When the body can't cool down: What are the risks of extreme heat?
Headlines about how Europe’s brutally hot summer has resulted in thousands of deaths have probably got your attention lately. But do you know why extreme heat is so dangerous?
To understand the impact of extreme heat on the human body, we spoke with Dr Nicole Vargas, who is a Senior Lecturer in Exercise Physiology at The Australian National University School of Medicine and Psychology.
Her research seeks to further our understanding of how we manage and can better mitigate the effects of heat stress during extreme heat events.
A recent study that she was involved in, led by Dr Michael Tong at the ANU National Centre for Epidemiology and Population Health demonstrated that in the ACT, emergency room visits resulting from extreme heat will increase, while visits from extreme cold will decrease in the future.
This future is happening faster than you might think.
In Europe, 2026 heatwaves have been linked to thousands of deaths, according to the head of the World Health Organization.
Closer to home, according to the Bureau of Meteorology, most Australian capital cities face an 80 per cent or greater chance of experiencing historically high maximum temperatures this coming summer, driven by one of the strongest El Niño events on record.
To help us prepare, Dr Vargas breaks down how our bodies fight the heat—and what happens when they lose the battle.
How we manage heat
Our bodies are resilient.
Whether we are walking under the midday sun or exercising, we constantly navigate different levels of environmental heat and physical stress.
“When we're doing physical activity, we're producing metabolic heat within our body that increases our core temperature,” Dr Vargas explains. "But these things aren't necessarily a bad thing,” she says as our body responds and reacts to those changes.
The human body maintains a stable core temperature of about 37°C using a process called thermoregulation.
According to Dr Vargas, the body relies on three distinct pathways to keep us from overheating.
Our first defence is behavioural, including conscious choices such as seeking shade, turning on air conditioning, drinking ice water, or wearing loose clothing to stop our core temperature from spiking.
When behaviour isn't enough, our internal physiological systems kick in, and we start sweating. This is our second defence.
“Every single droplet of sweat that we're evaporating actually takes heat away from the body,” Dr Vargas says. “That's our most beneficial way to cool the body in terms of our physiological mechanisms that we have.”
Simultaneously, our third defence, the cardiovascular system springs into action. Redistributing warm blood from the body's core out to the skin and allowing excess heat to dissipate into the air.
When we lose the battle
The environment still plays a role in whether these biological systems work effectively.
“Our skin temperature usually sits at around 32 to 34 degrees Celsius,” Dr Vargas says. “If the air temperature is above that, we're not able to transfer heat to the environment.
“Worse yet, if the air is highly humid, it is already saturated with water vapour. This reduces the moisture gradient between our skin and the air, preventing sweat from evaporating.
“If sweat can't evaporate, it can't cool you down.”
Why extreme heat is dangerous
When our body’s cooling systems fail, this triggers a dangerous domino effect.
Dr Vargas notes that extreme heat impacts our body in three interconnected ways from a physiological perspective.
The first threat is thermal strain—the relentless rise of your core body temperature.
"If we're not able to release heat into the environment, a continued increase in core body temperature can lead to heat exhaustion."
Dr Vargas says this phase triggers dizziness, confusion, and a feeling of being unwell.
"If our body temperature continues to rise, that can lead to heat stroke."
At this critical point, the major organs in the body do not receive enough oxygen, causing widespread inflammatory response, cell death and major organ failure.
The second threat is cardiovascular strain, or an overworked heart.
Dr Vargas explains that as the body redirects blood to the skin to eliminate heat, the heart is forced to pump much faster to compensate for dropping blood pressure and fluid loss.
"It may exacerbate risk, especially for people with underlying cardiovascular conditions."
The final threat is severe dehydration.
Dr Vargas says a combination of reduced blood flow to the kidneys as well as greater dehydration from sweating, especially if fluids are not being replaced, can lead to acute kidney injury or in extreme cases, kidney failure.
What can we do to keep safe?
When reviewing heatwave advice from Australian State and Territories, Dr Vargas believes that some advice is outdated or lacking evidence, for example, eating cold meals or taking a cold shower.
She says this may help people improve their thermal comfort but does not actually mitigate the heat strain on the body.
She instead recommends the scientifically proven cooling strategies:
Seek air conditioning
Finding somewhere with air conditioning to shelter remains the best option if possible. This is especially critical for vulnerable individuals, ensuring they are in a safe environment where their bodies can cope, and where they are surrounded by people who can help if needed.
Use pedestal fans
A large pedestal fan blowing directly on you is highly effective, but it has environmental limits. Fans work best when the indoor temperature is under 40°C, and with higher levels of humidity (60-80%). If the indoor temperature rises above this level, especially in low humidity regions (10-30%), fans should be avoided. In Canberra, it is generally safe to use a fan, except possibly in the afternoon on very extreme heat days.
Douse the skin with water
By wetting your skin, you are essentially mimicking sweat. Allowing you to get the cooling benefits of evaporation without draining your body's own hydration levels. You can do this easily with a wet washcloth or a handheld spray bottle.
Consider your hydration needs
For most individuals, drinking to thirst is usually okay. If you are working or doing activity outside, you should make sure you have some extra fluids (water and/or electrolyte drinks) to supplement your sweat loss and reduce dehydration. If you are on medication, or have underlying conditions, it may be important to discuss a hydration plan with your doctor.
If Australians get the summer that they are calling for?
Dr Vargas says some simple things matter and being prepared is important:
"Do you have medication in your refrigerator? If you have medication in your refrigerator, what's your game plan if you lose the electricity?
“It's those really simple things that people probably aren't thinking about because they haven't had to deal with it or manage it. But we are likely to see more extreme heat in our future, so having these conversations is important."