The heat is on: what does that mean for Australians?

Photograph: Mike Bowers/The Guardian

Last year was the hottest year in recorded history. Global average temperatures over 2023 nudged towards 1.5C above pre-industrial levels, and for two days in November, they reached 2 degrees above those levels.

With a hotter planet come more intense – and therefore more deadly – heatwaves.

Heatwaves kill more people than any other extreme weather event

Extreme heat is already the most deadly of natural hazards, causing or contributing to the deaths of more than 5,300 Australians between 1844 and 2010. And that doesn’t include the record-breaking heat of the past decade.

Heat is a selective killer. The majority of its victims are over 60 years of age, with those aged over 85 at greatest risk. It also targets those in socioeconomically disadvantaged areas; between 2000 and 2018, nearly one-third of all heatwave deaths in Australia occurred in the top 20% most socioeconomically disadvantaged areas of the country. Pregnant people and their unborn babies, as well as the very young, are also more at risk from severe heat.

Indigenous Australians appear to be more resilient to heat than non-Indigenous Australians, but there are limits. Remote Indigenous communities, particularly those in desert regions, are more vulnerable as temperatures reach inhospitable levels.

Heat deaths are also difficult to pin down, because they can be disguised as deaths from pre-existing diseases such as heart disease and diabetes. That means the figures we have are likely an underestimate.

Heat kills through dehydration, heart and organ failure

When heat shifts from pleasant to uncomfortable, humans sweat. It’s the most important cooling mechanism we have, and works by shedding heat as the salty water secreted by sweat glands evaporates on our skin. According to experts, we can sweat half a litre an hour.

To supply that sweat, the body increases blood flow to the surface of the skin by dilating the tiny capillaries throughout the skin. But we only have so much blood flow.

“Our heart has to maintain all of its other blood pressure considerations; supply muscle[s], [the] brain and all the other organs that it can,” says Dr Aaron Bach, an environmental physiologist at Griffith University in Brisbane. “This means that it has to pump harder and has to pump faster to do so.”

For hearts already compromised by age or disease, that can lead to cardiovascular collapse.

The loss of fluid from increased sweating also affects the kidneys’ ability to function normally, both because of dehydration and the drop in blood pressure from the increased blood flow to the skin and reduced blood flow to central organs.

Gut function also suffers with reduced blood flow and dehydration, which can cause it to release bacteria and inflammatory substances into the blood – leading to organ failure.

All these catastrophic events become more likely in the elderly, who have a lower ability to sweat – partially because they are also likely to have other medical conditions, such as heart disease or kidney disease, that puts them at higher risk. They are also often less able to take measures to cool themselves down, like travelling to a place with air-conditioning.

People with disabilities, who might be less able to do things to cool themselves down (like access a cooling shelter), and the very young are also at risk.

When does overheated and dehydrated become heatstroke?

It doesn’t only take heat to get dehydrated – not drinking enough water, even in cold temperatures, can cause it – but high temperatures and sweating make it much more likely. Mild dehydration comes with symptoms such as dizziness, nausea, dark brown or yellow urine and dry mouth. More severe dehydration leads to lethargy, irritability, paleness, cold hands and feet, faster breathing and confusion.

But in very hot weather, the big risk is heatstroke. Normal body temperature is around 36.5C to 37.5C. When it hits 40.5 degrees, you’re in dangerous territory where the body simply cannot compensate for the amount of heat it is exposed to.

That can present as profuse sweating or the cessation of sweating altogether; as confusion, dizziness, nausea, disorientation, and – as it worsens – loss of consciousness, seizures and death.

Don’t just look at the maximum temperature, look at the minimum too

Heatwaves are defined not just on the basis of how unusually hot it gets during the day over three days, but how cool it doesn’t get during the night.

“That’s the time where you would passively shed excess body heat,” says Prof Ollie Jay, a professor of heat and health at the University of Sydney. “The less you’re able to do that, you’re basically starting hotter the next day.”

Hotter nights mean your body, and particularly your heart, have to work harder to keep you cool. It means lower-quality sleep, which means starting the next hot day already exhausted. It also means more dehydration, especially if using fans to keep cool overnight.

What’s the deal with ‘feels like’ when it comes to temperatures?

Communicating temperature and how hot it will make us feel is a tricky thing, because the picture is far more complicated than the single figure we get in the nightly forecast. That represents what’s called dry bulb temperature, which is ambient air temperature in the shade.

But “if you’re out in the sun, you’re exposed to 12C to 15C hotter temperature[s], and that’s because you are gaining heat via radiation,” Jay says.

Wind speed also affects how hot you feel, because wind – whether natural or from a fan – increases the amount of evaporation from the skin and so helps you to feel cooler.

If it’s humid, that sweat will have a harder time evaporating, so temperatures will feel a lot hotter than the digits would suggest. That’s measured by what’s called “wet bulb” temperature – literally a thermometer with a wet bulb.

The limit of human survivability is thought to be around 35C wet bulb temperature, which is 35C with 100% humidity; “you can probably survive in it for about six hours before you die of heatstroke, no matter what you’re doing,” Bach says.

The lower the humidity, the higher the temperature can get before we reach that fatal threshold.

What we’re doing and what we’re wearing also affect how hot we feel. If we’re engaged in physical activity, our muscles generate heat, and our body has to work harder to get rid of that heat. And if we’re not dressed appropriately, that also makes things worse.

“Clothing acts as a barrier to heat dissipation, so it blocks the conduction or convection of heat away from the body,” Jay says. “Also, if something has a high evaporative resistance, it stops that sweat evaporating as well.”

How to keep cool: water on the feet, ice on the neck

The best methods for keeping cool work on the person rather than their surroundings, and also take into account power supply, cost and environmental impact.

For example, while fans can help cool the skin, they require electricity and are therefore useless during power cuts that can happen during heat waves. There’s also evidence worries about electricity bills stop people using electric fans, even in heatwaves.

Fans also increase evaporation of sweat, which can increase the risk of dehydration. Misting fans and evaporative coolers can also help, but only in dry conditions and when there’s power.

“Immersing your hands and your feet in tap water is a really effective strategy at reducing core body temperatures,” says Bach. Our hands and feet have a lot of surface area, so that maximises the exchange of heat into the water.

Icy towels – wet towels wrapped around crushed ice, or which have been soaked in a bucket of icy water – placed on the neck are also good, “because blood is flowing close to the surface of the skin and it’s flowing to the brain and that blood is coming straight from the heart,” Bach says.

Given the average volume lost from the body, Jay says that as a general guide on hot days we should drink 250ml of water every hour, and that thirst is not always a reliable guide to dehydration.

At a civic level, there is a greater focus around the world on the need for “cooling shelters”; places such as libraries, community centres and schools with air-conditioning, power and water where people can go during a heatwave. But there are barriers to be overcome to make those accessible and welcoming to everyone, including the less-mobile, the elderly or people experiencing homelessness.

“The solutions that we have for all these different subgroups and people are going to be different,” Jay says. “Once we find solutions that work, we then have got to figure out how to change practice and policy at the public health level and at the individual level as well.”