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A 29°C kitchen is an easy night for the expo pass and a real risk for the dish pit — same thermometer, two very different situations.
There's no thermometer over the flat-top, only one somewhere on the wall by the door. That single reading has to stand in for six stations that have nothing in common: the expo checking tickets, the line running three pans at once, the dish pit standing in steam, and the cook working right beside the fryer and the grill. On a heatwave night that wall thermometer shows the same number everywhere, and yet one station is harmless and another is dangerous.
The gap isn't in the air alone. Humidity determines how well your body can cool itself by sweating — in a steamy dish pit, evaporation barely works. Radiant heat off an open flame or a hot flat-top adds a heat load your skin feels but no wall thermometer ever registers. And how much heat your own body generates depends on how hard the work is: someone plating dishes builds up far less internal heat than someone hauling 20-litre stock pots.
That's why occupational medicine doesn't work with one fixed 'too hot' number — it uses an index that weighs all three together: WBGT (wet bulb globe temperature), measured against a limit that gets stricter as the work gets heavier. A certified reading needs an instrument most kitchens don't own. What every kitchen does have is a thermometer and a hygrometer, and that's enough to build a solid estimate of where you actually stand.
This guide walks through the five numbers behind that estimate, with a calculator at the bottom where you plug in your own kitchen. Everything runs in your own browser — nothing is sent or stored. The limits are occupational-health reference values, not legal standards; what applies where you operate depends on your own country's workplace-heat rules.
Why this catches kitchens out every summer
A kitchen runs hot even without a heatwave: ovens, ranges and fryers keep the air temperature structurally higher than the dining room. Once outside temperatures climb past 28-30°C, mechanical extraction can no longer carry the surplus away, and heat builds up over the course of a service — the peak usually isn't at opening, but somewhere in the second hour, once every burner has been on for two hours straight.
The staff working hardest is often standing exactly where the heat is worst: at the flat-top, the grill and the fryer, where radiant heat off an open flame stacks on top of the air temperature. That's precisely the combination that pushes WBGT up fastest — heavy work raises the body's own heat output right when the environment is doing the least to help shed it.
And it's one of the few kitchen hazards that builds rather than strikes. A cut happens in a fraction of a second; heat stress creeps up over an hour or more, with symptoms — headache, cramping, confusion — that are easy to wave off as 'just a busy service'. By the time someone is clearly unwell, the window to quietly step in and fix it has usually already closed.
Free guide The complete staff guide Everything about restaurant staff, from hiring to wellbeing, in one overview. Read the guideThe 5 numbers
Each number builds on the one before it: from what the thermometer says, to what your body actually feels, to what you do about it on the rota.
1. Your thermometer is lying by up to 6°C, and not always in the same direction
Air temperature is only one of three pieces. Humidity slows down evaporation of sweat — the main way your body actually cools itself — and radiant heat off an open flame or a hot flat-top adds a heat load your skin feels that no wall thermometer ever measures. Together they set an estimated heat load that can sit well away from what the display shows.
The graphic below shows three stations reading the exact same 29°C on the wall thermometer. The line stays close to that reading, the dish pit's steam pushes it up sharply, and the flat-top's radiant heat sends it well past its own limit — while the expo, with a lower base air temperature and light work, stays comfortably under its own.
Three stations read the exact same 29°C on the wall thermometer. Humidity and radiant heat decide which of them is actually dangerous.
The black line marks the limit for the workload at that station. The flat-top and the dish pit read the same air temperature as the line, but radiant heat (flat-top) and humidity (dish pit) push them well past it.
2. The safe limit isn't one number — it drops with the work
How much heat your body can absorb doesn't only depend on the environment — it depends on how much heat the body itself is producing. Someone checking tickets at the pass builds up little internal heat; someone hauling 20-litre stock pots or moving bags of ice generates plenty on their own. Occupational medicine works with four workload categories, each with its own limit that strictly drops as the work gets heavier.
That's why 'is it too hot to cook' has no single answer: the same 29°C sits comfortably under the limit for the expo and already well over it for someone hauling heavy pots. The ladder below lines up all four limits with a real example job on each rung.
Four workload categories from occupational medicine, each with its own safe limit. The same ambient temperature is harmless for one job and already too much for another.
Limits are reference values from widely used occupational-health tables for continuous work, not legal standards. What applies where you operate depends on your own country's rules.
3. Once you're over the limit, it's a ratio, not a wall
'Too hot' isn't an on/off switch. As the estimated heat load climbs further past that job's limit, the recommended balance between working and resting tightens: from working straight through with extra water breaks, to 45-minute blocks with a 15-minute rest, all the way to 15-minute blocks with 45 minutes of rest once things genuinely start to go wrong.
