As the UK Prepares to Write its First Rules on Heat at Work, New Research Finds Deadly Heat Well Below 35°C

During the first heatwave of the year, I read the following headline: “Non-survivable heatwaves: What scientists just learned about extreme temperatures and wet-bulb temperature threshold.” My first thought was uncharitable: Oh good, I thought. They got it wrong. The number that tells us how hot is too hot for a human being to survive. Splendid. I clicked through and read the paper, hoping to find the correct number.

I was wrong. The number wasn’t wrong. The number didn’t really matter. It never did.

The Number

Wet-bulb temperature sounds like something from a plumbing catalogue but it is, in fact, the lowest temperature the air around you can be reduced to by evaporation alone. This is a technical way of describing sweating. It’s measured by a thermometer with a wet sock on it.

Your body dumps excess heat mainly by evaporating sweat off your skin. If the air is already saturated with moisture, evaporation slows and eventually stops. At that point you have no way left to cool down.

In 2010, two climate scientists, Steven Sherwood and Matthew Huber, worked out where that boundary sits. Their answer was a wet-bulb temperature of 35°C. Six hours at that level and a healthy adult dies of heat stroke.

They were careful about their assumptions. A person naked, at rest, in shade, with unlimited water and unlimited airflow. It was a statement about physics, under perfect conditions. It was not a statement about biology, physiology, or about sunlight, or how anyone actually lives.

This distinction got lost somewhere between the journal and the general public. The figure escaped, took on a life of its own and has spent fifteen years being quoted as a survivability threshold.

What the New Paper Did

In March 2026, a team led by Sarah Perkins-Kirkpatrick, with physiologists including Jennifer Vanos and Ollie Jay, published in Nature Communications under a title with absolutely no hedging: “Deadly heat stress conditions are already occurring.”

Their approach was simple. Rather than model the future, they went back to six real heatwaves: Mecca 2024, Bangkok 2024, Phoenix 2023, Mount Isa 2019, Larkana 2015 and Seville 2003. They asked whether the conditions had actually been survivable for the people standing in them. Not by wet-bulb temperature but using a model built around human physiology: how much sweat a body can produce, how much of it can evaporate, what the sun adds and how all of that changes with age.

Not one of those six events came anywhere near the famous wet-bulb temperature of 35°C and in every single one, deadly thresholds were crossed for older people out in the sun.

Phoenix, Arizona

This is the part that has been rattling around in my head ever since.

Across the studied cities, peak wet-bulb temperatures ranged from 30.85°C in Larkana down to 24.32°C in Phoenix.

Twenty-four degrees. Nearly eleven degrees below the famous 35°C line. A number so unremarkable that if you’d been monitoring wet-bulb temperature that month, you’d have concluded Phoenix was fine.

Phoenix was not fine. That July, 24% of rolling six-hour windows were non-survivable for older people in direct sun. A quarter of the month.

Then the numbers on the ground. Maricopa County recorded 645 heat related deaths in 2023. A 52% increase on the year before and the most ever counted. Almost two thirds of them were among people aged 50 and over.

And then the detail that stopped me properly. Three quarters of those deaths happened outdoors.

Why Dry Heat Matters

Here is the thing I hadn’t understood and which I suspect most people haven’t either.

Wet-bulb temperature measures how much cooling the air will permit. In very dry air, it permits an enormous amount, so the wet-bulb number stays low and everything looks safe.

There is a second limit that nobody was tracking: how much sweat a body can actually make. This is a real, finite ceiling and in extreme dry heat, you hit it long before the air runs out of capacity. You are not killed by humidity. You are killed by your own maximum output: then by dehydration, then by the sun on your skin, which the standard wet bulb measurement doesn’t include at all, because thermometers are kept in the shade.

Older people hit this ceiling sooner, because sweating declines with age.

