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Scientific Reports volume 14, Article number: 26976 (2024)
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This study investigates the risk of severe heat stress and associated potential water losses in professional soccer players, considering as well the oxygen content of the inhaled air in the context of the 2026 FIFA World Cup. For the 16 stadiums, hourly values of biometeorological indices (adjusted Universal Thermal Climate Index – UTCI, Water loss – SW and Oxygen volume – Ov) were calculated. UTCI adjustments included modifications to activity levels, movement speeds and clothing configurations to better reflect the level of thermal stress on soccer player during a match. Ten out of the sixteen sites of the 2026 FIFA World Cup are at very high risk of experiencing extreme heat stress conditions. The highest risk of uncompensable thermal stress due to very high average hourly UTCI values above 49.5 °C and excessive water loss (> 1.5 kg/h) occur in the afternoon in stadiums located in Arlington, Houston (USA) and in Monterrey (Mexico). The results of this study will enable optimization of match schedules at individual venues, taking into account the health risks associated with extreme heat stress, but also the physiological reactions to heat potentially affecting the performance of players on the pitch.
In 2026 the FIFA World Cup will be held in three countries, i.e. USA, Canada and Mexico, for the first time in history. Consequently, venues will be scattered across the American continent. Large meridional and latitudinal extensions, as well as differences in the altitude will be a challenge for the soccer players taking part in the tournament, due to the need for quick adaptation to the various local climates.
Environmental conditions have a significant impact on the physiological responses of athletes during physical activity and exercise1,2,3. Scientific literature has pointed out various negative effects on soccer players performing at high relative humidity at the World Cup in Brazil in 2014, or at high air temperature in Russia in 2018456. For the World Cup in Qatar in 2022, in order to ensure the health and safety of the athletes and spectators under the heat stress, air-conditioned stadia were built and the entire event was postponed to a colder season. A recent review7 lists soccer among the sports associated with a high risk for heat stress concerning the Paris 2024 Olympic Games. Intense physical activity of soccer players during a match causes a significant thermoregulatory strain, which may be further intensified by the thermal environment. Heat stress in particular is a serious threat for highly motivated athletes, because they have very limited options for compensation during the competition8. When performing in warm and humid conditions, players’ heat production from intense physical activity often exceeds their ability to dissipate excessive heat, which may increase the risk of exertional heat illness9,10. In response to prolonged exercise in a hot environment, intense sweating is triggered, which may result in dehydration of the body11,12. It is estimated that dehydration above 2% of the body weight adversely affects cognitive functions, physical and intellectual capabilities, and also inhibits the body’s thermoregulatory mechanisms13, while dehydration of 3–4% of body weight significantly compromises soccer players’ performance14. The body’s physiological responses to heat stress may impair specific motor tasks15. In particular, psychomotor skills diminish, the speed and precision of movements decreases, and concentration disorders appear2,16. Peripheral fatigue resulting from heat stress may have a negative impact on the total distance players can cover during a match, as well as on the distance covered at high intensity, the number of accelerations, sprints and jumps17,18. In addition, the altitude of sports facilities influences the well-being of athletes and their performance in the competition. With a rising height a.s.l., the barometric pressure decreases, resulting in a reduced air density and a lower partial pressure of the inspired oxygen19. For the players living and training near sea level, exercising above 1200 m a.s.l. was observed to have negative effects on their endurance20, whereas ascending for matches to moderate altitudes (above 2000 m a.s.l.) carries the risk of aerobic performance impairment, especially in repeated sprints, as well as the risk of altitude headache or acute mountain sickness occurrence21,22. Therefore, it is recommended to monitor the thermal environment before and during sports events to help plan the hydration and acclimatisation process, select appropriate clothing in accordance with the expected biothermal conditions, as well as to protect the athlete’s health during competitions in the heat7,23,24.
FIFA recommends the use of the WBGT index (Wet Bulb Globe Temperature) to determine the safety standards during a soccer match7,25. If WBGT is higher than 32 °C, cooling breaks are mandatory in both halves of the match22. Despite the undisputed popularity of the WBGT index in soccer-related events, it is considered an imperfect measure of heat load on athletes, as it is prone to underestimating the heat stress level26,27. WBGT does not incorporate the most important factors specific to sports, i.e. metabolic heat production and the effects of body movement on relative air velocity, also it does not adequately reflect the additional thermal load that people experience when sweat evaporation is limited by high humidity or poor airflow28. WBGT can be measured in situ with appropriately calibrated globe thermometer and wet-bulb thermometer or, in ex post analysis, it can be calculated from meteorological data, using approximation formulas. However, as shown by d’Ambrosio Alfano et al.29, the mathematical modelling of the natural wet bulb temperature, which is a component of the WBGT index, may deliver more than one solution, and as this leads to high uncertainty, an indirect determination of WBGT appears a questionable approach.