That's not a theoretical schedule — it's what an experienced chef already does by instinct on a heatwave night: rotating people off the flat-top, sending someone on break a little early, moving the heaviest tasks to the quietest moment. The number just makes visible what otherwise happens by feel alone, and turns it into an actual break plan.
4. 0.25 to 1 litre an hour is the number nobody tracks
Fluid loss through sweat climbs fast in the heat, and thirst is a delayed signal — by the time someone feels thirsty, the deficit is already there. The rule of thumb scales with the risk band: from a quarter litre an hour for light, safe work up to a full litre an hour at extreme heat load, spread across the hour in small sips rather than gulped down in one go.
Drinking more than a litre an hour doesn't help and can actually be dangerous — the body can't process the surplus fast enough. Water alone is fine for short shifts; on long, heavy shifts in the heat a little salt or a sports drink helps the body actually hold onto the fluid rather than losing it again.
5. The five-minute window before it becomes an emergency
Heat exhaustion announces itself: headache, cramping, nausea, confusion, heavy sweating that suddenly stops. At that stage, the window to step in — water, shade, a cooler spot, less exertion — is still wide open. Once someone stops reacting clearly, stops sweating despite the heat, or loses consciousness, it's no longer heat stress but heat stroke: call emergency services immediately and cool them actively, rather than waiting to see if it 'passes'.
Heat also puts extra strain on the heart, in much the same way physical exertion does — anyone with an existing heart condition carries a higher risk on a day like this. What to do for a colleague who collapses fully is covered separately, outside heat-specific care, in our guide to cardiac arrest and the chain of survival in a restaurant.
Calculate the risk in your own kitchen
Enter your kitchen's air temperature and relative humidity — both usually already sit on a cheap combined thermometer/hygrometer you can buy for a few pounds. Then pick the heaviest task at that station, and flag whether someone is working right next to an open flame, a flat-top or a fryer.
The calculator adds the humidity correction and, where it applies, the radiant-heat correction to the air temperature, compares that against the limit for the workload you picked, and derives a work:rest ratio and a water guideline from the gap.
Heat stress calculator
Enter your kitchen's temperature and humidity, then pick the heaviest task at that station.
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This is a simplified estimate based on air temperature and humidity, not a certified WBGT measurement. Legal compliance needs a calibrated instrument — use this as a decision aid, not proof of anything.
Try changing the numbers: see what happens if you drop the humidity by ten points, or move someone from a radiant-heat task to one without. That difference is exactly what better ventilation or a simple task swap buys you.
Don't bookmark a screenshot — come back on the next heatwave day and enter it fresh. Conditions in your kitchen change with the weather, not with the season alone.
What to do this week, this month and this season
You don't need to change everything at once. This order starts with what's measurable today and builds from there.
This week — measure instead of guessing
- Buy a combined thermometer/hygrometer for a few pounds and hang it by the flat-top, not by the door.
- Run the calculator above for your hottest station on an averagely busy night.
- Ask your team which symptoms they already recognise — a headache by the end of service is more 'normal' than it should be.
- Put a jug of water and cups somewhere visible next to every heat source, not just in the staff toilet.
This month — build it into the rota
- Agree with your team who can rotate off the flat-top or grill once an hour during a heatwave service.
- Schedule the heaviest prep tasks — washing large pans, moving pots — for the coolest part of the day.
- Decide who's the first to flag symptoms in a colleague, rather than relying on someone to say it themselves.
- Repeat the measurement on the hottest day of the month and compare it with your first reading.
This season — fix the cause
- Get your extraction and make-up air checked to see if they're sized for the heat your equipment actually produces.
- Consider a fan or an air curtain at the heaviest station if a structural fix isn't feasible this year.
- Add heat stress to your onboarding plan for new staff, so it's an ordinary subject rather than a taboo.
- Review your terrace setup too: someone standing in full sun outside carries a different risk than someone at the indoor flat-top.
What can change tomorrow
No restaurant owner decides in the morning to let their team work in dangerous heat — it happens because nobody has the number that says when it's gone too far. A thermometer and hygrometer costing a few pounds, together with the calculator above, gives you that number.
This isn't about cancelling a service the moment it gets warm outside. It's about knowing which station is the hottest today, who's standing there today, and whether that person can step away from the flat-top once an hour without the pass falling apart. That's a rota question, not a weather forecast.
The ventilation that actually carries the heat away is a separate subject — work out here how urgent that is for your kitchen first, then move on to the air balance that fixes the problem structurally instead of having to work through it every heatwave again.