So: extremely hot and very dry conditions turn out to be just as deadly as hot and humid ones but dry air always produces a low wet-bulb reading. That is just what the measurement is. It climbs when humidity climbs. In a dry heatwave, humidity does not climb. So the more dangerous the dry heat gets, the calmer the wet bulb number looks.

The number wasn’t wrong. It measures what it measures, accurately. We’ve just been interpreting it incorrectly.

Throwing Shade

Buried in a paper about non-survivable conditions is one of the most encouraging and important findings: Shade changes almost everything. Across all six events, the number of non-survivable periods dropped dramatically for anyone who was able to get out of direct sun. Over Europe in 2003, older people in the shade faced barely any non-survivable conditions at all.

Not air conditioning. Not expensive infrastructure. Shade.

Phoenix makes the case. Three quarters of the deaths there were outdoors and two thirds of those were in urban areas. Six percent of the urban deaths happened at a bus stop.

The remaining quarter was a different failure with a different answer. Every single indoor heat death in Maricopa County that year happened in a room with no working cooling. The average air temperature at those scenes was 39°C. Ninety-five percent of the people who died indoors were over 50. An air conditioning unit was present in nearly nine cases out of ten and in 85% of those, it was broken.

A room without cooling or ventilation is not shade. It’s a box that has been absorbing sun all day. “Go inside” is not, on its own, advice.

The Counting Problem

There’s a reason none of this shows up in national statistics.

Heat rarely kills by heat stroke. It kills by triggering cardiac events, renal failure and strokes. Those are what go on the death certificate, which means every heat mortality figure is a modelled estimate, not a count. The Lancet Countdown puts the global figure at an average of 546,000 deaths a year. The authors of this paper are blunter still: reported mortality, they say, may severely underrepresent the true signal, particularly in densely populated and lower-income regions.

A cause of death that doesn’t appear on death certificates doesn’t appear in risk registers, or budget lines, or ministerial priorities either.

So we’re not just failing to solve this. We’re currently failing to even count it properly.

Communication Breakdown

I work in communication. A big part of my job is noticing when a message isn’t landing and figuring out why. While the public conversation has spent fifteen years focusing on a single wet-bulb figure, it has never been the number that governs heat at work. That job belongs to something else entirely and has done since 1957.

Since then, militaries, industrial safety inspectors and sports governing bodies have used something called Wet Bulb Globe Temperature. A different thing entirely, with a confusingly similar name. It combines humidity, radiant heat from the sun and air temperature into one figure which is adjusted for how hard you’re working, what you’re wearing and whether you’re used to the heat. It was built to stop US Marine recruits collapsing in training and is now an international standard, ISO 7243. It’s a British Standard too.

Wet Bulb Globe Temperature danger thresholds sit in the mid-to-high twenties. It triggers graded responses: slow down, rest, stop, rather than a single number.

The knowledge exists and has existed my entire life. It just never made it into the public conversation.

It wasn’t scientists or bureaucrats getting something wrong. It was a communications failure.

The View From Here

Britain has no maximum workplace temperature in law. The 1992 Workplace Regulations require only a “reasonable” temperature. The minimums everyone quotes, 16°C or 13°C for heavy work, are not in the Regulations at all. They sit in the Approved Code of Practice, which is exactly the document now being reopened.

This is now in play. On 8th July, Sir Stephen Timms told Parliament that the Health and Safety Executive will consult later this year on updating the Approved Code of Practice, potentially including a maximum recommended working temperature. On 14th July, Adrian Ramsay led a Westminster Hall debate on preparedness for extreme heat. The Climate Change Committee recommended maximum temperature regulations in May, though it stopped short of naming a figure. The TUC and Unison want a right to stop work at 30°C, or 27°C for strenuous jobs. A petition for a binding 25°C limit runs until December and sits at around 7,500 signatures, short of the 10,000 needed to force a government response. By 9th July the Met Office had confirmed eight days at or above 34°C this year, the most ever recorded in Britain. The heat carried on after that.