Therefore, various other models of human heat balance and alternative biothermal indices to assess thermal stress for the needs of sports activity have been considered7,30. One of the most well-known heat stress indices in the United States is the heat index (HI), which has previously been applied to study the impact of heat stress on hyperthermia fatalities in American football players31. Although recent laboratory experiments confirm, that appropriately modified heat index can accurately predict the onset of thermoregulatory failure32, its ability to determine the environmental risk for athletes diminishes as metabolic heat production increases with greater activity levels33. Vanos et al.34 investigated the effectiveness of the modified COMFA model in predicting the energy budget and assessing thermal strainof a conditioned athlete, while Grundstein et al.35 used the COMFA model to assess the potential impact of weather conditions on a fatal case of exertional heat stroke in an American football player. Honjo et al.36 used UTCI to assess thermal comfort along the marathon course of the 2020 Tokyo Olympics, whereas Havenga et al.37 used UTCI to assess the increased risk of heat stress during the Comrades Marathon in South Africa. In the context of soccer, the assessment of thermal comfort has so far been carried out mainly from the spectators’ perspective, using the PET index38,39, while studies taking into account the impact of biometeorological conditions on players were rare – Konefał et al.5 was the first to use the UTCI index for this purpose. However, the above mentioned indices were not developed specifically for athletes and in their current form have their limitations in sports applications, due to the underlying restrictive assumptions or the inability to modify certain physiological or behavioural factors that are inadequate in a sports context7,30. Nevertheless, thanks to the flexibility of the multi-node thermophysiological UTCI-Fiala model40, that forms the basis for the UTCI index, it is possible to optimise the UTCI for sports application. Therefore, in this study we intend to apply the recently introduced procedure by Bröde et al.41,42, which allows the UTCI to consider higher activity levels up to a rate of metabolic heat production of 285 W/m2 and modified thermal insulation of the clothing corresponding to the conditions during soccer matches.
An appropriate environmental stress risk assessment is essential for all outdoor sporting events7, especially when they take place in summer or in hot climates, because excessive heat stress during intense physical activity impairs athletes’ performance, may lead to overexertion of their body and, in extreme cases, fatalities8,19. Therefore accurate, hourly information on the atmospheric stimuli intensity that players can expect at the 2026 FIFA World Cup venues may be useful for national team training staffs in order to optimally and safely prepare players physically to perform in various, variable and often unfavourable climatic conditions. Hence, this study aimed to investigate the risk of severe heat stress conditions and associated potential water losses in professional soccer players, considering as well the oxygen content of the inhaled air in the context of the 2026 FIFA World Cup in North America (USA, Mexico, Canada).
The 2026 FIFA World Cup will be played in three countries on 16 stadiums located in different parts of the North American continent. Latitudinal extent of the area is almost 30°, while the longitudinal extent equals to 51.85°. The elevation above the sea level ranges from 2 to 2240 m (Tab. S1 in the Electronic Supplementary Material). According to the Köppen-Geiger climate classification43 the stadiums are located in nine different climate zones (six in Cfa, two in each: Csa & Csb, one in each: Am, BSh, Cfb, Cwb, Dfa, Dfb) (Fig. 1). Therefore, throughout the tournament, the players will be forced to move between different climatic conditions, in which the degree of environmental stress will vary significantly.
Location of the 2026 FIFA World Cup stadiums within the Köppen-Geiger climate type classification. Own elaboration using ArcMap ver. 10.8.2 based on the World Bank data44 available under Creative Commons Attribution 4.0.
For the 16 venues of the 2026 FIFA World Cup, hourly values of biometeorological indices (Universal Thermal Climate Index – UTCI and Water loss – SW, cf. detailed methods below) were calculated in order to characterise the heat stress on the players and potential water losses due to sweating, respectively. The study considered two scenarios: situations in which athletes would be inside the stadium, and the time they would spend outside, moving around the area (Fig. 2). Previous measurements and estimates of soccer players’ energy consumption45 had indicated a wide range of activities during the match associated with varying levels of metabolic rate. In the context of soccer players’ activity at the stadium, the following levels of metabolic heat production were assumed in addition to the moderate activity level of 135 W/m² implemented in UTCI46: 65 W/m2 for sedentary resting and 285 W/m2, corresponding to pre-match exercise and playing with the average movement speed of 1.75 m/s, respectively. UTCI adjustment values to these activity levels had been developed previously41,42,47. In the case of the SW index, an additional level of 450 W/m2 was analysed, which was assumed relevant for the average activity of soccer players in various positions on the pitch during the match, moving at an average speed of 2 m/s48. For the above situations, when the players are on the stadium pitch, due to limited air flow in semi-open spaces, the calculations assumed that the actual wind speed was 60% of the wind speed outside the stadium, in accordance with previous measurements49. For the normal activity of athletes outside sports facilities, the assumed light to moderate level of metabolic heat production was 135 W/m2, which corresponds to a person walking at a speed of 1.1 m/s (being likewise the reference level for the standard UTCI index). Hence, this scenario can also be treated as representative not only for athletes, but also for the training staff, service workers and supporters who will come to watch the matches. Additionally, considering the location of venues in Mexico high above the sea level, which is accompanied by lower atmospheric pressure and thus lower partial pressure of oxygen in the air, the oxygen content in the air was determined using the Ov (Oxygen volume) index.
Scheme of the research workflow (own elaboration).
