Open Water Swimming's Temperature Rules: 16°C Is Settled, 31°C Is Not

The protocol before the numbers

Temperature is measured two hours before the race, at three designated points around the course, at a depth of 40 centimetres, and the official figure is the average of the three.

Other sports legislate for heat with a composite measure — see heat rules in sport.

Every clause there is doing work.

Three points, because a 10km course is not one body of water — a sheltered corner and an exposed turn can differ by degrees.

40 centimetres, because the surface film is warmed by sun and cooled by wind, and a swimmer’s torso is not at the surface.

Two hours before, because a decision to cancel or delay has consequences — broadcast, transport, athletes’ warm-up — and a limit checked at the start line would be a limit nobody could act on.

Notice what that last one costs. A reading two hours old is not the water the swimmers race in. On a hot morning, 30.5°C at measurement can be over 31 by the second lap. The protocol trades accuracy for actionability, deliberately, and the trade is on the dangerous side.

The cold limit has a number behind it

Research published in the Journal of Applied Physiology put 20 trained swimmers in a flume at competition pace and progressively lowered the temperature. The finding: 15.0°C was the lowest at which swimmers could maintain core body temperature. No significant difference by gender, and none by wetsuit type.

The rule is 16°C.

That one-degree gap is the interesting part. World Aquatics is explicit that the margin exists because the laboratory is not a lake: wind, currents, and a field containing swimmers considerably less cold-tolerant than 20 trained subjects in a flume.

Below 18°C, wetsuits are mandatory — not optional, not permitted. A swimmer who would rather race without one does not get the choice.

That is a well-constructed piece of regulation. It has a measured failure point, a stated safety margin, an explanation of what the margin is for, and a mandatory intervention above the limit rather than at it.

The heat limit does not

The maximum is 31°C, set in 2013 on the basis of research by the University of Otago for FINA, World Triathlon and the IOC.

It exists because of Fran Crippen, who died during a 2010 FINA 10km World Cup race in Fujairah from what is believed to have been hyperthermia. Other athletes in the same race were treated for heat-related illness.

The limit has been contested ever since by swimmers and coaches who consider 31°C too high — and in January 2026 World Aquatics issued a request for proposals for research to establish a safe maximum water temperature for open water swimming.

Commissioning that research is a governing body saying, in the most formal way available to it, that it does not currently know the answer.

Why heat is genuinely harder

The asymmetry is not institutional negligence. Cold and heat fail differently, and only one of them fails in a way a laboratory can pin down.

Cold has a threshold. Below some temperature, a body producing heat at a fixed rate loses it faster than it makes it, and core temperature falls. You can find that point in a flume, as the study did.

Heat has no equivalent. It fails by accumulation: a swimmer working hard for two hours generates heat continuously, and the only route out is the water. When the water is near skin temperature, that route closes — slowly, without an obvious moment, and at a rate that depends on how hard the individual is racing.

So the dangerous variable is not the water alone. It is water temperature multiplied by effort multiplied by duration — and effort is the one thing a competition guarantees will be maximal.

This is why the 31°C debate never resolves. The number pretends to be a property of the water when the risk is a property of the race.

The rules that follow from the same worry

The temperature protocol is one part of a safety structure that also includes mandatory feed stations, craft accompanying the field, and the referee’s power to stop a race. Swimmers may be disqualified for obstruction, cutting the course, or breaching feeding-zone rules — provisions that keep the field from turning a survival exercise into a contact one.

This is the same design as triathlon’s drafting rules: in an endurance event held in the open, a large share of the rulebook exists to stop the field’s own behaviour from compounding the environment’s risk.

Feeding matters more here than in any pool event. A 10km race takes most of two hours; nobody completes it without taking on fluid, and in warm water the feed is not nutrition but thermoregulation.

For the pool version of the sport, see swimming rules. For a governing body imposing an equipment answer to a physiological problem, see the 2010 swimsuit ban — the same organisation, the same year as Fujairah, acting decisively on the thing that affected records and slowly on the thing that killed someone.

What the 2026 review will have to decide

Any new maximum has to answer a question the 2013 figure ducked: is the limit about the water, or about the race in the water?

A pure water limit is easy to administer and inevitably wrong at the margins, because it treats a 5km sprint in a current and a 10km championship in flat water as the same exposure.

A limit that accounts for duration and effort would be more defensible and much harder to operate — it would mean shortening races, changing start times, or capping exposure rather than simply cancelling.

The precedent for that already exists in athletics and triathlon, which use composite heat measures rather than a single figure. Open water swimming has resisted it, partly because water is a more forgiving environment than air right up until the moment it stops being one.

Sixteen years after Fujairah, the sport is finally asking the question properly. That is worth saying plainly, in both directions: it is progress, and it is very late.