Formula 1 closes September with an announcement that sounds like a bureaucratic formality: a “heat hazard” has been declared for the round in Malaysia, officially called the Bahrain Grand Prix—a race bearing one country’s name held in another. This is the fifth such case in two seasons: before Malaysia, the hazard was declared in Singapore and Austin, Austria and Monza.
The rule is simple. If the official weather service forecast puts the Heat Index above 31 °C, teams must fit the car with a driver cooling system, and the minimum car weight rises from 800 to 805 kilograms. The driver can choose whether to wear the vest—a fireproof shirt threaded with tubing carrying chilled fluid. But at first glance, opting out only looks advantageous: the five kilograms of hardware stay in the car either way.
And almost nobody is buying into this rule. Drivers whose profession is enduring hardship are turning down technology developed specifically for them. The rule paid for cooling with weight, removed any argument over seconds, made it all the way to international certification—and yet no line formed at the checkout. The reason for the refusal turned out to be more interesting than the rule itself.
It all began at the 2023 Qatar Grand Prix. The drivers finished the race at the limit: Esteban Ocon vomited right in the car, Logan Sargeant retired with heat exhaustion, several needed medical assistance. The FIA then launched an analysis, and in December 2024 the World Motor Sport Council approved a new procedure.
The first discrepancy in the sources appears immediately—and it is not a small one. RaceFans writes that the air temperature in Qatar reached 32 °C; McLaren’s press office says 44 °C. Both figures refer to the same Sunday. The explanation is probably that one outlet uses weather-station data while the other uses track and cockpit temperatures, but that is beside the point here: there is still no official measurement everyone cites.
Then comes the threshold. The sporting regulations say it literally: “The Heat Index will be greater than 31.0 °C at some point during the race or sprint.” The Heat Index is not air temperature but a combined measure of temperature and humidity—the “feels like” heat. Humidity matters more than degrees: the air inside the cockpit cannot absorb evaporated sweat, and the entire physiology of the issue rests on that.
The press almost always simplifies the rule when retelling it: “temperatures of 31 °C and above”—that is how Formula 1, McLaren, and Crash.net all put it. In the 2024 draft system, the threshold was described differently—30.5 °C ambient air temperature. The final 31.0 °C refers to the index, not the thermometer.
The vest itself was not made by the FIA. By then, American company Chillout Motorsports was cooling drivers in NASCAR and WEC and managing battery temperatures in Formula E; its Cypher Pro Micro Cooler had to be shrunk down to fit inside an F1 car. The kit is a box containing a micro-compressor, evaporator, and condenser, plus a fireproof shirt threaded with roughly 48 metres of tubing around the chest and back.
The tubing proved hardest of all. It had to be made from four different materials to combine thermal conductivity, flexibility, and fire resistance: in the cold, the tube goes stiff; in the heat, it softens; in a fire, it melts. A separate FIA fire-resistance test nearly killed the project, while the hardware’s weight came out almost comical—the system itself weighs just under 2 kg, the shirt another roughly 300 grams.
Then comes the engineering drama almost invisible from outside. Originally, the FIA wanted an honest-to-God refrigerator: a heat exchanger, pumps, perhaps a small compressor with refrigerant. That setup was abandoned after experiments in mid and late 2024—it was too complicated for a car with no spare room. What won was a setup FIA single-seater director Nikolas Tombazis calls total loss: ice melts in a tank, and that is that.
The variation between teams turned out to be enormous. “For some, the system works for 15 minutes; for others, almost the whole race,” Tombazis says. That is exactly why drivers’ opinions differ so much: they are not arguing about the principle, they are comparing different devices. To keep an arms race from breaking out, the system was designated an Open Source Component: anyone can copy another team’s design, while the weight penalty was deliberately written into the rule so no one would try to save half a kilo on cooling.
The scars of that evolution are visible in the 2026 technical regulations. A continuous-operation system must remove at least 200 watts from the driver at an air temperature of 40 °C. A system with a cold reserve must carry at least 1.1 megajoules and finish the race with its reservoir colder than 10 °C. Solid CO₂—dry ice—is explicitly prohibited; the liquid inside may only come from a short list: air, water, salt solutions, or propylene glycol. Refrigerant, if there is any, must have a global-warming potential below 10.
Back to the weight. The rule adds 5 kg to the minimum for races and sprints, and 2 kg for practice and qualifying. At the same time, the technical regulations require that the combined mass of the “heat-related parts”—the difference between normal equipment and equipment with the system, plus the system itself—must be no less than 5 kg.
Skip the vest, and you pay exactly the same. All other components, including the cooling fluid, must remain in the car, and any shortfall is made up with 0.5 kg of ballast in the cockpit. So a driver who does not wear the vest gains not a single gram: he carries the same system and the same weight, he just does not use the result.
Oscar Piastri, the only McLaren driver who wears the vest, says exactly that out loud. “The weight is being added to the car anyway, so if the system works and cools you down a little—why not?” The argument over seconds, which would predictably have arisen in any other case, was closed before it began. So the drivers’ refusal is not about speed at all.
To understand what they are arguing over, you need numbers. A cockpit at a hot round means air from 36 to 66 °C and a microclimate beneath the race suit above 50 °C. In the 2025 season, moderate heat stress was recorded in at least 7 races out of 24. The metabolic heat generated by a driver is estimated at 400–600 watts: he drives at 65–75% of maximum oxygen uptake, while a 90-minute race burns roughly 540 kilocalories. Sweating is useless under those conditions: sweat absorbed by the fireproof suit barely cools at all and simply drains water—around a litre per hour.