Meteorological data including Mean Radiant Temperature were obtained from ERA5 reanalysis50 and ERA5-Heat reanalysis51. The data sets were acquired from the Copernicus Climate Change Service (C3S), an Earth observation system managed by the European Union. The data files were downloaded from Climate Data Store in a netCDF format. Both reanalyses offer global-coverage data in a latitudinal and longitudinal resolution of 0.25°. To ensure the best possible representation of conditions at the chosen stadiums, data from the grid points closest to each venue were downloaded. The temporal range of the dataset was limited to hourly data from months of June and July, covering the years 2009 to 2023. The sample was trimmed to the period from the 11th of June to the 19th of July, each year, to correspond to the planned time frame of the FIFA World Cup in 2026. Additionally, for each venue, all data entries were marked with an appropriate timestamp in local time, to ensure the correct representation of changes of parameters throughout the day.
The meteorological data included air and dew point temperature at 2 m a.g.l., eastward (U) and northward (V) wind components at 10 m a.g.l., atmospheric pressure and global solar radiation at the horizontal surface. Additionally, resultant wind velocity, relative humidity and water vapour pressure were calculated from the initial data. Resultant wind speed and vector were calculated in accordance with the ERA5 Confluence Manual52. The average daily values of the basic meteorological parameters at all sixteen stadiums of the 2026 FIFA World Cup are summarized by boxplots in Fig. 3.
Distribution of meteorological parameters in the vicinity of the 2026 FIFA World Cup venues from 11th June to 19th of July covering the years 2009–2023.
The UTCI53 was designed as an equivalent temperature (in °C) defined as the air temperature of a reference environment causing the same dynamic physiological response as the actual meteorological conditions46. The reference environment is characterised by an air temperature equal to the mean radiant temperature, low air movement (0.5 m/s wind speed 10 m above ground), relative humidity of 50%, but a vapour pressure capped at 20 hPa for air temperature above 29 °C. The physiological response to thermal stress was determined from the output of an advanced model of human thermoregulation40, which had been coupled with an adaptive clothing model in which clothing insulation changes depending on the air temperature, resembling the typical clothing behaviour of European urban population54. The model predicts physiological heat strain responses, e.g. core temperature, by solving the bioheat equations governing the heat transfer between the compartments of a segmented humanoid body shape40. Validation studies addressing the prediction accuracy had revealed a root-mean squared error (rmse) of 0.3 °C concerning core temperatures, 1.4 °C concerning skin temperatures and 0.2 kg/h for sweat rates40,55,56. Simulation runs covering the relevant range of meteorological conditions were performed for a person assumed walking 4 km/h corresponding to a metabolic rate of 135 W/m2 or 2.3 met (1 met = 58.15 W/m2) for 2 hours of exposure. For assessment purposes, a scale classifying the UTCI values into ten categories of thermal stress was added to the operational procedure46. The assessment scale was developed based on comparisons of variables describing the thermal state (including temperature sensation and effectors of thermoregulation of the human body) and established ergonomic limit criteria46,57. In this study, UTCI was calculated for the given meteorological database from the air temperature (t), wind speed at 10 m a.g.l. (va,10 m), mean radiant temperature (Mrt) and humidity expressed as water vapour pressure (vp) using the regression equation provided by the UTCI operational procedure46.
Concerning a refined UTCI assessment of heat stress, a closer inspection of the core temperature increase above its resting value at the end of the 2 h exposure (Δtre) depending on UTCI (Fig. 4A) revealed that a UTCI value of 49.5 °C, determined by segmented linear regression analysis58, marked a tipping point associated with a distinctive increase in Δtre. As this point is used as a criterion for the transition from tolerable to uncompensable heat stress24,59, we evaluated the predicted UTCI values for exercising persons against the UTCI limit value of 49.5 °C in addition to the heat stress categories shown in Fig. 4A.
While the reference UTCI clothing model shown in Fig. 4B was considered relevant for low (65 W/m²) and moderate (135 W/m²) levels of activity, it tended to overestimate clothing insulation for heavily exercising persons considered in this study at UTCI temperatures below 27 °C. ISO 992060 provides clothing insulation values for ensembles comprising briefs, socks, shoes, short- and ¾ length-sleeved shirts and shorts ranging from 0.41 to 0.52 clo with a median value of 0.48 clo. Therefore, we capped the clothing insulation to 0.48 clo for the high activity level (285 W/m²) and used simulations performed with the UTCI-Fiala model specifically for these conditions41,42 to derive adjusted UTCI index values as explained below. Specific simulations were also available to account for a zero-walking speed (vw=0 m/s) of resting persons (65 W/m²), which will reduce the relative air velocity at body level (var,1 m) resulting from a combination of walking speed and a wind speed at 10 m above the ground (va,10 m). This requires careful consideration in modelling heat stress, as the convective and evaporative heat transfer depend on var,1 m, which had been calculated for the UTCI reference (135 W/m²) using vw = 1.1 m/s. Figure 4C depicts these mutual dependencies for the different simulated levels of activity and illustrates our approach to compensate for the higher movement speed vw = 1.75 m/s associated with the high activity condition in UTCI calculation. More specifically, as indicated by the blue lines in Fig. 4C, for a given original wind speed input to UTCI concerning the high activity level (285 W/m²), we searched for the wind speed (va,10 m) yielding identical relative air velocity (va,1 m) under reference conditions (vw=1.1 m/s) and used this va,10 m in the UTCI calculations.
