Every summer produces the same news cycle and the same explanation. A heat dome parks over a continent, the death toll climbs, and the coverage settles on a familiar shorthand: the elderly are frail, they have heart conditions, they live alone. All true. It is also, physiologically speaking, a bit of a dodge.
The numbers behind the cycle are not small. An analysis from ISGlobal in Barcelona, published in Nature Medicine, estimated 62,775 heat-related deaths across 654 European regions in the summer of 2024 alone.[1] Stack that on the same team's figures for 2022 and 2023 and the three summers total roughly 181,000 deaths. The mortality rate in people over 75 ran several times higher than in every other age group combined.
So the question worth asking is not whether heat is more dangerous with age. It plainly is. The question is why — and for two decades the mainstream physiological answer has been surprisingly deflationary.
The old answer: it isn't really age
In a widely cited 2003 review, Penn State physiologist W. Larry Kenney surveyed what was then known about aging and temperature regulation and reached a conclusion that still shapes the field. Yes, older adults die in heatwaves at far higher rates. But studies that carefully separated chronological age from the things that travel with it — fitness level, body composition, chronic disease — found that "thermal tolerance appears to be minimally compromised by age."[2]
That is a genuinely useful correction, and it aged well for a long time. It reframed heat deaths as a problem of illness, medication and social isolation rather than an inevitability of birthdays. It is also the reason a lot of otherwise careful health writing has been comfortable saying that a fit 70-year-old handles heat about as well as a fit 30-year-old.
A study published this year in the Journal of Applied Physiology puts real strain on that idea.
Six hours at 43°C, sixty-one people, one clean slope
An Australian-Canadian team led by Harry Brown at the University of Canberra, with collaborators at Sydney's Heat and Health Research Centre and the Montreal Heart Institute, recruited 61 adults spanning ages 20 to 79 — deliberately across the whole adult lifespan rather than the usual young-versus-old comparison.[3]
Each wore an accelerometer for a week, completed a graded exercise test to establish peak oxygen uptake, then spent six hours in a chamber at 43°C and 25 percent humidity, performing intermittent light activity meant to mimic ordinary daily tasks. Core temperature was measured rectally, alongside skin temperature, heart rate, whole-body sweat rate and a battery of cognitive tests.
The central result is a slope, not a threshold. Each additional decade of age was associated with a further 0.08°C rise in core temperature (95% confidence interval 0.04 to 0.13) and roughly 6 grams per square metre per hour less whole-body sweating. Both effects were statistically robust, and both ran continuously across the sample.
Extrapolated across a lifetime, that is the difference between a 20-year-old and someone approaching 80 ending a hot afternoon around half a degree apart in core temperature under identical conditions. Half a degree sounds trivial. In a body already near the top of its thermoregulatory range, with less cardiac reserve to spare, it is the margin between uncomfortable and dangerous.
The more important finding is where the slope starts. There is no inflection point at retirement age. The decline is already underway in the forties and fifties, in people who feel entirely fine in a heatwave and have no reason to think of themselves as heat-vulnerable.
Fitness helps. It does not rescue you.
The same study tested the reassuring hypothesis directly, and the answer is nuanced enough to be worth stating precisely.
Higher cardiorespiratory fitness did measurably help. Every additional 10 mL/kg/min of peak oxygen uptake was associated with 11 g/m²/h more sweating and an end-of-exposure heart rate about 4 beats per minute lower. That is a real reduction in cardiovascular strain, and it is one more entry on the long list of reasons VO2 max keeps out-predicting almost every other health marker.
But fitness did not offset the age-related rise in core temperature. The authors' own framing is that cardiorespiratory fitness moderates age-related heat susceptibility without preventing it. Weekly moderate-to-vigorous activity and sex showed minimal association with the outcomes at all.
One more detail deserves mention, because it cuts against an easy narrative. Cognitive performance did decline with age — but independently of the heat exposure. The heat did not selectively degrade older participants' thinking. Whatever the risk is, it is thermal and cardiovascular, not a story about heat fogging older brains faster.
The surprising part: the wiring is fine
If sweating falls with age, the intuitive explanation is that the nervous system is failing to call for it. A second 2026 study, published in The Journal of Physiology, went looking for exactly that — and did not find it.[4]
Thomas Deshayes, Daniel Gagnon and colleagues at the Montreal Heart Institute recorded skin sympathetic nerve activity directly, along with local sweat rate and skin blood flow, in 15 adults aged 60 to 78 and 16 aged 21 to 40. They measured the onset threshold: the core temperature at which the body finally decides to start sweating and open up skin blood vessels.