The effects begin before the driver notices them. Deterioration in visuomotor coordination has been recorded with a core-temperature rise of just 0.8 °C. Executive function, working memory, and information-processing speed drop off around 38.5–39 °C—before obvious symptoms of overheating appear. One separate line: an overheated, dehydrated body tolerates G-forces worse—in a classic 1979 experiment, heat stress cut G-tolerance by 0.3 g. A driver struggling to keep his head up through a corner drives worse, and he is no safer for it.
A The Race reporter tried measuring it on himself: 45 minutes on an exercise bike in the climate chamber at the University of Roehampton, at 31 °C and 70% humidity, heart rate around 160, wearing a fireproof race suit. Down 1.232 litres of fluid, down a kilogram in body mass, core temperature up 1.4 °C—to 38.9 °C. Extrapolated to a full race, that comes to 2–3% of body mass lost.
Some solutions are obvious and cheap. In a 2025 study, eight drivers completed two hours of race simulation at 40 °C and 50% humidity in full FIA equipment: with cold water, core temperature rose by 1.17 °C, against 1.69 °C with room-temperature water; all eight completed the cold protocol, while only two finished the warm one. In other words, a bottle of ice water works. The problem is that the big bets here are being placed on the vest.
And here is what made digging worthwhile. The first peer-reviewed controlled comparison of cooling technologies for race drivers was funded by Mercedes itself: the Spartan Motorsport Performance Lab at Michigan State University, published in Medicine & Science in Sports & Exercise in March 2026. Fourteen participants trained to the standard of professional drivers, six conditions, an hour on an exercise bike at 65–75% of maximum heart rate in full FIA equipment.
The result was uncomfortable for the regulations. The liquid-cooling vest did not merely fail to lower core temperature compared with the control—it was higher: 38.58 °C against 38.32 °C with no cooling at all. The experiment’s best result came from blowing cold air beneath the race suit—37.51 °C—but that sort of thing is not permitted in Formula 1.
The mechanism offered to explain it sounds almost mocking toward the notion of “just pour cold water on it.” By cooling the skin, you fool the hypothalamus: it thinks the body is coping and dampens its own heat-dissipation mechanisms—vasodilation and sweating. The metabolic heat is still being produced at the same rate and remains inside. The effect is confirmed by adjacent research: an ice collar around the neck in 35 °C heat improves comfort but changes neither core temperature nor heart rate nor sweating.
Then physiology is joined by reserve-capacity arithmetic. The regulations require 1.1 megajoules of cooling; if the system removes the stated 200 watts, that is enough for roughly 92 minutes—formally, almost a race distance. In practice, in Davis and co-authors’ study, the vest worked for around 30 minutes, after which it “began to warm”: the reservoir is depleted faster than the calculation promises.
And the sensation of cooling masks deterioration: in the vest experiment, more people made it to the end than without it—even though temperature was higher in that very condition. Another line from the same study, this time about reliability: system failures were recorded in roughly 15% of runs over 15 years of field observation.
This is precisely the scenario drivers retell in their own words. Charles Leclerc: “The ice runs out—and then hot water goes into the vest. It is not a pleasant feeling.” Isack Hadjar: “Too many tubes, too much stuff in the cockpit. It works really well for about ten minutes, then it is warm again.” Andrea Kimi Antonelli suffered a system failure in Austin and, by his own description, was “literally boiling” in the car.
From there, everything turns into an argument about the nature of safety, and this is the story’s most alive part. The FIA has no intention of backing down. Tombazis explains that the goal is not comfort but making it impossible for a race to be cancelled because of heat: “The worst thing that could happen is finding ourselves in a situation where the system does not really work and we are forced into extreme measures like cancelling a round.” He attributes the drivers’ complaints to the “tardiness” of some teams, which still have not got their devices right.
The drivers are no less direct. Max Verstappen: “It should be the driver’s choice. The FIA always brings up safety, but then let’s talk about what can actually be improved—for example, pit-lane entry. That is more important than a vest in the car. I do not like tubes on my body.” Lewis Hamilton initially said air conditioning in the car was not needed at all: “We are highly paid athletes, we have to work hard to be able to withstand that heat.” A year later—more sharply: “No driver has ever died from overheating… Let it remain our choice.” Lando Norris simply lists it: he does not wear the vest, does not use a water bottle, does not drink during the race at all.
But there is no unity even among those refusing. George Russell, director of the Grand Prix Drivers’ Association, has worn the vest even where no heat hazard was declared—and paid for it in weight. “With nearly 60 °C in the cockpit and humidity close to 90%—it is a sauna, so we welcome the concept.” Esteban Ocon, who vomited right in the car in Qatar 2023 and later became a supporter of the system himself, takes the argument to its conclusion: “If a driver has spent all night over the toilet, he will not go to the FIA to complain. He will race. And if the vest helps him avoid passing out and crashing into someone else—that is safety.”
There is another discrepancy sitting right there, and I am leaving it as it is. Since 2026, the press has described the vest as “fluid pumped through a tank of dry ice”—while the 2026 technical regulations explicitly prohibit dry ice. A scientific review in the GSSI journal claims that from 2026 the vest must be worn at every round regardless of the forecast; the sporting regulations and the FIA’s September clarifications say the opposite—opting out is still permitted, provided ballast is carried. I am not going to smooth out those contradictions: it looks as though the sources are describing different years and different versions of the same rule, but I have not found a definitive document that reconciles them all.
What remains, in the end, is this. The regulations solved exactly the problem they could solve: they paid for hardware with mass, removed the performance-seconds doping from the equation, and left the choice intact. One variable remains that no rule can close off—trust in a device that lasts 15 minutes for some teams and almost an entire race for others. And that is perhaps the best indicator of where the technology stands: when the gap between the best and worst examples determines whether to wear it or not, the question is not yet one of regulations.