The relevance of walking speed concerning the interplay of metabolic heat production with convective and evaporative body cooling is illustrated in Fig. 4D by the intersection of the curves of UTCI adjustments (∆UTCI) for low sedentary activity (65 W/m2, vw = 0 m/s) with the curve for moderate activity (135 W/m2, vw = 1.1 m/s) at about UTCI = 10 °C. Below 10 °C, body cooling at higher metabolic rate with walking speed increasing var,1 m and, thus, increasing heat loss will dominate, and ∆UTCI for the walking person will stay below the corresponding value for a sedentary person.
Finally, as carried out previously in a recreational context47, UTCI adjustment values (ΔUTCI) shown in Fig. 4D, which had been derived earlier41,42 accounting for modified activity levels under assumed clothing configuration were added to the UTCI values calculated from the meteorological input by the operational procedure46.
(A) Core temperature increase from rest at the end of exposure (Δtre) predicted for UTCI heat stress categories with the tipping point from tolerable to uncompensable heat stress estimated at 49.5 °C by segmented linear regression. (B) Adaptive clothing model with insulation (Icl) depending on UTCI as applied for a low (65 W/m²) and moderate (135 W/m²) activity level compared to the ‘soccer clothing’ used with a high activity level (285 W/m²). (C) Relation between wind speed at 10 m above the ground (va,10 m) and the resulting air velocity at body level (var,1 m) considering different walking speeds (vw) for low (65 W/m²), moderate (135 W/m²) and high (285 W/m²) activity levels, respectively. The reference conditions for the UTCI calculations (135 W/m²) imply a walking speed fixed to vw = 1.1 m/s. Here, the blue lines illustrate the strategy for accounting for a higher walking speed (vw = 1.75 m/s). When UTCI is obtained for a high activity level, the original wind speed input (va,10 m = 1.5 m/s in the example) is replaced by a compensating higher value (resulting to va,10 m = 2.56 m/s in the example). For the low activity level (65 W/m²) specific simulations with vw = 0 m/s were available41,42. (D) Adjustment values (∆UTCI) considering different activity levels for UTCI calculated for the reference conditions41,42.
Water loss due to sweating (SW, in g/h) is an index derived from the Man-environment heat exchange model – MENEX_200561,62. It was estimated based on the values of potential evaporative heat loss (Epot, in W/m2) under the given environmental conditions (provided that the relative air humidity dropped to 5%):
According to the MENEX_2005 model, potential evaporative heat loss (Epot) depends on the difference in water vapour pressure in the air (vp’) corresponding to the assumed 5% relative humidity (RH) and saturated water vapour pressure on the surface of the skin (vps), as well as on the evaporative heat transfer coefficient (he), skin wettedness coefficient (w) and the dimensionless coefficient of attenuation of evaporative heat transfer due to clothing (Ie). The influence of the metabolic heat production (M) on sweating60, was considered when estimating Epot by:
where vp’ can be calculated from the air temperature (t) as follows using the modified Tetens’ formula:
and vps depends on the value of the skin temperature (ts):
.
The skin temperature was predicted using the empirical formula of Błażejczyk63,64:
The further variables were estimated by the MENEX_2005 model as follows:
where the conductive heat transfer coefficient of the clothing (hc’) is:
and the coefficient of the convective heat transfer (hc) is equal to:
Clothing insulation (Icl) was fixed to 0.4 clo, while the metabolic heat production (M) was analysed for three levels in correspondence with the UTCI analysis: 65, 135, 285 W/m2 and in addition at 450 W/m2, which refers to the average activity level of the players (regarding different positions on the pitch) during the competition48.
In order to establish a SW index threshold above which hypohydration may lead to exertion and impair the athletes’ performance, we adopted the criterium of 3% body mass loss due to sweating during the whole match, according to Deshayes et al.65 and Nuccio et al.14. Taking into account the average weight of 77 kg recorded for players during the FIFA Championship 2018 in Russia5, SW values above 1.5 kg/h should be associated with dehydration.
Oxygen volume (Ov, in g/m3) is an indicator of the environmental stress on the human respiratory system, which determines the oxygen mass in the air64,66. It depends on the temperature and humidity of the air, but it is primarily determined by the atmospheric pressure, which decreases with the increase of the altitude. Ov index was calculated according to67 using Eqs. (11–13) derived from the Clapeyron equation:
where k is the oxygen content (in g/mol) in one mole of humid air equal:
R is ideal gas constant of 83.14 hPa∙dm3/mol∙K and T is air temperature in K.
After transformation, this formula takes the following form:
A special attention was paid to the Estadio Azteca in Tlalpan (Mexico), as it is the only stadium that is located at a moderate altitude (above 2000 m a.s.l.), where due to oxygen deficiency in the inhaled air, an impairment of the aerobic capacity and endurance performance may occur21.
Descriptive statistics summarizing hourly values of UTCI, SW and Ov indices dependent on the location and the time period were calculated using the R statistical computing environment68. Reanalysis data were processed using the ncdf4 package69. Radar plots presenting the distribution of mean hourly UTCI and SW values for each level of the metabolic heat production were created using the ggradar package70. The probability of uncompensable heat stress and dehydration was calculated as relative frequencies of exceeding the stress criteria thresholds defined above for UTCI and SW, respectively.
A list of all used symbols including units is provided in supplemental Table S2. The study did not involve human participants and was conducted in accordance with the guidelines approved by the Wroclaw University of Health and Sports Sciences Ethical Review Board (agreement number: 12/2021).
