Objectives Beach Soccer World Cup 2015 (BSWC15) was held in Espinho, north of Portugal, from 4 to 19^th of July. Health and Performance Unit of Portugal’s Football Federation was in charge of planning and delivering medical assistance in the event. A heat stress monitoring programme was implemented and all heat-related medical forfeits during the event were recorded.

Methods BSWC15 hosted 192 athletes from 16 teams. A total 32 matches were played during the event. Wet-bulb globe temperature (WBGT) and relative humidity (RH) were measured on field before and after each match. The mean value obtained was used in this analysis. Heat stress was evaluated with two indices: WBGT and WBGT + RH. According to the first method, the level of risk associated with heat was evaluated as “unrestricted” (< 22°C), “low” (22–28°C), “high” (28–30°C), “very high” (30–32°C) and “stop play” ( > 32°C). As the venue was located in the beach, high humidity and moderate dry heat conditions were expected. WBGT alone could underestimate the risk of heat stress under such conditions, warranting the use of the second method (that categorises the risk of heat stress as low, moderate and high).

A medical forfeit was defined as the withdrawal of an athlete from competition for medical reasons. In all cases the local medical office evaluated whether the withdrawal was prompted by symptoms of heat illness and/or dehydration. If so, the player was asked if he had suffered from diarrhoea or symptoms suggestive of gastroenteritis during any of the preceding five days. This information was annotated in a score sheet.

Results Mean air temperature was 22.0°C (min 18.9°C, max 30.1°C) with mean WBGT being 25.6°C (min 20.2°C, max 30.5°C). Mean RH was 83.6% (min 62.3%, max 90.2%). Taking into account WBGT only, 28 games were played under low, 2 under moderate and 2 under high environmental stress. Applying WBGT + RH classification system, over the 32 matches, 4 were played under moderate, 25 under high and 3 under excessive environmental stress.

There were 3 cases of heat-related illnesses. Depending on the classification system, they all happened in games played under low (WBGT) or high (WBGT+RH) environmental stress conditions. Two of the reported situations involved players from Oman and Madagascar. The other was a Russian player who admittedly suffered from diarrhoea in the previous 2 days.

Conclusion Few games were played under high environmental stress (according to WBGT) and none of the heat-related illnesses happened in those. This may either mean that top-level beach soccer players are adapted to these conditions or they are able to modulate their activity pattern during matches in a hot and humid environment. One of the cases is easily explained by the diarrhoea history. The others involved players from teams that were probably the most “amateur” in the tournament. The lack of physical conditioning may explain such occurrence.

In humid environments WBGT alone may underestimate environmental stress, as shown by the fact that adding RH shifted most of the matches from low to high risk of heat stress.