Before any intervention, those thresholds did not differ between the groups — not for nerve activity, not for sweating, not for skin blood flow. The older nervous system was issuing the order at the same body temperature as the younger one. The deficit lies downstream, in how much sweat the glands actually produce and how much blood the heart can send to the skin once the order arrives.
That distinction matters, because a signalling failure would be hard to train. A downstream capacity problem might not be.
Seven days of hot baths
Which is what the second half of the study tested. Both groups then completed seven consecutive days of hot water immersion — 90-minute baths at 40°C, a passive protocol requiring no exercise, which matters for people who cannot easily train in the heat.
Afterwards, the older adults were starting to sweat and vasodilate at meaningfully lower core temperatures: thresholds fell by 0.19°C for nerve activity, 0.15°C for sweating and 0.23°C for skin blood flow. Resting core temperature dropped and heart rate during heat exposure fell in both groups.
And the size of those gains in the 60-to-78 group was statistically indistinguishable from the gains in the 21-to-40 group. Heat acclimation — a body of research built almost entirely on athletes and soldiers[5] — appears to work about as well at 75 as at 25.
What this does and doesn't license
It would be easy to convert this into a wellness prescription, and we would rather not.
The acclimation study involved 31 people in a laboratory. Ninety minutes in a 40°C bath every day for a week is a substantial cardiovascular load — the same load that makes regular sauna use look so good for the heart is also the reason it is not a neutral thing to self-prescribe if you have cardiovascular disease, kidney impairment or take medication affecting blood pressure or fluid balance. The authors themselves flag that the minimum effective dose is unknown and question whether such protocols are feasible at scale. Nobody has shown that acclimating older adults reduces heatwave deaths; that trial does not exist.
Heat adaptations also decay. In athletes the benefits fade substantially within one to two weeks of stopping, which makes this a pre-season strategy rather than something to attempt once a heat warning is already issued.
There is, though, a genuinely encouraging finding sitting alongside all of this. The same ISGlobal group estimated that Europe's heat mortality burden in 2023 would have been 80 percent higher without the adaptation that has already occurred since 2000 — and more than 100 percent higher in people aged 80 and over.[6] Air conditioning, cooling centres, warning systems and better clinical awareness are already preventing an enormous number of deaths.
The practical read
The two 2026 studies do not overturn Kenney's correction so much as sharpen it. Age alone does erode heat tolerance, gradually and from early adulthood. Fitness blunts it without removing it. And the erosion is not a hardware failure — the control system still works, and still responds to training, at 78.
What follows from that is unglamorous. If you are over about 50, treat heat as something your body has measurably less margin for than it did at 25, regardless of how fit you feel. The interventions that actually save lives in a heatwave remain boring and effective: staying out of peak-afternoon heat, cooling the skin with water rather than relying on a fan alone in very dry heat, keeping fluid intake up, reviewing medications with a clinician before summer, and checking on people who live alone. Building the aerobic base that shows up in how well you hold on to strength and function with age pays here too, just not as much as you would hope.
And if this summer wrecked your sleep, that is not incidental — heat is one of the more reliable ways to lose the deep sleep that does the most repair work, which we cover in our piece on sleep and longevity. The rest of the unexciting groundwork is in our guide to daily longevity habits.
Common questions
Why are older adults more sensitive to heat?
Two things change with age. Sweat output falls — roughly 6 grams per square metre of skin per hour less with each decade — and the heart has less spare capacity to push blood to the skin, which is how the body dumps heat. In the 2026 chamber study, core temperature rose about 0.08°C more per decade of age. Across a lifetime that compounds to roughly half a degree between a 20-year-old and a 79-year-old in identical conditions. Chronic illness and medications such as diuretics and anticholinergics add further risk on top of the physiology.
Does being fit protect you from heat as you age?
Partly, but less than most people assume. Every 10 mL/kg/min of extra cardiorespiratory fitness was associated with 11 g/m²/h more whole-body sweating and a heart rate about 4 beats per minute lower at the end of a six-hour heat exposure. Fitness did not, however, offset the age-related rise in core temperature. Fitness moderates heat strain without cancelling it, so a fit 70-year-old still needs to take a heatwave seriously.
Can older adults still acclimatise to heat?
Yes. After seven consecutive days of hot water immersion, adults aged 60 to 78 began sweating and dilating skin blood vessels at a lower core temperature, with threshold reductions of 0.15°C to 0.23°C, and the size of that improvement was statistically indistinguishable from the young group's. The study involved only 31 people in a laboratory, and daily 90-minute hot baths are a genuine cardiovascular load, so anyone with heart or kidney disease should talk to a clinician before attempting anything similar.