We call a 40°C day uncomfortable. The likelier cost today is to your judgement.

The country has been told, in the Met Office's most serious language, that the next two days are dangerous: a red extreme-heat warning, temperatures expected to break the June record, parts of southern England forecast close to 40°C. The advice that follows is about the body: drink water, stay out of the midday sun, check on older neighbours. All correct. But it skips a quieter cost that sits between "fine" and "ill", and it is the one most likely to reach a desk job today: heat makes us think worse.

This is not a figure of speech, and it isn't pop science. A 2025 review pooling dozens of studies of workplace heat found consistent evidence that it impairs cognition and raises the risk of injury — on the order of a 1% increase in injury risk for every 1°C rise — and that it drags on productivity for a large share of exposed workers. Laboratory studies are more specific: as people heat up, their working memory, their attention and their reaction times slip, and their error rate climbs. One Swiss study, tracking two decades of accident records, estimated that extreme temperatures cause roughly 2,600 extra workplace accidents a year there, and pointed at a culprit with nothing to do with the daytime office: a bad night's sleep in the heat.

My instinct is to be precise rather than alarmist, so here is the honest nuance. It is not simply "hot equals stupid". Mild warmth can even sharpen simple, physical reaction tasks — there's an inverted-U to it. What degrades, reliably, is the effortful work: holding several things in mind at once, weighing options, sustaining attention on something dull, making a careful decision. Which is, of course, exactly the kind of work more and more of us are left doing as the routine tasks get automated away. The work that is most "human" is the work heat hits hardest.

That reframes a familiar phrase. "Too hot to work" is something we say about builders and warehouse staff, and rightly: outdoor and manual workers carry by far the greatest risk, and this piece is not about minimising theirs. But as the human share of work tilts towards judgement and attention, extreme heat stops being only a danger to the body on the scaffold and becomes a drag on the mind at the laptop too. The question this publication keeps returning to — what we do when we can't, or don't, work — has a small, literal, same-day version today: for a couple of afternoons, a great deal of cognitive work will quietly be done less well, and most of the people doing it won't notice.

Britain is oddly unprepared for this as a workplace question. There is no legal maximum working temperature here — guidance asks only that indoor workplaces be kept "reasonable" — so the response is left to individual employers, most of whom treat heat as a comfort issue to be solved with a fan. The more useful framing, on a day like today, is to treat your own attention as a resource that depletes in the heat: not because you're soft, but because the evidence says careful decisions get worse as the room gets hotter.

**Why it matters here.** On the hottest June days on record, the risk most office workers will actually run isn't heatstroke — it's a worse decision, a missed detail, a slower afternoon. Knowing that is the difference between pushing important work through the heat and moving it into the cool of the morning.