LLast year was the hottest year in history. Global average temperatures rose towards 1.5 degrees Celsius above pre-industrial levels in 2023, and for two days in November they reached 2 degrees above that level.
With a hotter planet comes more intense – and therefore deadlier – heat waves.
Heat waves kill more people than any other extreme weather event
Extreme heat is already happening most deadly natural hazardswhich caused or contributed 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 victims are over 60 years old, with people over 85 years of age at greatest risk. It also targets people in socio-economically disadvantaged areas; between 2000 and 2018, almost a third of all heatwave deaths in Australia occurred in the top 20% most socio-economically disadvantaged areas of the country. Pregnant people and their unborn babieslike the very youngestare also more at risk from intense heat.
Indigenous Australians appear to be more resilient warmer than non-indigenous Australians, but there are limits. Remote indigenous communities, especially those in desert areas, are more vulnerable as temperatures reach inhospitable levels.
Heat deaths are also difficult to diagnose because they can be disguised as deaths from pre-existing conditions such as heart disease and diabetes. That means the numbers we have are probably an underestimate.
Heat kills due to dehydration, heart and organ failure
When the heat changes from pleasant to uncomfortable, people sweat. It is the most important cooling mechanism we have and works by releasing heat as the salty water secreted by the sweat glands on our skin evaporates. According to experts, we can sweat half a liter per hour.
To supply that sweat, the body increases blood flow to the skin’s surface by dilating the small capillaries throughout the skin. But we only have a limited blood flow.
“Our heart must maintain all its other blood pressure considerations; supply muscle(s), brain and all other organs it can,” says Dr. Aaron Bach, an environmental physiologist at Griffith University in Brisbane. “This means he has to pump harder and faster to do that.”
For hearts already compromised by age or disease, this can lead to cardiovascular collapse.
The loss of fluid from increased sweating also affects the ability of the kidneys to function normally, both due to dehydration and the drop in blood pressure due to increased blood flow to the skin and decreased blood flow to central organs.
Bowel function also suffers from reduced blood flow and dehydration, which can release bacteria and inflammatory substances into the blood, leading to organ failure.
All of these catastrophic events become more likely in the elderly, who have less ability to sweat – in part because they are also likely to have other medical conditions, such as heart or kidney disease, that put them at greater risk. They are also often less able to take measures to cool themselves, such as traveling to a place with air conditioning.
People with disabilities, who may be less able to do things to cool themselves (such as accessing a cooling shelter), and the very young are also at risk.
When does overheating and dehydration become heatstroke?
Not only does it take heat to become dehydrated – not drinking enough water, even in low temperatures, can cause it – but high temperatures and sweating make it much more likely. Mild dehydration is associated 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 heat stroke. The normal body temperature is between 36.5 and 37.5 degrees Celsius. When it hits 40.5 degrees, you are in dangerous territory where the body simply cannot compensate for the amount of heat it is exposed to.
That can present such as profuse sweating or complete cessation of sweating; such as confusion, dizziness, nausea, disorientation and – as this worsens – loss of consciousness, seizures and death.
Don’t just look at the maximum temperature, but also at the minimum temperature
Heat waves are certain not just based on how unusually warm it gets during the day for three days, but also how cool it doesn’t get at night.
“That’s when you passively lose excess body heat,” says Prof. Ollie Jay, professor of heat and health at the University of Sydney. “The less you can do that, the warmer you will start the next day.”
Hotter nights mean your body, and especially your heart, has to work harder to keep you cool. It means poorer quality sleep, which means you start the next hot day already exhausted. It also means more dehydration, especially if fans are used to stay cool at night.
What about ‘feels like’ when it comes to temperatures?
Communicating the temperature and how warm it will make us feel is a tricky thing, because the picture is much more complicated than the single figure we get in the overnight forecast. That represents what is called dry bulb temperaturewhat the ambient air temperature is in the shade.
But “when you’re in the sun, you’re exposed to 12 to 15 degrees higher temperatures, and that’s because you’re gaining heat through radiation,” says Jay.
Wind speed also affects how warm you feel, because wind – natural or from a fan – increases the amount of evaporation from the skin, thus helping you feel cooler.
If it is humid, it will be more difficult for sweat to evaporate, so the temperature will feel a lot higher than the numbers suggest. This is measured by the so-called “wet bulb” temperature – literally a thermometer with a wet bulb.
It is thought that the limit of human survival is round 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 do,” says Bach.
The lower the humidity, the higher the temperature can get before we reach that fatal threshold.
What we do and what we wear also affects how warm we feel. When we are physically active, our muscles generate heat and our body has to work harder to get rid of that heat. And if we are not dressed properly, it also makes things worse.
“Clothes act as a barrier to heat dissipation, thus blocking the conduction or convection of heat away from the body,” says Jay. “If something has a high evaporation resistance, it also stops sweat from evaporating.”
How to stay cool: water on the feet, ice on the neck
Best methods to keep cool working on the person rather than their environment, and also taking into account power supply, costs and environmental impact.
For example, although fans can help cool the skin, they require electricity and are therefore useless during power outages that can occur during heat waves. There is also evidence that concerns about electricity bills prevent people from using electric fans, even during heat waves.
Fans also increase the 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 power is available.
“Immersing your hands and feet in tap water is a very effective strategy for lowering body temperature,” says Bach. Our hands and feet have a large surface area, which maximizes heat exchange with the water.
Icy towels – wet towels wrapped around crushed ice, or soaked in a bucket of ice-cold water – placed on the neck are also good, “because the blood flows close to the surface of the skin and flows to the brain and that the blood comes straight from the heart,” says Bach.
Considering the average volume the body loses, Jay says that as a general guideline we should drink 8 ounces of water every hour on hot days, and that thirst is not always a reliable indicator of dehydration.
At a societal level, there is increased attention around the world to the need for “cooling shelters”; places like libraries, community centers and schools with air conditioning, electricity and water where people can go during a heat wave. But barriers need to be overcome to make it accessible and welcoming to everyone, including the less mobile, the elderly or those experiencing homelessness.
“The solutions we have for all these different subgroups and people are going to be different,” Jay says. “Once we find solutions that work, we need to figure out how to change practice and policy at the public health level and also at the individual level.”