Radar charts in Fig. 5 illustrate the mean hourly UTCI values in relation to the locations and activity levels, whereas supplemental Figure S1 presents the corresponding distributions as boxplots. The highest thermal stress was estimated to occur at all stadiums between 2 and 5 p.m. local time, with an exception of Miami, where the highest UTCI values were observed between 11 a.m. and 12 p.m. (Fig. 5, Fig. S1). In terms of low (65 W/m2) and moderate (135 W/m2) activity levels, the highest average UTCI values occurred in Arlington at 4 p.m. (37.7 ± 3.7 °C and 38.4 ± 3.6 °C, respectively) (supplemental Table S3). However, when accounting for the additional heat load resulting from the greater physical exertion of the soccer players during the match (285 W/m2), the average hourly UTCI values at 10 venues reached the ‘extreme heat stress’ level (> 46.0°C). The highest average UTCI values corresponding to the heat stress during a match, exceeded 50°C and were recorded in Arlington (2–6 p.m.) and Houston (2–4 p.m.). In contrast, the lowest heat stress for the time of the scheduled Championships was found to be at night and in the morning. Among all the 2026 FIFA World Cup locations, thermoneutral conditions (UTCI ≤ 26.0°C) for low and moderate activity levels occurred during day hours only in Seattle, Tlalpan and Vancouver (Fig. 5), although in the case of intense physical activity, the average hourly UTCI value in these three stadiums was far from thermal comfort and also exceeded 40°C (supplemental Table S3).
Radar charts of mean hourly UTCI values from 11th June to 19th of July (2009–2023) at sixteen locations of the 2026 FIFA World Cup, calculated for 3 levels of metabolic heat production.
For low and moderate levels of activity strong and very strong thermal stress could be expected most frequently in Arlington, Houston and Monterrey, while in Tlalpan these categories of thermal stress did not occur at all (Fig. 6). Thermoneutral conditions for low intensity (65 W/m2) were the most frequent in Vancouver, followed by Tlalpan and Seattle. Biothermal conditions in all locations at night generally provided no thermal stress, as heat stress was not recorded until morning hours. For activities of moderate intensity (135 W/m2) usually no thermal stress occurred early in the morning, with the exception of Miami and Houston, where even at night and in the morning hours at least moderate heat stress prevailed. Increasing the intensity of physical exercise (up to 285 W/m2) resulted in a significant increase in the intensity of heat stress at all locations. In Arlington, Atlanta, Houston, Kansas, Miami and Monterrey in the hours when the 2026 FIFA World Cup matches will most probably be played (between 3 and 10 p.m.) the overwhelming majority constituted very strong and extreme heat stress conditions.
Frequency of individual UTCI categories of heat stress in the different locations, presented as hourly slots. Results for three levels of physical activity: 65, 135 and 285 W/m2.
Radar charts in Fig. 7 illustrate the mean hourly SW index values in relation to the locations and activity levels, whereas supplemental Figure S2 presents the corresponding distributions as boxplots. The highest water loss due to sweating was predicted at all locations between 2 and 5 p.m. local time (Fig. 7, Fig. S2). In June and July, in the period when the 2026 FIFA World Cup matches are scheduled, the highest mean values of the SW index occurred in Arlington, followed by Monterrey and Houston. Even at rest (65 W/m2), players were expected in these three stadiums to lose on average more than 500 g of water per hour in the afternoon. For the highest levels of activity (450 W/m2) between 1 p.m. and 6 p.m. the average sweat rate in these locations exceeded 1 kg/h (Arlington – 1.2 ± 0.3 kg/h, Monterrey – 1.1 ± 0.3 kg/h, Houston – 1.1 ± 0.2 kg/h), while in Atlanta, Kansas, Los Angeles and Philadelphia hourly SW values in these hours were on average in the range 0.9–1.0 kg/h during the play. However, in particular cases the maximum hourly SW index values could be many times larger. In the analysed period the highest predicted hourly SW for 450 W/m2 metabolic heat production was obtained for Los Angeles during the 2018 North American heat wave, on 6th July 2018 at 2 p.m. and equalled 13.6 kg/h (supplemental Table S3), which was far beyond the maximal reported sweating rate of a soccer player of 4 l/h71 and should be interpreted as reaching the limits of the maximum physiologically possible water loss by the human body through sweating. The lowest expected water loss due to sweating for the highest activity level (450 W/m2), was obtained in the coolest location – in Tlalpan (on average 0.6 ± 0.05 kg/h at 3 p.m.). Relatively low sweat rates could also be expected during matches played in Vancouver and Seattle, where the highest average hourly SW values were 0.7 ± 0.1 kg/h in both venues, although for these stadiums the maximum predicted hourly SW values exceeded 2.1 kg/h (at 5 p.m.) and 1.7 kg/h (at 3 p.m.) (supplemental Table S3).
Radar charts of mean hourly SW values from 11th June to 19th of July (2009–2023) at sixteen locations of the 2026 FIFA World Cup, calculated for 4 levels of metabolic heat production.
