Australia’s southern states are baking in the most significant heatwave since the 2019-20 black summer. Temperatures surpassing 40C have been recorded in many parts of the country, including Melbourne and Adelaide.
But experts warn that the heat people actually experience could far exceed what is forecast – because the Bureau of Meteorology measures air temperature in the shade.
“Depending on what surface you’re standing on and the radiant heat, it could be up to 10C hotter,” says Dr Kim Loo, the NSW and ACT deputy chair of the Royal Australian College of General Practitioners.
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Heatwaves have been described as a “silent killer” because they do not leave the same large-scale visible scars as bushfires and floods – but extreme heat is the most common cause of weather-related hospitalisations and deaths in Australia.
What effect does extreme heat have on the body? Here are the warning signs of heat stress to look out for, and what to do to minimise health risks.
What are the symptoms of heat-related illness?
Extreme heat results in a spectrum of problems progressing from heat stress to heat exhaustion and heatstroke.
“Heat stress is where you feel uncomfortable from heatwaves. You get weakness, tiredness, cramps and dizziness,” Loo says. “If you feel that, you need to get yourself out of the sun and start drinking [water].”
Heat stress can then progress to heat exhaustion. “Your temperature can go up [above] 38C … you can feel nauseous [and experience] vomiting, headache and fainting,” Loo says. People with heat exhaustion may have reduced sweating due to insufficient fluid.
There may also be behavioural changes, Loo adds. “If you get a little bit of heat stress you might be really cranky and irritable. But if you see someone really aggro, it could be because they’re going from heat stress to heat exhaustion.”
Further stress from extreme heat may result in renal failure or cardiovascular collapse, or heatstroke, a diagnosable medical emergency.
People suffering heatstroke can have core body temperatures above 40.5C and may show confusion, reduced alertness and have red, hot dry skin that no longer produces sweat.
“Heatwaves have caused the most hospitalisations related to extreme weather over the last decade, and the risk of hospitalisation increases with age,” says Dr Michael Wright, president of the RACGP.
How to stay cool and prevent heat stress
Wright’s advice is “to drink plenty of water, try to stay indoors if possible during those peak UV hours of 11am to 3pm … check in on friends and family members who are more vulnerable to the impact of heatwaves”.
Older people, babies, people with chronic illness or who are pregnant or breastfeeding are more susceptible to the effects of extreme heat.
Public health authorities suggest staying well hydrated – drinking six to eight glasses of water a day, and avoiding alcoholic, hot or sugary drinks. Urine that is a “clear to light straw colour” is a good guide you’re drinking enough.
People with heart failure and other medical conditions that may require limiting fluid intake should consult their medical practitioner.
Avoid or reduce strenuous activities, and carry water with you if you have to go outside.
Dr Aaron Bach, an environmental physiologist at Griffith University, suggests “fully closing up your house when it’s at its coolest”.
“First thing when you wake up: close all the windows, close all blinds, try and keep that cool air inside the house if you don’t have air conditioning,” he says. “But eventually … your house will become hotter than outside.”
Fans can safely be used in combination with other measures in indoor temperatures up to 37C. Beyond that, it’s best to seek out somewhere with air conditioning, such as a local library or shopping centre.
In environments that aren’t too humid, wetting the skin can also help. Bach says 80% of the heat the body loses during exercise is due to sweat evaporating. “When you wipe sweat off your skin, you’re losing cooling potential, because you want that to naturally evaporate.”
“Adding water to the skin, whether it be through a wet cloth, a spray bottle, jumping in the shower” can help to cool the body down, he says.
If you can, cool your bedroom to give your body a chance to recuperate while you sleep, Bach adds. “The real risk comes when you just have no real break from that unrelenting heat.”
What does prolonged heat do to the body?
When it’s hot, the body dissipates heat through the skin by increasing blood flow to the extremities, and sweating.
Doing so results in a drop in central blood pressure because the body is sending blood to the peripheries, Bach says. To compensate for that fall in blood pressure, the heart has to work hard to deliver oxygen and nutrients to vital organs.
While this is manageable for short periods of time, a heatwave – defined by the BoM as at least three consecutive days of above-average temperatures – puts extended stress particularly on the cardiovascular system and kidneys.
“It’s the prolonged nature of these high temperatures that make a heatwave a problem,” Bach says.
“The bulk of people who die from heatwaves in Australia is due to cardiovascular collapse,” Loo, a GP in western Sydney, says. The condition often occurs in those over 65 or with pre-existing conditions. “If someone’s got a vulnerable heart and you don’t get enough blood pumping into it, you get heart failure.”
In addition to cardiovascular collapse, kidney failure and heatstroke are also causes of heat-related death.
Kidney failure can occur when someone is dehydrated and not adequately replenishing the fluid they are losing. “You can lose 1 or 2 litres of water in one hour with sweating,” Loo says.
Heatwaves caused 1,009 deaths in Australia between 2016 to 2019.
