In the last days of 2018, Dr Kim Loo did something she had never done in 30 years of treating patients: she wrote “heat” on a death certificate.

One of her long-time patients had gone out in a heatwave, and was later found dead back at his home in western Sydney. Heart attack was the official cause – he was 81 with cardiovascular and lung problems – but heat was “so obviously a contributing factor that I wrote it down too,” says Loo. “It shouldn’t have happened. It was heartbreaking. No one should die in a heatwave.”

Extreme heat has killed more people than any other natural disaster in Australia, but experts say many deaths by “the silent killer” still go under the radar – even as they increase. Today, Australia is one of the fastest-warming corners of the world, 1.5 degrees hotter than it was before the Industrial Revolution. Doctor and researcher Dr Simon Quilty says some places, including the remote Northern Territory where he works, are starting to cross temperature thresholds beyond human endurance.

So, what does heat do to the body, how do you stay cool, and can you adapt?

What happens to our bodies when they get hot?

Humans might live in all kinds of climates, from the Arctic chill to the searing Sahara desert, but we have evolved to survive with an average core body temperature of 37 degrees (give or take half a degree). Too cold and we get hypothermia. Too hot and it’s hyperthermia – dangerous overheating.

Professor Ollie Jay, director of The Heat and Health Research Incubator at the University of Sydney, has studied both phenomena, having lived in wintry Wales and Canada, and now running the Thermal Ergonomics Laboratory in Australia.

When the outside temperature stays within our so-called “thermoneutral zone” of about 20 to 25 degrees, our bodies don’t need to do anything much to maintain our core body temperature. But when the weather warms up, Jay says, two main defence adaptations kick in. And both start in the skin.

The first we know well: sweat. The second is just below the surface – blood vessels dilate to pump blood up to the skin and allow more heat to vent out of the body.

Sweat alone does not cool you down, says Jay, who has worked with Cricket Australia and the Australian Open on their heat policies to protect players. It’s sweat evaporating from your skin that does the trick. But if it’s humid, with lots of moisture in the air, or not much airflow, that becomes harder. Eventually, you can hit a point where your sweat can’t evaporate properly and your core body temperature will keep ticking up and up.

What are ‘wet bulb’ and ‘black globe’ temperatures?

As climate change drives both hotter and wetter weather around the globe, experts are looking beyond the standard temperatures issued by the Bureau of Meteorology to “wet bulb” temperature. That measures humidity as well as heat, using a thermometer covered in a wet cloth to gauge evaporation. The number will be lower than the weather forecast, but every degree has a bigger impact.

In fact, the highest wet bulb temperature humans can survive is about 35 degrees – beyond that we can no longer cool ourselves via sweat evaporation and can die after a few hours, even in the shade drinking water. “Thirty-five degrees [wet bulb] is about survivability, everyone will die,” explains Jay. “But before that, we have severe impacts on liveability, and the most vulnerable can die.”

A 2020 study found that global warming is pushing up wet bulb temperatures faster than expected. Parts of the Middle East and south Asia are approaching the deadly 35-degree threshold (even briefly crossing it) and will probably regularly cross it by 2075. Up north in Australia, Quilty says, people in humid Katherine often fare worse at 35 degrees on the regular forecast than those in the drier Alice Springs do at 40. “And when you get high humidity with heat, it’s incredibly dangerous. In 2019 in Katherine, we had 54 days above 40 degrees. The average is six.”

Jay says wet bulb temperature is an “elegant” way to underscore the importance of humidity but there are other factors to consider too, such as wind and sun.

Wind will affect how well your sweat can evaporate. “We all know that sweet relief of a breeze,” says Jay. But without it, you’re in even more trouble. As one scientist pointed out, wet bulb temperature assumes maximum airflow, the equivalent of “a naked, healthy adult standing in the shade with gale-force winds blowing on them while they were drinking gallons of water”.

Meanwhile, “black globe” temperature measures solar radiation. “In summer, it’s about 15 degrees higher in the sun than in the shade,” Jay says. “But the temperature you get from the bureau is the shade so using that alone as an indication of your risk is fraught with problems.”

