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

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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° than 18°C (-1.07° (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.
Original languageEnglish
Pages (from-to)1078-1084
JournalBritish Journal of Sports Medicine
Issue number17
Early online date1 Mar 2019
Publication statusPublished - 1 Sep 2019


  • How cold is too cold

    Rights statement: This article has been accepted for publication in British Journal of Sports Medicine, 2019 following peer review, and the Version of Record can be accessed online at

    Accepted author manuscript (Post-print), 888 KB, PDF document

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