The probability of uncompensable heat stress was assessed by the relative frequency of UTCI values exceeding 49.5 °C (Fig. 4A, Fig. S1). The highest values (up to 70%) were predicted to occur in Arlington and Houston between 2 and 6 p.m. (Fig. 8A), although an elevated risk level (more than 30%) was also apparent at morning hours. In Monterrey the probability of uncompensable heat stress was likewise high, exceeding 50% in the afternoon, but the heat stress risk decreased abruptly after 6 p.m. Excessive water loss predicted by the SW index for the 2026 FIFA World Cup locations affected only 3 stadiums to a moderate extent. The highest dehydration risk, defined as the relative frequency of SW values above the threshold of 1.5 kg/h (Fig. S2), exceeding 15%, was observed in Arlington between 2 and 6 p.m., followed by Monterrey and Houston (Fig. 8B), however in Arlington approximately 10% probability of excessive water loss persisted in the evenings.
The probability (%) of (A) uncompensable heat stress assessed by UTCi and (B) dehydration occurrence assessed by SW index during intensive physical activity from 11th June to 19th of July (2009–2023) at sixteen locations of 2026 FIFA World Cup, divided into 4-hour periods.
Only two near sea level locations – Seattle and Vancouver were characterised by Ov higher than 275 g/m3 (Fig. 9A). The highest located venues, Guadalajara (1566 m a.s.l.) and Tlalpan (2240 m a.s.l.) showed the lowest predicted oxygen volume of all locations (on average 225.9 ± 1.6 and 202.3 ± 1.3 g/m3 respectively). In Tlalpan the lowest mean hourly Ov index values (198.3 ± 1.5 g/m3) were obtained in the afternoon (2–3 p.m.) and similarly in Guadalajara at 3 p.m. (221.1 ± 1.8 g/m3), while the highest rates occurred at night and early in the morning (205.6 ± 1.5 g/m3 and 229.4 ± 1.2 g/m3 respectively) (Fig. 9B).
(A) Boxplots of the oxygen volume distribution in the vicinity of the 2026 FIFA World Cup stadiums, and (B) means and standard deviations of hourly oxygen volumes estimated for Guadalajara and Tlalpan, respectively, from 11th June to 19th of July (2009–2023).
This study aimed to investigate the risk of severe heat stress conditions and the associated potential water losses in professional athletes, considering as well the oxygen content in the inhaled air in the context of the 2026 FIFA World Cup in North America (USA, Mexico, Canada).
The research goal proposed in this study fits into current research on the relationship between environmental conditions and human performance, widely discussed in the literature7,72,73,74,75. In this study, these relationships refer to a well-trained and physically efficient male athlete, who participates in a large and popular soccer tournament. The environmental conditions, apart from the match venue, team strength and match result, are one of the most important variables that influence the physical performance of players in highly competitive soccer matches76. Previous research has shown that the higher the level of heat stress during a competition, the harder it is for a team to succeed2,4. Therefore, the data analysed in this study emphasize the need for seriously considering the prevailing climatic conditions as one of the most important factors that will influence the athletic performance during the 2026 FIFA World Cup. At the same time, due to the significant heat stress on the athletes in the summer, the biothermal conditions should be considered by sport events’ organizers and coaching staffs as one of the most important factors concerning athletes’ health and well-being.
Although athletes practicing outdoor sports often have to deal with difficult and changeable weather conditions, the huge diversity of the biothermal conditions during the World Cup in the USA, Canada and Mexico, resulting from the different geographical locations of the stadiums, will be unprecedented in the history of the FIFA World Cup. The distances between the individual stadiums, reaching up to nearly five thousand kilometres, mean that the matches will be played in nine different types of climate43, and therefore they will be accompanied by different levels of environmental stress on the players. However, during the 2026 FIFA matches, the players will be experiencing heat stress of varying intensity not only due to the geographical location of the stadium or the match time schedule, but also due to the intensity of their physical activity9. In this context, this study applied thermal stress simulation taking into account the level of the metabolic heat production41,42. Although for UTCI application, the highest possible level of the metabolic heat production for the soccer players was only 285 W/m2, corresponding to playing with an average running speed of 1.75 m/s, the intensity of predicted thermal stress affecting the players was considerably high. This additional heat load resulting from the greater physical effort of the soccer players during the match caused the average hourly UTCI values in 10 stadiums to exceed 46.0 °C, which corresponds to extreme heat stress during daytime hours46. Additionally, the highest average UTCI values exceeding 50 °C were recorded in Arlington (2:00–6:00 p.m.) and Houston (2:00–4:00 p.m.). Staying in such conditions may place a heavy burden on the body, result in a partial failure of the body temperature regulation processes and, consequently lead to a rise of the core temperature, followed by the occurrence of heat exhaustion, potentially even heat stroke9,77. Therefore, during matches at the 2026 FIFA World Cup, for the safety of the players, it will be necessary to use treatments that reduce the impact of heat stress, including regular additional breaks for cooling of the body and adequate fluid replenishment7,22. The risk of strong and very strong heat stress occurrence at the 2026 FIFA World Cup should also be considered a threat for the substitute players and supporters, as in the specific locations, e.g., in Arlington, even when simulating low (65 W/m2) and moderate (135 W/m2) activity levels, the highest average UTCI values exceeded 37 °C.
