How cold is too cold? Establishing the minimum water temperature limits for marathon swim racing

Objectives - To provide a rationale for minimum water temperature rules for elite and sub-elite marathon swim racing and highlight factors that make individuals vulnerable to excessive cooling during open water swimming.

Methods - 12 lean competitive swimmers swam for up to two hours, three times in different water temperatures between 14 °C and 20 °C, wearing standard swimming costumes and hats. Rectal temperature (Tre), oxygen consumption, perception of cold, and performance were measured.

Results - In 16°C half the swimmers did not complete a two-hour swim in 16°C; four became (or were predicted to become) hypothermic within two hours. In 18 °C, three quarters completed the swim; three became (or were predicted to become) hypothermic. In 20 °C, one swimmer was predicted to become hypothermic in under two hours. The mean linear rate of fall of Tre was greater in 16°C (-1.57°C.hr-1) than 18°C (-1.07°C.hr-1) (p = 0.03). There was no change in swimming performance during the swims or between conditions. Most of the cooling rate could be explained by metabolic heat production and morphology for both 16 °C (R2 = 0.94, p ˂0.01) and 18 °C (R2 = 0.82, p ˂ 0.01) conditions. No relationship was observed between Tre and perception of thermal sensation (r = 0.25, p = 0.13) and there was a weak correlation between Tre and thermal comfort (r = 0.32, p = 0.04).

Conclusion - We recommend that 16 °C and 18 °C water are too cold for elite marathon swim racing. Fédération Internationale de Natation rules were changed in 2017 to make wetsuits compulsory below 18 °C and optional below 20 °C.