Heat: The silent killer stalking Australia's summer

Sanaa (right) and her younger sister Lana live in one of Sydney's urban heat islands

For Sanaa Shah's family, summer in Australia is worlds apart from the carefree coastal lifestyle depicted in travel brochures.

"We don't have a close-by beach to go to," the 20-year-old says.

"We can't escape the heat here."

Instead, sweltering days often leave her "locked indoors" with a crippling migraine and trigger heavy nosebleeds for her younger sister.

Her home is in an inland area of Sydney where temperatures can climb 10C higher than seaside suburbs - a result of its geography, lack of green spaces and abundance of heat-trapping surfaces.

That region - western Sydney - has one of the fastest-growing urban populations in the country, as well as rising poverty rates.

And weather data shows that one in every 10 summer days there already surpasses 35C.

Heat is known as Australia's "silent killer" because it's deadlier than all other natural disasters combined yet leaves behind no visual clues as to the scale of its devastation.

But its impacts aren't being felt equally, with more than 60% of such deaths occurring in underprivileged communities like Ms Shah's, according to a climate modelling firm.

Now, experts say that without government intervention, "social inequality" will play a defining role in who survives Australia's increasingly blistering temperatures.

'Urban heat islands'

Australia defines a heatwave as three or more days in a row of unusually high day and night temperatures.

It's amid those hotter conditions that the body can struggle to cool down, prompting a range of illnesses, including heatstroke, which can effectively cook the organs if untreated.

People who are elderly, disabled, or have underlying medical issues are most at risk - but anyone who can't seek out cooler conditions can face lethal consequences.

As the driest inhabited continent, Australia is particularly vulnerable to extreme heat

According to official data, heat claimed almost 300 Australian lives and sent 7,000 people to hospital in the past decade.

But an Australian National University study argues the true numbers are vastly under-reported because death certificates only record certain information. It found heat had contributed to 36,000 Australian deaths between 2006 and 2017.

Much of the danger exists in what scientists call "urban heat islands".

They're built-up areas covered in materials that amplify heat, such as concrete, asphalt, and rows of dark-roofed homes which attract sun and raise household temperatures.

Western Sydney - where 2.5 million Australians live - is a prime example.

Its position at the foothills of Sydney's Blue Mountains shields it from cooling coastal breezes, and many residents live in buildings with inadequate insulation and feel cost of living pressures.

Ms Shah says summertime can be "isolating", as many in her community are forced to "shelter indoors" or find themselves shackled to their air conditioners.

"That in itself is a privilege though, because lots of people can't afford to keep their AC running, so maybe they go to the library or the local shopping centre to escape the heat," she explains.

Now as she studies the impacts of climate change at university, Ms Shah questions why her suburb was seemingly built to "work against the local environment".

"We have lots of tightly compacted homes here with dark roofs that were built quickly and cheaply with poor insulation. And if you drive around, you won't see any trees or green spaces," she says.

"I just don't think that long term sustainability was ever considered."

And she's not alone in her concerns.

Located a 45-minute drive northwest of Sydney, Sanaa's suburb was built in 2007 to accommodate the city's growing population

Nearly 50% of residents quizzed in Australia's largest survey on the health impacts of heatwaves felt their suburbs had been built in ways which increased heat.

The nationwide survey, conducted last year by charity Sweltering Cities, also found that almost 70% of people reported feeling unwell on hot days.

The charity provides information to people living in urban heat islands on where to seek refuge in extreme temperatures, or how to use cheaper cooling techniques such as homemade window coverings or outdoor awnings.

What can be done?

Authorities have made efforts to combat the problem. Australia's Bureau of Meteorology recently put in place a heatwave warning system to alert communities about incoming hot weather and the associated health risks.

State and local governments across the country are investing in revegetation projects to provide more urban green cover. In Melbourne, two "chief heat officers" have even been appointed to find ways to make the city more weather-resilient.

But Emma Bacon, who founded Sweltering Cities, argues that meaningful progress will require bigger policy shifts.

She wants the federal construction code to be updated to ensure that up-to-date climate data informs development; a review of heatwave emergency plans in every state; and a ban on dark roofing nationally.

Last year, New South Wales tried to implement a requirement for lighter-coloured roofs to be used on all new homes to improve energy efficiency.

However, the policy was scrapped amid concerns from property developers that the changes would make it harder to deliver affordable housing in what is already one of the most expensive markets globally.

For Ms Bacon it was yet another "short-sighted" example of lawmakers failing to take the dangers of heat seriously.

"The way we plan our cities today decides how many people die in the heatwaves of tomorrow," she says.

"Unlike in other disasters, whether you stay safe in extreme temperatures comes down to your resources and what your home and workplace looks like."

More extreme fire, drought and heatwaves will hit Australia in the coming decades

There is also criticism of Australia's reliance on coal and gas to underpin its electricity grid and economy.

Since taking office last year, Prime Minister Anthony Albanese has promised to slash emissions much faster than his predecessors, but his government has also approved several new coal mines.

"We're having a much more honest conversation about climate change than a few years ago, but we're also opening up fossil fuel developments and literally adding fuel to the fire," Climate Council researcher Simon Bradshaw says.

Mr Bradshaw, who studies extreme weather, warns that unless Australia "weans itself off coal, oil and gas" this decade, "a lot more people will soon be exposed to lethal heat".

"It will keep getting hotter, but every ton of carbon we leave in the ground will reduce the severity of heatwaves in the future," he says.

But carbon cuts will have to happen at a household level too, experts say - which means teaching people different ways to cope on stifling days, rather than immediately turning to their air conditioning.

"We can't stay locked inside this vicious cycle, whereby we respond to hot weather with mass air conditioning use, fuelled by electricity that is predominantly generated by fossil fuel power plants," Prof Ollie Jay from Sydney University explains.

Prof Jay - who works in a specially-made climate chamber that simulates extreme heatwaves - is urging Australians to adopt "low resource" cooling strategies this summer.

The most effective include using fans to focus on reducing the body's temperature rather than that of the broader home environment, wetting the skin or dunking the feet in a bucket of water, or putting ice in a towel and resting it on the back of the neck.

These are science-backed methods that can have a big carbon impact, he says. But crucially they're affordable and practical - which for Ms Bacon and her team, as they check in on people, is essential.

"We'll be doing everything we can to be present in hot suburbs and keep the communities we work with safe," she tells the BBC.

"But we also shouldn't be living in a country where people die in heatwaves because they're poor or vulnerable."