Millions of people in Sydney and Adelaide are among residents across Australia who have been blanketed by bushfire smoke in recent days.
Outside of immediate fire zones, should they be worried? How damaging is bushfire smoke? And can it really be worse than having a cigarette?
Who is this affecting?
Friday marked the third day this week that Sydney – Australia’s largest city – has been covered by acrid smoke from fires.
Air quality has exceeded “hazardous” levels, and paramedics responded to dozens of related calls on Tuesday alone.
But bushfires are burning near populated zones across the country. In Adelaide – a city of 1.3 million – locals were advised to stay inside on Thursday.
How harmful is bushfire smoke?
It comes largely from natural sources – trees, leaves and other ground vegetation – and comprises small particles, gases and water vapour.
The gases include carbon monoxide, carbon dioxide and nitrogen oxide – all are most present nearer to a fire.
Bushfire smoke is not as bad as industrial pollution but it is still harmful, says Associate Prof Brian Oliver, an expert in respiratory disease from the University of Technology, Sydney.
“Any smoke that is produced as the by-product of something burning is noxious and bad,” he tells the BBC.
In Sydney – which is receiving smoke from a blaze 150km (93 miles) inland – most danger lies in “ultrafine” particles which can travel vast distances on the wind.
How bad has Sydney’s air quality been?
Fine particle matters – known and measured globally as PM2.5 – are invisible to the human eye. They are coated in chemicals such as lead and are most worrying because they penetrate deep into the lungs.
Australia’s clear air standard is a PM2.5 level of eight micrograms per cubic metre. By comparison, smoking a single cigarette produces 20 micrograms per cubic metre.
But on Tuesday, in Sydney’s north-west, the PM2.5 reading was as high as 734 micrograms – the equivalent of about 37 cigarettes.
And for firefighters and even some people who live closer to the blazes, exposure would have been “10-15 times higher”, says Associate Prof Oliver.
“You have to feel for those people out there in those conditions,” he said.
In one part of Sydney on Tuesday, the air quality index – which takes in the particle reading, ozone levels, and other chemicals – exceeded Delhi’s pollution when it reached “unbearable” levels earlier this month., he added.
What is the impact on health?
In these circumstances, even healthy people can have small breathing issues and feel irritations in the eyes, nose, throat and lungs.
Most sensitive are children, the elderly and smokers, while those with asthma, heart and lung problems can see exacerbated symptoms – such as chest tightness and difficulty breathing.
Standard face masks are ineffective when it comes to blocking out fine particles, according to health authorities. They warned people to avoid exercising outdoors this week – a suggestion that many have ignored.
But this breathing discomfort is, fortunately, mostly short-lived, says Associate Prof Oliver.
Bushfire pollution does not typically do as much lasting damage as traffic or industry pollution – because conditions often lift with a change in weather.
However many fires have been burning for weeks, and authorities warn that there is no immediate end in sight. If the smoke becomes constant, then the particles could eventually have the same effect as cigarettes.
“A cigarette is basically a plant that we purposely inhale. And in bushfires, it’s another plant that we’re inhaling the smoke from, so it’s not surprising the health effects are actually quite similar,” said Associate Prof Oliver.
What about ‘thunderstorm asthma’?
Smoke is an obvious health hazard that can travel vast distances from the fire front. But the weather stoking the flames has created other issues too, such as dust storms.
In Victoria, officials issued an “extreme” warning this week for a phenomenon known as “thunderstorm asthma”.
This is where strong winds create more pollen in the air, which can lead to an outbreak of asthma attacks. In 2016, nine people died in Melbourne from such an outbreak.
Reporting by the BBC’s Frances Mao