A decision is coming about what shape a heat rule takes. Having just spent weeks reading about what happens when you reduce the complex science of human survival in the face of extreme heat to one simple, almost arbitrary figure, I would like to gently suggest we not do that.

An air temperature alone ignores humidity, sunlight, exertion and clothing: the same four things wet-bulb temperature was missing in Phoenix. The instrument to argue for is a graded mechanism: a simple threshold for ordinary workplaces, plus proper ISO 7243 assessment wherever heat, humidity or protective equipment are factors.

To be clear, this is a disagreement about the instrument, not about the ambition.

I am not the only one making it. The British Safety Council has argued that the 1992 Regulations should be updated to cover excessive heat, but through statutory guidance setting practical trigger temperatures and proportionate controls rather than a single fixed maximum.

The unions are absolutely right that people need protecting. I’m just saying we should protect them with something that works.

What To Do?

Learn the difference between the two numbers, because they are not the same measurement and only one of them is about you. It takes ninety seconds and it will change how you read every heat story from here on.

• Wet-bulb temperature describes the air and the weather. It tells you how much cooling the atmosphere will still allow through evaporation. It rises with humidity and it stays low in dry heat, however hot the day is.
• Wet-bulb globe temperature describes the load on a human body. It includes the sun, the wind, how hard you are working and what you are wearing. It’s what safety inspectors and armies have used for seventy years.

Check on older people during hot weather and remember that being indoors without ventilation isn’t necessarily sheltering from the heat.

If you’re in the UK, respond to the HSE consultation when it opens. Consultations are, of course, decided by whoever bothers to turn up.

If you’re a corporate or political leader:

• Adopt ISO 7243-based Heat Assessment across your operations and supply chain, before regulation rather than waiting for it.
• BUILD SHADE. It is the cheapest life-saving intervention and the evidence for it just got considerably stronger. Trees, canopies and shelters, at bus stops, in playgrounds, over loading bays and queues.
• Put Heat in Your Risk Disclosure as a workforce and continuity risk, with exposed headcount quantified.
• Count the Deaths. Governments should fund heat-coded mortality surveillance. Employers should record heat in their own incident and absence data. We cannot manage what we have decided not to measure.
• Legislators: Pick Numbers That Measure the Right Things. A temperature limit only protects people if it measures what actually harms them. Air temperature on its own does not. It says nothing about humidity, which decides whether sweat can evaporate and it says nothing about direct sun, which can load more heat onto a body than the air does.

ISO 7243 only covers workplaces, yet most people who die of heat are not at work. That is the next argument and we might not be ready for it yet.

Every margin in this crisis has turned out to be narrower than we thought. The carbon budget. The tipping points and now, the human body. Not because our biology changed but because we were reading the wrong number. Phoenix, that July, had a wet-bulb temperature of just 24.32°C. Nothing on the instrument sounded the alarm but the heat killed people anyway.

Heat survivability is not about a fixed temperature. It changes with what surrounds a person: whether there is shade, whether the air is moving or not and whether anyone is checking these things.

Which means it is not only a property of the weather. It is something we can influence. A tree on the right side of a bus stop. Well ventilated rooms. Shifts starting earlier. Rest and rehydration breaks. Siestas. None of these are breakthroughs. They are decisions, most of them cheap and each one has a big impact.

We spent the last fifteen years watching for 35°C to arrive but in many ways, that number was always arbitrary. The better question was always how much survivability we are prepared to build, how and for whom.