In western Sydney, Loo says, temperatures are often “6 to 10 degrees hotter than the rest of Sydney”. She describes the rows of black roofs, of baking fake turf and concrete, and few trees to keep out the heat. Penrith was the hottest place on Earth during the height of the Black Summer bushfires (although records tumbled again this January when the outback town of Onslow, Western Australia, hit 50.7 degrees).

Jay’s team is working on a more robust approach to heat risk that looks at someone’s age, sun exposure, activity, even their clothing.

As we age, we sweat less. And, as we move around, our muscles generate heat inside our body. “So this heat needs to be vented out through the skin,” Jay says. “If you encased yourself in a suit – imagine the thickest, most insulating suit – you’d cook yourself in six hours just sitting down. So for people working outdoors in the sun, maybe wearing protective clothing, moving a lot, suddenly these other factors become very important.”

How does heat kill you?

Our 37-degree core temperature is the optimum level for our body’s internal chemistry to function, Jay says. So, while a difference of a degree or two might not feel like a lot outside, inside it can quickly send things off kilter. (“We are always about 4 or 5 degrees away from catastrophe.”)

At a high enough temperature, our cells can collapse and die, as the bonds holding key proteins together snap. “Think of cooking a steak,” says Quilty. “It gets tender, the proteins start to denature.”

When the external temperature stays high, it starts to infiltrate the body, Quilty says. “In the last five years [in the Northern Territory], we’ve seen temperatures so extreme that we will die if we can’t shelter properly, we will cook. People doing manual work long-term in hot temperatures can get damage to their kidneys over time, too. That’s literally the kidneys cooking from the inside.”

Heatstroke is a medical emergency, adds Loo: “We treat it like a heart attack.” It happens when the amount of heat inside the body drives up our core temperature to a critical level. That’s beyond, say, a short-lived fever that the body might induce to fight a nasty infection. “The skin gets red-hot. Someone could get a dry, swollen tongue; they might be intensely thirsty; they’re often aggressive because their brain isn’t functioning properly; and others walk away not realising what’s happening, but from there it can go very quickly to seizure and loss of consciousness.”

Someone with heat stroke should be cooled down as quickly as possible. They may be rushed to hospital, where doctors may rehydrate with an intravenous drip.

But, if you stay at an extreme temperature too long, there is a point of no return, Loo and Jay say, where the damage to the body becomes too great. “There’s been core body temperatures of 42 degrees measured in people who have died of heatstroke,” Jay says.

When it comes to this kind of acute heatstroke – a body pushed beyond its limits – deaths can happen among young, otherwise healthy people.

Professor Ollie Jay

The two responses – sweating and blood vessel dilation – that help cool our bodies also cause much of the damage during such an event, he says.

The redirection of blood away from internal organs to the skin can deprive them of oxygen and put more pressure on the heart. Humans are about 60 per cent water, and “like any piping system”, says Jay, “things affect the pressure”. “More blood to the skin means our central blood pressure will go down. To compensate, our heart rate will climb as it pumps faster to get the blood everywhere it needs to go.”

Meanwhile, as we sweat, we can become dehydrated. We lose water and our blood thickens – “that means our heart now has to do more with less, adding even more pressure”.

In our gut, if the blood flow gets too low, the protective membrane holding in the toxins there can start to break down – and “nasties can leak out”, says Loo. That can set off a cascade of problems as the body mounts an immune defence. “If the gut leaks – remember we have E. coli in there – the body will have an inflammatory response,” she says, and that can lead to widespread clotting and even organ failure.

She points to the 2004 case of an Australian soldier dying during military training up north in extreme heat and humidity, and cases of labourers reluctant to complain about hot conditions ending up with rhabdomyolysis from heat exertion – “that means their muscles are liquefying”.

When it comes to this kind of acute heatstroke – a body pushed beyond its limits – Jay says deaths can happen among young, otherwise healthy people. In 2001 US football player Korey Stringer died from heatstroke during training. An institute dedicated to studying the illness now bears his name.

“Heatstroke is what we worry about with sport in summer, for example,” says Jay. “But heatstroke is a small proportion of the people who actually die in heatwaves.”

So, how else does heat kill?

Mostly, heat kills by setting off a pre-existing condition. “Often, it triggers a heart attack,” says Jay. Even before our core temperatures hit heatstroke territory, all that extra stress on our body, or “heat strain”, can overwhelm. “If you have plaque building up in your coronary arteries, for example, not enough blood will be able to get through to deliver oxygen around the heart. Or, the strain could set off an electrical issue in its wiring.” The redirection of blood to the skin can also spell trouble for the kidneys, causing renal failure.

