the polar bears in the Arctic are showing up with residues of hydrocarbons that come from coal burning in China," she said.

"We can track emissions from specific smokestacks through radio-labelling and see where it's deposited.

"Smoke doesn't follow any national boundaries. And we only have one planet."

The intensity of the bushfires, she said, were a "consequence of years of inaction".

She cited the failure of many governments, including those in Australia, to "accept the reality of the need to reduce greenhouse gases".

"You have a tremendous opportunity when it comes to renewable energy in Australia," she said.

Labor leader Anthony Albanese last week backed coal exports to continue from Australia.

"If Australia stopped exporting today there would not be less demand for coal - the coal would come from a different place," Mr Albanese said.

Dr Davis said it was true that coal would continue to be burnt "in the short term".

"That's why you need the COP 25 to take action."

COP 25 is a UN climate change conference that's been held over the past fortnight in Madrid.

"World leaders need to get together and figure out a way to make it more equitable, but there will be winners and losers," she said.

She said a concerted effort and commitment to reduce carbon intensity was needed "across the board".

"And frankly, you need a carbon tax. All of us need a price on carbon."

She said coal was a "major and valuable export" for Australia, but it "really does have a damaging effect overall".

A move away from coal exports should be "encouraged by economic incentives".

"Many people have said coal does not really have an economic future in Australia. There has to be a transition."

She had seen enough persuasive information to know that "renewable energy is a realistic option".

"A concerted effort organised with economic incentives for renewable energy makes a great deal of sense. It requires leadership."

She said nuclear power shouldn't be taken off the table, but it was far from ideal.

"We're still dealing with the consequences of Fukushima," she said.

The consequences of the Chernobyl disaster had been "devastating for decades".

In 1997, she and colleagues published a peer-reviewed article in The Lancet medical journal about the public health effects of greenhouse gases.

"We estimated that by 2020 there would be 8 million avoidable deaths from fossil fuels and air pollution. 2020 is coming and I'm afraid that was an underestimate. We didn't even include the devastating impact of fires, floods and storms."