SINGAPORE - Australia's recent bush fire crisis caused nearly A$2 billion (S$1.98 billion) in smoke-related health costs, from premature deaths to emergency admissions for asthma, researchers say, a record for any fire season in the country and a pointer to future health costs as the planet heats up.

The fires burnt about 8 million ha of forests and farmland in an unusually long fire season lasting more than six months from August last year to March this year, blanketing major cities including Sydney, Melbourne and Canberra in hazardous smoke for weeks to months on end, and affecting millions of people.

Pictures of famous Sydney tourist spots - from Bondi beach to the Sydney Harbour Bridge - all but blanked out by smoke shocked the nation and the world. Just to the south, the capital Canberra recorded some of the worst air quality in the world. During early January this year, the city's Air Quality Index peaked at 5,185 - more than 25 times above hazardous levels.

The researchers say the health costs and risk to lives attributable to bush fire smoke have the potential to increase dramatically as the frequency and intensity of wildfires increases with a hotter climate.

"The global trend of longer fire seasons with more extreme fire weather is leading to fires that are unusually frequent, severe and, in some cases, economically destructive," say the authors in the study published on Monday (Sept 21) in the journal Nature Sustainability.

The researchers found that health impacts of smoke pollution are not always included in cost assessments of bush fire impacts, which generally focus more on the costs of fire-related injuries, housing and infrastructure losses and fighting the fires.

To get a better idea of the direct health costs, they looked at the number of cases referred to hospital emergency departments for asthma, admissions to hospital for cardio-respiratory disorders and deaths from all causes linked to bush fire smoke in populated southern regions of Australia.

They compared the total economic costs associated with deaths and hospital admissions for each fire season (October through March) for a 20-year period from Oct 1, 2000.

They found that the smoke-related health costs of the 2019-2020 fire season totalled a record of A$1.95 billion, involving an estimated 429 smoke-related premature deaths, 3,230 hospital admissions for cardiovascular and respiratory disorders, and 1,523 emergency cases for asthma.

The total cost was well above the next highest estimate of A$566 million in 2002-2003 and more than nine times the median annual bush fire associated costs of A$211 million for the previous 19 years, they conclude.

The authors note that they don't account for likely ongoing costs of medication, ambulance call outs, reduced productivity or mental health burden, which are important contributors to the economic impacts of fire disasters.

"For these reasons, the health and economic assessments we present are likely to underestimate the true burden."

And the long-term costs of bush fire smoke exposure are unknown -be it Australia or elsewhere.

Bush fire smoke contains irritants that affect the eyes and the lungs, causing coughing and can exacerbate heart and lung conditions. Tiny particles in the smoke less than 2.5 microns in diameter can lodge deep inside the lungs and get into the bloodstream.

Health experts say the long-term exposure to high levels of PM2.5 particulate matter is still unclear, although research suggests it could increase cases of heart disease and stroke.

Still, the findings are a useful guide for authorities in Australia, the United States, Indonesia and other places increasingly affected by larger and more intense fires.

Indonesia's record fires of 2015 cost the economy more than US$16 billion (S$21.7 billion), the World Bank says. More than 500,000 Indonesians suffered respiratory ailments, while the smoke haze affected regional neighbours.

"An important feature of wildfire-related air pollution is that it is transnational," the authors note.

Smoke from Australia's fires affected air quality in New Zealand, while smoke from the ongoing US West Coast fires has spread to the other side of the country and parts of Europe.

The scale of threat from bush fires on large human populations is a powerful reason to step up efforts to fight climate change, the authors conclude, as well as to improve bush fire management practices.