INTRODUCTION: Heat-and-moisture exchanging masks (HMEs) are commonly used by athletes experiencing cold-air induced airway obstructions such as exercise-induced asthma to negate symptoms in training and competition. These masks have been demonstrated as an effective intervention at wide ranging intensity levels and duration for preventing airway obstructions (3). A large proportion of cross-country skiers experience asthma symptoms whilst training and competing in cold conditions (1, 2). As large numbers of athletes in cross country skiing and other outdoor winter endurance sports train and compete in cold conditions around the globe, links have been inferred between prolonged cold air inhalation and the development of exercise-induced asthma (2). The aim of this study was to investigate the effect of an HME on the performance (defined as distance completed) of healthy people during a maximal self-paced exercise test. METHODS: Twenty-three healthy, well-trained participants (eight females, 15 males; age 18-53 y) performed two simulated competition efforts at -15 degrees Celsius in randomized order either with or without mask first. The protocol was 5 min each at 65, 70 and 75% of VO2 Max followed by 3 min at 90% VO2 Max and a further 12 min at 65% before a 5 min pause to simulate a competition warm up followed by a 4 min maximal self-paced running test. A t-test was performed to compare performance outcomes. Two-way repeated measures ANOVA was used to examine effects of trial order and gender on performance. Linear regressions were used to investigate relationships between key physiological and biometric data and the effect of the HME on performance. All statistical analyses were performed with R using the jamovi interface. The study was conducted according to the declaration of Helsinki and approved by the regional ethics committee. RESULTS: Participants ran significantly further without HME (Mean: 931m, SD: 106m) than with HME (Mean: 918m, SD: 110m, p=0.039). No significant effect of trial order was observed (p=0.816). Body mass negatively correlated with the magnitude of the effect of the mask on performance; participants with higher body mass had a greater negative effect of the mask (r2 = 0.215, p= 0.026). No relationships were found between the effect of the HME and sex, age, ventilation, absolute or relative VO2peak. CONCLUSION: Our findings suggest that HME usage hinders maximal running performance in healthy subjects to an extent that may be sufficient to alter competition outcomes. Potential effects of sex, body mass and performance level should also be investigated further. References: 1. Larsson, K. et al. (1993), Br Med J, 307(6915), pp. 1326–1329. 2. Eriksson, L. M. et al. (2018), Scand J Med Sci Sport, 28(1), pp. 180–186. doi: 10.1111/sms.12879. 3. Nisar, M. et al. (1992), Thorax, 47(6), p. 446. doi: 10.1136/thx.47.6.446.