🐰Down the Rabbit Hole🐰

Research

Perkins-Kirkpatrick, S.E., Gregory, C.H., Vanos, J.K., Baldwin, J.W., Staudmyer, H., Guzman-Echavarria, G. & Jay, O. (2026). Deadly heat stress conditions are already occurring. Nature Communications 17, 2590. https://doi.org/10.1038/s41467-026-70485-1

Sherwood, S.C. & Huber, M. (2010). An adaptability limit to climate change due to heat stress. PNAS 107(21), 9552–9555. https://doi.org/10.1073/pnas.0913352107

Vanos, J. et al. (2023). A physiological approach for assessing human survivability and liveability to heat in a changing climate. Nature Communications 14, 7653. https://doi.org/10.1038/s41467-023-43121-5

Vecellio, D.J., Wolf, S.T., Cottle, R.M. & Kenney, W.L. (2022). Evaluating the 35°C wet-bulb temperature adaptability threshold for young, healthy subjects (PSU HEAT Project). Journal of Applied Physiology 132(2), 340–345. https://doi.org/10.1152/japplphysiol.00738.2021

Vecellio, D.J., Huber, M. & Kenney, W.L. (2024). Why not 35°C? Reasons for reductions in limits of human thermal tolerance and their implications. Temperature. https://doi.org/10.1080/23328940.2024.2399952

Wolf, S.T., Cottle, R.M., Fisher, K.G., Vecellio, D.J. & Kenney, W.L. (2023). Heat stress vulnerability and critical environmental limits for older adults. Communications Earth & Environment 4, 486. https://doi.org/10.1038/s43247-023-01159-9

Meade, R.D. et al. (2025). Validating new limits for human thermoregulation. PNAS 122(14), e2421281122. https://doi.org/10.1073/pnas.2421281122

Yaglou, C.P. & Minard, D. (1957). Control of heat casualties at military training centers. A.M.A. Archives of Industrial Health 16(4), 302–316. https://pubmed.ncbi.nlm.nih.gov/13457450/

Standards, Law & Policy

ISO 7243:2017. Ergonomics of the thermal environment: assessment of heat stress using the WBGT index (adopted as BS EN ISO 7243:2017). https://www.iso.org/standard/67188.html

Health and Safety Executive. Temperature in the workplace: heat stress. https://www.hse.gov.uk/temperature/employer/heat-stress.htm

Health and Safety Executive. Workplace health, safety and welfare: Approved Code of Practice and guidance (L24). https://www.hse.gov.uk/pubns/books/l24.htm

The Workplace (Health, Safety and Welfare) Regulations 1992, regulation 7. https://www.legislation.gov.uk/uksi/1992/3004/regulation/7

House of Commons Library (2026). Working in hot weather: What does the law say? CBP-10925. https://commonslibrary.parliament.uk/research-briefings/cbp-10925/

UK Parliament written question 3463, 20 May 2026, on a maximum workplace temperature. https://questions-statements.parliament.uk/written-questions/detail/2026-05-20/3463

Extreme Heat: Preparedness. Westminster Hall debate, 14 July 2026, Hansard. https://hansard.parliament.uk/commons/2026-07-14/debates/B155CB0B-23CF-4EDD-8C08-DCCF330F7DFD/ExtremeHeatPreparedness

Climate Change Committee (2026). A Well-Adapted UK. https://www.theccc.org.uk/publication/a-well-adapted-uk/

Petition 770354, maximum working temperature of 25°C. https://petition.parliament.uk/petitions/770354

Data & Reporting

Romanello, M. et al. (2025). The 2025 report of the Lancet Countdown on health and climate change. https://lancetcountdown.org/2025-report/

Maricopa County Department of Public Health. 2023 Heat Related Deaths Report. https://www.maricopa.gov/ArchiveCenter/ViewFile/Item/5820

Met Office (2026). Prolonged spell of hot weather continues into next week. https://www.metoffice.gov.uk/about-us/news-and-media/media-centre/weather-and-climate-news/2026/prolonged-spell-of-hot-weather-continues-into-next-week

British Safety Council (9 July 2026). Legal maximum working temperature: HSE to review workplace code of practice. https://www.britsafe.org/safety-management/2026/legal-maximum-working-temperature-hse-to-review-workplace-code-of-practice