Or heat can kill in more unexpected ways. Taking cocaine is always dangerous but scientists have found you’re more likely to have a fatal overdose if using on a hot day, for example.

“Someone could have a seizure while driving and die in a crash because, unbeknown to them, their epilepsy medication has cooked [in the glovebox] and stopped working in the heat,” adds Quilty. “That won’t be listed as a heat death, but heat is absolutely a factor. Lots of medications like insulin are dependent on staying at a cool, stable temperature. ” During that scorching 2019 summer, remote NT medical centres struggled to cool the water in their life-saving kidney dialysis machines.

‘Heatwaves don’t offer the dramatic visuals of other natural disasters. But they can also be harder to escape.’

All these factors can make it hard to narrow in on heat deaths, Jay says, though he thinks attribution is improving. It wasn’t until the Chicago heatwave of 1995, when 739 people died who could only have perished from the heat, that the seriousness of this “silent killer” really hit home.

“You can’t see it. Heatwaves don’t offer the dramatic visuals of other natural disasters. But they can also be harder to escape.”

According to research by Quilty and others, heat-related deaths in Australia are at least 50 times higher than the official numbers reported. Of the 1.7 million deaths in Australia between 2006 and 2017, exposure to excessive heat was listed as a factor in fewer than 1000. But, under analysis, heat was found to play a role in almost 37,000 of them.

Quilty and Loo are part of a growing chorus of doctors pushing for weather events to be included on death certificates. In June 2021, as a record-breaking “heat dome” settled over Canada and killed hundreds, a doctor made headlines for listing “climate change” as the reason he admitted a 70-year-old patient with heatstroke to hospital. To underline the danger, cities such as Athens and Seville are now considering naming heatwaves in the same way they do cyclones.

Who is most at risk in the heat?

Older people, who sweat less and tend to have underlying conditions, are particularly vulnerable but so are children, who cannot always thermoregulate as efficiently as adults, Loo says. “Pregnant or breastfeeding women and babies, too, need to watch for dehydration in particular. People isolated by mental health, people with chronic disease.”

Jay agrees. “Those suffering most are the most vulnerable. When it’s hot, we turn on the aircon. That’s great if you can afford an aircon – or a home, for that matter. Then, sometimes, it’s the elderly person with no family on the top floor of an apartment complex who dies, and they don’t get found for days, even weeks. It’s all very sad.”

Loo says communities that are more connected, where neighbours check on each other, are more likely to survive natural disasters. “I dread summer. I know my patients will be sicker. A patient of mine recently had a newborn, she has no aircon in her rental, she’s had to hike to the shops every day to escape.”

‘If it doesn’t cool down below 30 degrees at night, that’s putting huge physiological stress on the body, but the mind too.’

In the Northern Territory, Quilty says, people often live in poverty, in poor housing without insulation or airconditioning, sometimes without windows. They are regularly hit by power failures. “They can’t shelter from the heat.” Emergency rooms fill up. But the impact isn’t just on physical health. There’s also a well-known “mango madness” season up north in the hottest month of the year, he says, when suicide rates, self-harm and domestic violence all spike. “If it doesn’t cool down below 30 degrees at night, that’s putting huge physiological stress on the body, but the mind too. You’re more impulsive, you can’t think as clearly.”

Loo says many people ignore the early warning signs of dehydration and heat exhaustion, such as nausea, muscle cramps and irritability. “That’s when people really need to rest in the shade and drink. If in doubt, check the colour of your wee.” Darker urine, especially orange or brown, is a sign you’re dehydrated. No colour at all may mean you’re drinking too much water. “That sometimes happens with marathon runners,” says Jay, “and it can be dangerous too”, diluting your sodium levels (one of the major electrolytes).

In October 2021, at the release of a major report in The Lancet on the health implications of climate change, doctors around the world warned of a growing heat toll, paramedics with “burns on their knees from kneeling down to care for patients with heatstroke” and “far too many patients dying” in emergency departments. The number of hours in a year in which it is safe to work and exercise outside is shrinking as temperatures climb, the report found. In 2020, at least 295 billion collective hours of potential work were lost because of extreme heat.

