A cooling kit is not mandatory, but any driver who chooses not to use it has to carry 5kg of ballast in their car to ensure he does not have a competitive advantage.
The system uses a cooling tank and pump, usually situated in the car’s nose, that delivers a cooled liquid, such as glycol, through a system of pipes in a fireproof top worn under their overalls.
Some teams and drivers prefer not to use the system because of two key compromises.
It is difficult to keep the liquid cool enough for long enough so that it lasts for an entire grand prix – a problem exacerbated in Singapore because it is the longest race of the year.
And some drivers find the cooling top uncomfortable in the tight confines of the cockpit.
If the liquid cannot be kept cool, it reverts to car temperature, which is considerably higher than ambient. It is not unusual for temperatures in an F1 cockpit to rise above 40C.
Mercedes are one of the teams who have decided against running the system because they preferred to devote the resource that would have been involved in making it robust enough to last a race into car performance.
Governing body the FIA has granted teams an allowance under the cost cap of $50,000 (£38,000) if they can make a system that keeps the liquid cool for an entire grand prix, and a further $75,000 (£57,000) if they could make the top comfortable.
Mercedes deputy team principal Bradley Lord described the allowance as “small”, adding: “We didn’t believe that allowance was sufficient for us to be able to develop a solution that would be robust enough to last for the entire race.”
Singapore follows the Austrian, Italian and last weekend’s Bahrain Grand Prix in Malaysia in being declared a heat-hazard race this year.
The Austrian and Italian races were held amid the Europe-wide heatwave that led to record temperatures across the continent.
The temperatures in Malaysia and Singapore are comparable with typical temperatures in south-east Asia. Had the heat-hazard designation existed before 2025, they would have fallen into that category numerous times in the past.