Safe cold-water thresholds while wearing wetsuits approved for open water swimming competitions

This study tested the hypothesis that the critical cold-water temperature (T_crit) at which core temperature can no longer be maintained while wearing an approved wetsuit is below the currently mandated water temperature of 16°C. We further aimed to examine the effect of wetsuit sleeves on T_crit. We recruited 20 trained swimmers (12 men, 8 women) who completed 120-min progressive cooling swim trials in a swim flume. Water temperature at the start was 16°C and decreased ∼0.15°C every 10 min while swimming at race-representative intensity and wearing a full-length wetsuit. A subset (<i>n</i> = 8) completed an additional swim trial comparing sleeved versus sleeveless wetsuits. The water temperature upon which core temperature could not be maintained (i.e., T_crit) was identified using segmental linear regression. Median T_crit was 15.0°C [95% confidence interval (CI): 14.8-15.3°C], significantly below the 16°C regulation (<i>P</i> < 0.001). No differences in T_crit were observed between genders [<i>P</i> = 0.78; men: 15.0°C (95% CI: 14.6-15.3°C); women: 14.91°C (95% CI: 14.6-15.4°C)] or wetsuit types (<i>P</i> = 0.90; sleeved: 15.1 ± 0.5°C; sleeveless: 15.1 ± 0.6°C). Multiple regression analysis revealed that body fat percentage (β-coefficient: -0.06088: <i>P</i> = 0.0390), metabolic heat production (β-coefficient: -0.004416; <i>P</i> = 0.0079), and body surface area-to-mass ratio (β-coefficient: -370.6; <i>P</i> = 0.0078) significantly explained T_crit (<i>R</i>² = 0.41, <i>P</i> = 0.008). Although our data demonstrate that trained swimmers can physiologically tolerate temperatures below current regulations, we recommend maintaining the 16°C mandate to provide appropriate safety margins for the diverse competitive population and variable real-world conditions encountered in open water swimming competitions.<b>NEW & NOTEWORTHY</b> Using a novel experimental protocol, this study provides empirical determination of critical cold-water temperature for competitive open water swimmers while wearing two different World Aquatics-approved wetsuits designs. Well-trained swimmers protected core temperature down to 15.0°C, below current 16°C regulations, which did not differ between sleeved and sleeveless wetsuits. These findings provide objective evidence supporting current conservative regulatory approaches.