Moreover, in this study the UTCI limit value of 49.5 °C was estimated as a turning point from tolerable to uncompensable heat stress, when the thermoregulatory system becomes inefficient and the core temperature starts to increase abruptly (Fig. 4A). During the 2026 FIFA World Cup, the highest probability of unacceptable thermal stress is to be expected in the afternoon in Arlington and Houston (almost 70%), followed by Monterrey (exceeding 50%), decreasing sharply after 6 p.m. Therefore, tournament organizers should optimally plan the time schedule of the matches for individual venues, as uncompensated heat stress may not only threaten the health of players, but also compromise the activity and effectiveness of the players on the pitch6. The biothermal conditions prevailing in all locations at night generally provide good conditions for the body’s regeneration during rest, as heat stress is not recorded until the morning hours, which is a fundamental determinant of the preparation for, and recovery from training or competition78,79. However, taking into account matches played late in the evening and the fact that players remain awake until late at night, it is worth mentioning that during moderate intensity activity (135 W/m2) in Miami and Houston, at least moderate heat stress is present for the whole day. Prolonging periods without cooling at nights can lead to heat accumulation within the body80, and, when combined with a lack of sleep, can increase the susceptibility to heat stroke9.
In response to a hot environment, body temperatures rise, and the thermoregulatory system responds to this by excreting sweat at the skin surface. In conditions of severe heat stress, the sweating process may be very intense and lead to significant loss of water from the body, up to 4 l/h81. In order to compensate for the water loss under heat stress conditions, re-hydrating the body is essential. In the context of soccer players, it should include starting competitions in a state of euhydration, preventing excessive dehydration during play, and replenishing remaining losses before the next effort19,82. It is well-recognized that fluid needs are individual and depend on factors such as personal sweat rate, exercise intensity, environmental conditions and duration of exercise83, nevertheless, the optimal hydration during exercise can be associated with a water loss of no more than 2–3% of body weight, while avoiding overhydration82. In this study dehydration was identified with the predicted sweat loss exceeding 1.5 kg/h. The results suggest that excessive water loss in the 2026 FIFA locations will affect prominently 3 stadiums, in Arlington, Monterrey and Houston, to a moderate extent. On the other hand, in particular situations the environmental conditions can promote faster evaporation of sweat, which, combined with intense physical activity, may generate a high demand for fluid replenishment. Nevertheless, although thermal stress could probably negatively affect the match performance, a player with a body temperature below 39 °C, who is adequately hydrated, is not expected to experience any significant deterioration in his sports skills18.
The oxygen content in the inhaled air is closely related to the biological conditions and altitude above sea level, and will influence the physiological performance of the athletes. When playing with a reduced oxygen content in the air, the players must compensate for this deficit by breathing more intensively. The sporting implication of playing in such conditions is most often a reduction of the maximum aerobic power of the players, who live near the sea level and are not well adapted21,84. Game statistics of the 2010 FIFA World Cup, as well as data from the 2011 FIFA U20 World Cup corroborate that the total running distance covered by teams during matches at moderate altitude was lower compared with matches played at sea level85, although the results of the International Study on Soccer at Altitude 3600 m (ISA3600) indicate that the negative effects of high altitude on soccer players are independent of whether the athletes come from high altitudes or sea levels86. Therefore, the oxygen volume index (Ov) can be used to assess the load on the respiratory system and, indirectly, to assess the exercise capacity of the soccer players. Under reference conditions (1000 hPa, air temperature 15 °C and water vapor pressure 8 hPa), the Ov index is 277 g/m3 64. The stadiums of the higher altitude, Guadalajara (1,566 m a.s.l.) and Tlalpan (2,240 m a.s.l.) have the lowest oxygen concentrations of all analysed locations (on average 225.7 and 202.3 g/m3, respectively), which corresponds to 81.5% and 73% of the values determined at sea level. In the future, the analysis of post-match statistics after the 2026 FIFA World Cup combined with information about oxygen volume in the inspired air will provide further relevant conclusions about the impact of the altitude on the exercise capacity of the soccer players.
While discussing the results of the present study, some limitations must be considered. Firstly, meteorological data from the ERA reanalysis characterises mean biothermal conditions in the close vicinity of the stadia, but certainly differ from the conditions prevailing on the stadium pitch. However, open roof stadiums may be regarded as outdoor spaces, while bearing in mind that the stadium thermal climate is affected not only by the environmental conditions in the area, but also by the indoor climate control from the air conditioning and ventilation units, which vary between stadiums and are hard to simulate38,49. The most difficult factor to estimate is the wind speed in the stadium, as the stadium surrounding geometry and wind direction modify significantly the wind field87. In addition, the wind speed also varies greatly within the pitch, which results from the geometry of a given stadium88,89. Therefore, as detailed research on the microclimate modifications inside a stadium due to its specific size and shape are very limited, and only few CFD (computational fluid dynamics) mathematical simulations are available for specific theoretical objects88,90, we had to made an assumption regarding the lower wind speeds in all analysed stadiums. To account for the limited air flow in general, the wind speed on the pitch was estimated to be 60% of the wind speed measured outside the stadium, based on Ghani et al.49 in situ measurements. Moreover, we have performed calculations assuming that the stadiums are not air-conditioned, which means that our analyses may overestimate the level of heat stress and water losses in some cases. However, for high metabolic rate, the effect of the wind speed limitation might be diminished, as an elevated movement speed of the players was considered, which will lower the influence of wind speed modifications.
