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High temperatures and humidity can not only impede athletic performance but also present health dangers for both amateur and professional athletes.(Image credit: FG Trade via Getty Images)
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As the World Cup commences across the U.S., Canada, and Mexico, experts have projected that extreme heat may significantly influence the competition’s outcome.
Approximately half of the scheduled contests — particularly those in the southern U.S. and low-lying regions of Mexico — face at least a 50% probability of experiencing “performance-impeding heat.” In preparation, many teams have undergone rigorous training in hot climates, and throughout the tournament, they will have access to cold-water immersion tubs, ice vests, and misting fans to aid in cooling players if they become overheated.
In contrast to these major sporting events, which possess the resources to adjust to extreme heat, community sports participants and recreational exercisers often have fewer alternatives. This reduced capacity to adapt might increasingly impact when, where, and how individuals can safely engage in physical activity, especially as climate change leads to more frequent and intense extreme warming events.
“The vast majority of individuals who participate in sports for competition, or simply for enjoyment, recreation, and leisure, have a fraction of the capacity [that professional sports organizers have] but confront greater challenges and risks,” Jessica Murfree, an associate professor in the Department of Exercise and Sport Science at the University of North Carolina at Chapel Hill, informed Live Science.
How hot is too hot for exercise?
Sports governing bodies typically employ the wet-bulb globe temperature (WBGT) to evaluate heat-related risks. This metric assesses the combined effects of temperature, humidity, direct solar radiation, and wind speed on the human body.
Incorporating humidity is crucial because elevated moisture in the air impedes sweat evaporation from the skin, which is the body’s primary cooling mechanism. The WBGT was developed by the U.S. military in the 1950s to prevent heat-related fatalities during training exercises. Today, it also informs safety protocols for outdoor laborers and athletes.
The American College of Sports Medicine categorizes the U.S. into three zones to account for varying degrees of heat acclimatization and establishes WBGT ranges that signify a “very high” risk within each zone. This threshold is a WBGT of 82.2 degrees Fahrenheit (27.9 degrees Celsius) or higher in the Southeastern and Southwestern states, 79.9 F (26.6 C) in the Central states, and 76.3 F (24.6 C) in the Northern regions.
For individuals who are not heat-acclimatized or possess lower levels of physical fitness, the health hazard increases at lower thresholds.
The Gulf Coast states — particularly Southern Texas, Louisiana, Mississippi, Alabama, and Florida — generally experience the highest WBGT readings. High-risk conditions commence in May in South Texas and South Florida, expanding northward and reaching their peak in July and August. Research indicates that the frequency of extreme humid heat events has more than doubled in most parts of the U.S. since 1980, and globally, climate change contributed approximately three weeks of dangerous humid heat in 2024 alone.
Average monthly WBGT in the southeastern United States between 1991 and 2020.
(Image credit: Thompson, D. (2026). Journal of Applied Meteorology and Climatology 65, 3; 10.1175/JAMC-D-25-0125.1 © American Meteorological Society. Used with permission.)
An individual’s general health and fitness, hydration status, and level of exertion must be taken into account, and systemic factors such as socioeconomic standing also play a part, Murfree commented. For example, lower-income households are more likely to reside in hotter urban settings and lack air conditioning, which means they might already be enduring heat stress at home.
While high humidity and heat prevent sweat from evaporating efficiently, the body possesses alternative methods for cooling down. It also redirects more blood flow to the skin, arms, and legs, facilitating heat dissipation through the skin. However, physical exertion under extreme heat conditions makes this process more challenging.
“Our body’s blood supply is finite, and we are simultaneously attempting to utilize it to power the muscular processes necessary for exercise, as well as to circulate it to the skin to aid in cooling,” stated Grant Lynch, a research fellow at the Heat and Health Research Centre at the University of Sydney.
When the body can no longer shed excess heat, symptoms of heat exhaustion — including lightheadedness, queasiness, head pain, and muscle spasms — may emerge. If the body continues to overheat, this can escalate to heat stroke, a severe and potentially fatal medical crisis where a person’s core body temperature climbs to 104 F (40 C) or higher. Heat stroke can lead to central nervous system impairment and cause delirium, convulsions, or coma.
In the U.S., exertional heat stroke is a primary cause of preventable fatalities during sports and exercise and is a particular concern for young athletes. Children, including teenagers, generate more heat relative to their body weight compared to adults and sweat less. Furthermore, they might feel less empowered to assert themselves on the field if they begin to feel unwell, Murfree noted.
Heat can be particularly hazardous for children and adolescents compared to adults.
(Image credit: Frazao Studio Latino via Getty Images)Reducing health risk in hot weather
The body possesses a degree of adaptability to extreme heat. At the University of Sydney’s thermal ergonomics laboratory, scientists utilize a climate chamber to assess the body’s capacity for adaptation by enhancing its blood plasma volume and reducing heart rate and resting temperature.
“They would participate in sessions lasting anywhere from five to 14 consecutive days of exposure,” Lynch explained. “They would be cycling or running for approximately 90 minutes to 2 hours each day, under conditions of [95 F to 104 F] 35 C to 40 C in the room for the entire duration.”
The advantages gained from this type of heat acclimatization are not enduring. Studies suggest that for every two days spent in a cooler environment, one day’s worth of heat acclimatization is diminished. For individuals who exercise only a few times per week in hot weather, “it’s highly improbable that you are physiologically adapting as much as you believe,” Lynch stated.
While the WBGT is regarded as the “benchmark standard” for monitoring heat safety, the specialized instrument needed to measure it might not be readily available to every community sports organization or individual engaging in exercise. The risk, Lynch cautioned, lies in many people focusing solely on ambient temperature to gauge danger, when numerous factors are involved and can compound.
Sports Medicine Australia, the nation’s governing body for sports medicine and sports science, has developed a tool to evaluate risk and propose safety measures. It integrates data automatically retrieved from weather stations — encompassing ambient temperature, humidity, wind speed, and solar radiation — with sport-specific details, such as typical attire and activity duration.
Generally, the sports organization advises maintaining adequate hydration before, during, and after physical exertion; wearing lightweight and permeable clothing; taking frequent rests using cold towels and misting devices when feasible; and postponing or canceling games when the risk level becomes excessively high. If an individual begins to feel overheated, prompt intervention is crucial to avert further heat strain.
“The most critical action, unequivocally, is to cease or reduce the intensity of your current activity, find shade, move to an area shielded from direct sunlight, and then douse your skin with water,” Lynch advised.
Profuse sweating, painful muscle cramps, nausea, dizziness, and cool, pale, clammy skin are all indicators that someone might be experiencing a heat-related ailment. Symptoms of heat stroke encompass extremely high body temperature; hot, dry skin or excessive perspiration; confusion; and loss of consciousness. These symptoms necessitate immediate cooling via a cold water bath and emergency medical attention.
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While rescheduling events to cooler periods of the day is a possibility, heatwaves are already intensifying in most regions, and this trend is anticipated to persist as average global temperatures escalate. Consequently, some researchers and sports organizations are investigating infrastructural modifications that could offer assistance: stadiums equipped with shaded and green areas, school grounds featuring grass instead of concrete, and artificial turf installations with integrated irrigation systems for use in school sports fields.
Murfree expressed a desire to see heat safety education incorporated into school curricula in regions of the U.S. susceptible to extreme heat, mirroring existing fire safety instruction.
“Sports often compel us to push our limits, to be faster and more resilient,” Murfree remarked. “It is perfectly acceptable to take breaks, pause, slow down, reschedule, and advocate for our bodies and our well-being in the heat.”
