As the World Cup kicks off across the U.S., Canada and Mexico, scientists warn that extreme heat could play a decisive role in determining outcomes, with nearly half of scheduled matches—particularly in southerly U.S. regions and low-lying parts of Mexico—facing at least a 50% likelihood of “performance-impairing heat.” While elite teams train intensively in hot conditions and have access to cooling resources like cold-water immersion tubs, ice vests, and misting fans, recreational athletes often lack such protections, increasing vulnerability to heat-related health risks, especially as climate change amplifies extreme warming events. “The majority of people who play sport for competition or just for fun, recreation and leisure have a fraction of that power but face greater challenges and harms,” said Jessica Murfree, an assistant professor at the University of North Carolina at Chapel Hill.
Sports governing bodies use wet-bulb globe temperature (WBGT) to assess heat risk, measuring the combined impacts of temperature, humidity, direct sunlight and wind speed. Developed by the U.S. military in the 1950s to prevent heat-related deaths in training camps, WBGT is now used for outdoor worker safety and athlete guidelines. The American College of Sports Medicine defines regional “very high” risk thresholds: 82.2°F (27.9°C) in Southeastern and Southwestern states, 79.9°F (26.6°C) in Central states, and 76.3°F (24.6°C) in Northern regions. Unacclimatized individuals and those with poor fitness face elevated risks at lower thresholds.
Gulf Coast states—including Southern Texas, Louisiana, Mississippi, Alabama, and Florida—experience the highest WBGT values, with high-risk conditions beginning in May in South Texas and South Florida before expanding northward and peaking in July and August. Studies show extreme humid heat events have more than doubled across much of the U.S. since 1980, and climate change added roughly three weeks of dangerous humid heat globally in 2024 alone.
Average monthly WBGT in the southeastern United States between 1991 and 2020.
Individual factors such as overall health, fitness, hydration, and exertion levels influence risk, as do systemic issues like socioeconomic status. Lower-income communities are more likely to reside in hotter urban environments without air conditioning, compounding heat stress exposure. While the body can partially adapt to heat through acclimatization, these changes aren’t permanent—for every two days in a cool environment, one day of heat adaptation is lost. For those exercising only a few times weekly in hot weather, “it’s pretty unlikely you are physiologically adapting as much as you think,” said Grant Lynch, a research fellow at the University of Sydney’s Heat and Health Research Centre.
When humidity is high, sweat cannot evaporate easily to cool the body. The body attempts to compensate by redirecting blood flow to the skin, arms, and legs to release heat, but physical exertion in extreme heat makes this harder. With limited blood volume, the body must simultaneously fuel muscle activity and facilitate cooling. Heat exhaustion symptoms include dizziness, nausea, headache and muscle cramps, while heat stroke—a potentially fatal medical emergency with core body temperature above 104°F (40°C)—can cause confusion, convulsions, coma and central nervous system dysfunction.
Youth athletes are particularly vulnerable; children produce more heat relative to body weight than adults while sweating less, and may struggle to advocate for themselves when feeling ill. In the U.S., exertional heat stroke is a leading cause of preventable death during sport and exercise.
Sports Medicine Australia developed a risk assessment tool combining weather data—including temperature, humidity, wind speed and solar radiation—with sport-specific factors like uniform type and activity duration. General safety recommendations include staying hydrated before, during and after activity; wearing lightweight, breathable clothing; taking frequent rest breaks with ice towels and misting fans when possible; and postponing or canceling activities when risk levels are too high. Immediate action is critical if someone begins to overheat: “Stop or reduce activity intensity, seek shade, and douse the skin with water,” Lynch advised. Heavy sweating, painful muscle cramps, nausea, dizziness, and cool, clammy skin indicate heat-related illness, while symptoms of heat stroke include very high body temperature, hot dry skin or profuse sweating, confusion, and loss of consciousness—requiring cold water immersion and emergency medical care.
As heatwaves intensify across most regions and are projected to continue rising with global warming, researchers and sports organizations are exploring infrastructure solutions such as stadiums with shaded and green spaces, grass schoolyards instead of concrete, and artificial turf with integrated irrigation systems. Murfree advocates for heat safety training in schools within extreme-heat-prone areas, similar to fire safety education.
“Sport often asks us to push ourselves harder, be faster and tougher,” Murfree said. “It’s OK to take breaks, stop, slow down, reschedule and advocate for our bodies and our wellness in the heat.”
This article is for informational purposes only and is not meant to offer medical advice.