So, what’s the best way to cool down?

In his custom-built climate chamber at the University of Sydney, Jay can simulate any heatwave – even recreate conditions as they were on the ground the day of that 1995 Chicago scorcher. From 90-year-olds to pregnant women, volunteers sweat it out in his lab to test different cooling tactics. It’s led to new work developing Australia’s first heat strategy for children’s sport and extra measures such as frozen towels and misting fans at the Australian Open. “I was surprised how well the misting fans work,” says Jay. But he adds that official government advice on how to beat the heat is still “littered with old wives’ tales”.

Fans sometimes barely rate a mention. And yet they use about 30 to 50 times less electricity than airconditioning and at temperatures below 35 degrees they can be very effective, helping sweat evaporation. Beyond 35 degrees, though, the World Health Organisation advises against fans – “to avoid the turkey in the oven effect”, where it’s thought hot air forced over your body will make you even hotter, like a convection oven. While Jay’s testing in the lab has shown fans can still be effective for young people at up to 40 degrees when it’s humid, airconditioning is often the best way to stay safe in extreme heat. “The problem though is that’s a bit of a maladaptation because aircons [when run by burning fossil fuels] create more greenhouse gases that create more global warming,” he says. “It’s a vicious cycle.”

Drinking ice-cold water won’t lower your core temperature. It will only make you feel cooler.

One of the best home remedies to cool down fast is applying water to the skin by dousing yourself, wearing a wet or frozen towel around your neck or a wet T-shirt, even submerging your feet in cool water, says Jay.But he adds that rigging up DIY evaporative cooling, with wet towels hanging in front of a fan, will probably just add humidity to a room. “It’s only a good idea if it’s very dry.”

Wearing loose clothing to allow airflow over the skin is the most important summer fashion choice you can make (although you can choose reflective white over heat-absorbing black). And staying hydrated is crucial to replace liquid lost through sweat.

But most sports drinks claiming to protect you by restoring electrolytes rarely make much of a difference, Jay says. “It’s something to keep in mind, but unless you’re Rafael Nadal in the Australian Open final losing kilos of sweat over a five-hour match, you’re probably not losing that many electrolytes.”

Jay also notes that drinking ice-cold water won’t lower your core temperature. It will only make you feel cooler. That’s because there are temperature-sensitive nerve endings in our stomach – and these link to our sweat glands, independent of our core temperature, to modulate sweating, Jay says. “So your gut will think you’re cold but that can stop you sweating when you actually need to.”

Similarly, alcohol doesn’t actually warm you up – that’s another sensory trick – but, as with the heat, it will dehydrate you and throw off your co-ordination (not to mention your thinking), so Loo says it’s one to avoid.

Can we adapt to the heat?

People can “acclimatise” to high temperatures. “That’s why our Olympians trained in the Darwin heat before competing in the Tokyo summer,” Quilty says.

Jay says these kinds of adaptations come into effect after about 10 consecutive days exposed to severe heat: you sweat more, your core temperature runs slightly cooler at rest, your blood volume is expanded a little, with more plasma.

“Even the rate of water the kidneys clears changes,” Quilty adds. “People constantly in airconditioning may be making themselves more vulnerable to heat stress.”

Still, Jay says, it’s unlikely that most of us routinely adapt to extreme heat during summer. “One-off hot spells don’t produce it, you need it to run on. People are looking at how people can adapt when they know a heatwave is coming. Whether, say, immersing yourself in a hot tub for 20 minutes a day five days ahead of a heatwave does anything.”

But even acclimatisation tends to decline fast. And there are limits to how much you can adapt. The worst heatwave on record, which killed more than 70,000 people in Europe in 2003, was considered a once-in-a-thousand-year event. But scientists say heat like that is likely to happen at least once a decade by the end of the century if we don’t transition to clean energy soon.

“In Australia, we always wait for the disaster to hit before we do anything,” says Loo, who has been lobbying the government for the past seven years for a faster energy transition, as well as better urban design to cope with heatwaves.

“We knew the [Black Summer] bushfires were coming. We know the heatwaves are here. All we need is a heatwave, COVID and smoke from another fire to hit [at once]. That’s my nightmare.”

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