As Europe's record-breaking hot weather eases, the extent of its deadly toll is starting to surface.
Heatwaves are a subtle disaster. People are not washed away or caught in the flames.
"I think of them as a creeping killer," said Lucinda Coates, senior research scientist at Risk Frontiers.
"You can't take a photo of it to illustrate a media story, they aren't dramatic."
But their quietness belies the horrifying numbers of lives they claim.
Heatwaves are Australia's deadliest natural disasters.
So how does the body respond to heat and what can we do to protect ourselves from extremely hot weather?
What happens to our bodies in extreme temperatures?
According to Ollie Jay, professor of heat and health at the University of Sydney and head of the Heat and Health Research Incubator, there are three main pathways that can lead to death when humans are exposed to extreme heat.
Be forewarned, they are all pretty nasty.
1. A critically high core temperature
One way the body responds to increased core temperature is to send blood towards the skin to help it cool down.
Dr Jay says it is "a thermoregulatory response called vasodilation".
But the body has got only so much blood, so this reduces the amount of blood available to the internal organs and, consequently, the amount of oxygen reaching them.
That in turn reduces the tightness of the bonds in the cells of the gut.
"So, you start getting something called 'increased gut permeability', which means that all the nasty stuff that normally stays inside your gut starts leaking out of the gut, and it enters the circulation," Dr Jay explains.
That sets off a sepsis response, mass clotting around the body, and can cause death.
2. Catastrophic cardiovascular event
"Again, we only have so much blood inside of the body," Dr Jay notes.
The heart rate increases as the body attempts to circulate a fixed amount of blood around the body at a faster rate.
"So, if you've got an underlying infirmity of the heart muscles because you've got cardiovascular disease, for example, then you're more likely to have a catastrophic cardiovascular event when you're exposed to the heat," Dr Jay says.
3. Dehydration and kidney failure
The main way our body tries to cool ourselves down is by sweating, but again the body only has limited resources. If we don't or can't top up the tank, we will progressively dehydrate.
Dr Jay warns "if you have an underlying kidney problem … let's say renal disease or chronic kidney disease, that might lead to a catastrophic renal collapse or kidney failure".
It is not just heat stroke.
A side note for all those shivering away in southern Australia at the moment — the cold can kill you too.
The key ways your body fights the cold is by drawing the blood in towards the core and by shivering.
"So, if we're deprived of energy for whatever reason, if you're stuck on the mountain, you've run out of food, then it becomes problematic," Dr Jay says.
"And if you're deprived of oxygen, it's a problem as well, because we need extra oxygen to fuel those muscle contractions through shivering."
If your temperature drops far enough you can become hypothermic, which can lead to death.
How deadly are heatwaves?
Given these heat-triggered deaths often present as other conditions, accurately determining how many people die as a result of a heatwave can be tricky.
"Heat-associated deaths or heat-related deaths, they're generally not well documented," according to Lucinda Coates.
Ms Coates has spent many years studying heatwave deaths. She says the best way of telling if a person has died from heat is through assessing coronial records.
But even in the coronial records, heat-related deaths can only be identified usually through a combination of an autopsy, plus an understanding of the circumstances around the death.
"What that means is that many deaths that are in fact heat-related, aren't necessarily going to be recorded as such."
An alternative method is to calculate the excess deaths during the heatwave period compared to a similar period without the heatwave.
"For example, that 2009 heatwave event, there were 374 excess deaths recorded for Victoria," Ms Coates said.
"But the number of deceased persons that were actually recorded in the coronial records for that time period is a lot less."
The issue with using excess deaths is that it involves a high degree of uncertainty; it is difficult to pin down who and how many people died because of a heatwave.
"It's a very sad thing to study. But I guess the satisfying thing for me in all of this is that before I did the study, people didn't really know the heatwaves were such an issue," Ms Coates said.
"Now we do; now we're doing things about it."
What can we do to cool down?
How we handle the heat comes down to both our physiology and our behaviour.
On the eve of a heatwave, there is not a whole lot you can do to improve your physiology, but you can definitely change your behaviour.
Nicole Vargas, lecturer in exercise physiology at the Australian National University Medical School, is an expert on our behaviour and heatwaves.
Behavioural responses to heat include things like drinking enough water, reducing physical activity and seeking shade.
"Whether or not you should drink cool water or thermoneutral [unchilled or around air temperature] water is to be debated," Dr Vargas said.
"It doesn't feel as palatable but when you're drinking cold water, you feel like you're drinking enough.
"But you don't drink as much as when you're drinking normal temperature water."
These behavioural responses are dependent upon our ability to sense that we need to respond.
"So, if vulnerable people have any issues with their thermal sensory signals, we might see that they're at increased risk of not engaging in certain behaviours," Dr Vargas said.
This is an area of ongoing research, but Dr Vargas says there is some evidence to suggest that older people have a reduced thirst sensation, which may inhibit their behavioural response.
Another barrier to responding effectively is socio-economic disadvantage.
Work done by Ms Coates has shown examples of people dying in their homes, where their air-conditioner has not been turned on.
But Dr Jay, whose work in this area has been published in The Lancet journal, says there are cost-effective measures people can implement to beat the heat, such as self-dousing, foot submersions, wearing wet clothing, using evaporative coolers, misting fans, electric fans, ice towels, and drinking water.
A word of warning though about electric fans; they are only useful up to a point when the air is very dry.
Without enough moisture around it gets to the point when it is so hot that the moving air is no longer helping to cool you down.
It's not getting better any time soon
Beyond what we are doing at a personal level, governments could do more to reduce climate change.
Yuming Guo, professor of Global Environmental Health and head of Monash Climate, Air Quality Research Unit, says policies need to be developed to mitigate the long-term shifts in temperatures and weather patterns.
"They need to make policy to reduce carbon emissions from industry and other sources," he says.
"A number of studies, including studies from my group, have clearly provided robust evidence that extremely high temperatures, including heatwaves, and air pollution including wildfire smoke, can increase risks of mortality and morbidity."
A 2021 study in Nature Climate Change found that approximately a third of warm-season heat-related deaths could be attributed to anthropogenic climate change.