We decided to use UTCI and modify it slightly to adapt it better for soccer players. However, the corresponding UTCI adjustments (∆UTCI) were determined for UTCI reference conditions with low wind speed (va,10 m = 0.5 m/s), 50% relative humidity and mean radiant temperature equalling air temperature. One of the core principles of UTCI conception was that identical index values should represent equal thermophysiological strain53. During UTCI development, this requirement had been validated, e.g., when defining the UTCI stress categories, which could be based on the strain responses to UTCI reference conditions, or when comparing physiological heat strain recorded in laboratory experiments covering a wide range of temperature-humidity combinations46,56. These previous findings may motivate the application of ∆UTCI in non-reference conditions. In addition, supplemental Figure S3, which summarizes previous analyses of the error associated with the applied procedure41, indicates a negligible bias and a root-mean squared error (rmse) of about 2–3 °C. Thus, the errors associated with using ∆UTCI in non-reference conditions fall within the range of uncertainties attributable to the application of the regression polynomial for UTCI calculation, with rmse = 1.1 C°46, and to calculating UTCI based on mean radiant temperatures estimated from routine weather station data, with rmse = 6 °91. These findings suggest that the application of ∆UTCI to non-reference climatic conditions in our study was appropriate.
In addition, as for now, UTCI can be adjusted only up to 285 W/m2 metabolic heat production, and as the soccer players may be physically exerting partially twice as much during a match, the heat stress level may even be underestimated. The highest running speed we analysed was 7.2 km/h, which corresponds to a match average. And although at high speed runs the players reach much higher speeds (> 25 km/h)92, most distance during the match is covered at lower speeds (< 7.2 km/h)93. Therefore, while our analysis does not take into account higher activity levels of the soccer players during the match (over 450 W/m2), it can be assumed that the real water losses can be even higher (although some of these losses can be compensated during the break of the match by fluids supplementation). Additionally, the heat stress levels we determined may be better tolerated by well-trained athletes with higher levels of physical fitness than by an average person, to whom the UTCI index scale applies. Augmenting current thermal indices by including the effects of individual characteristics like physical fitness and status of heat acclimation might enhance their prospective applicability to athletes performing in extreme environments7,30. Furthermore the UTCI thermal stress categories, although related to specific responses of the thermoregulatory system to biothermal conditions, have not previously been tested in the context of competitive athletes’ performance. Therefore, they cannot be treated directly as thresholds for a given exertion omission, but can constitute warnings of the rising intensity of environmental heat stress in relation to a potential health hazard94. Our results should be treated with caution also in relation to senior members of technical service or coaching staff staying on the stadium field during the matches. With an advancing age both the thermoregulatory capacity and fluid regulation deteriorate, leading to an increased risk of hyperthermia and dehydration in older adults95.
Our results indicate that ten out of all sixteen 2026 FIFA World Cup venues are at very high risk of experiencing severe heat stress conditions. The highest predicted average hourly UTCI values and excessive water loss due to unacceptable heat stress will occur in the afternoon in stadiums located in the southern regions of the United States (Arlington, Houston) and in Monterrey (Mexico). This outcome may be a guideline for the tournament organizers to optimize the schedule of match time and additional breaks for cooling and re-hydration at individual stadiums. In this context, the reduced oxygen content in the inhaled air at Guadalajara (1,566 m a.s.l.) and Tlalpan (2,240 m a.s.l.) will also require additional consideration.
By means of biometeorological indices, especially those taking into account increased metabolic heat production due to elevated intensities of exercise, it is possible to characterise the level of thermal stress and the associated health risks for soccer players during big sport events that take place over large geographical areas, like the 2026 FIFA World Cup. The results shall allow a better training planning and pre-match adaptation, as well as provide a basis for optimising the schedule of the matches in terms of avoiding the threat of excessive thermal strain.
The datasets used and analysed during the current study are available from the corresponding author marek.konefal@awf.wroc.pl on reasonable request.
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Institute of Geography and Spatial Organization, Polish Academy of Sciences, Twarda 51/55, Warsaw, 00-818, Poland
Katarzyna Lindner-Cendrowska
Department of Climatology, Faculty of Geography and Regional Studies, University of Warsaw, Krakowskie Przedmieście 30, Warsaw, 00-927, Poland
Kamil Leziak
Leibniz Research Centre for Working Environment and Human Factors at TU Dortmund (IfADo), Dortmund, Germany
Peter Bröde
ErgonSim–Human Thermal Modelling, Robert-Bosch-Str. 20, 72469, Messstetten, Germany
Dusan Fiala
Department of Human Motor Skills, Wrocław University of Health and Sport Sciences, Paderewskiego 35, Wrocław, 51-612, Poland
Marek Konefał
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Conception and design: K.LC., K.L., M.K., data collection: K.LC., K.L., M.K., data analysis and interpretation: K.LC., K.L., M.K., P.B., writing original draft: K.LC., K.L., M.K., P.B., supervision: D.F., K.LC., M.K., P.B., editing and review: D.F., K.LC., K.L., M.K., P.B.
Correspondence to Marek Konefał.
The authors declare no competing interests.
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Lindner-Cendrowska, K., Leziak, K., Bröde, P. et al. Prospective heat stress risk assessment for professional soccer players in the context of the 2026 FIFA World Cup. Sci Rep 14, 26976 (2024). https://doi.org/10.1038/s41598-024-77540-1
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DOI: https://doi.org/10.1038/s41598-024-77540-1
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