Dangerously hot and humid: Rising temperatures in the US make outdoor exercise hazardous

As the World Cup kicks off across the U.S., Canada and Mexico, scientists have predicted that extreme heat could play a decisive role in determining the outcome.

Nearly half of the scheduled matches — especially those in more southerly areas of the U.S. and low-lying parts of Mexico — have at least a 50% likelihood of "performance-impairing heat." To prepare, many teams have been training intensively in hot conditions, and during the tournament, they will have access to cold-water immersion tubs, ice vests and misting fans to help cool players down if they overheat.

Compared with these major sporting events, which have resources to adapt to extreme heat, community sports participants and recreational exercisers often have fewer options. This lack of adaptability may increasingly affect when, where and how people can safely exercise, especially as climate change makes extreme warming events more frequent and intense.

"The majority of people who play sport for competition or just for fun, recreation and leisure have a fraction of that power [of professional sports organizers] but face greater challenges and harms," Jessica Murfree, an assistant professor in the Department of Exercise and Sport Science at the University of North Carolina at Chapel Hill, told Live Science.

How hot is too hot for exercise?

Sports governing bodies typically use wet-bulb globe temperature (WBGT) to assess heat risk. It measures the combined impacts of temperature, humidity, direct sunlight and wind speed on the human body.

Taking humidity into account is key because, with more water in the air, it's harder for sweat to evaporate off the skin and cool the body. WBGT was developed by the U.S. military in the 1950s to prevent heat-related deaths in training camps. Nowadays, it also shapes safety guidelines for outdoor workers, as well as athletes.

The American College of Sports Medicine divides the U.S. into three regions to account for varying levels of heat acclimatization and defines WBGT ranges that pose a "very high" risk in each region. That threshold is a WBGT of 82.2 degrees Fahrenheit (27.9 degrees Celsius) or higher in Southeastern and Southwestern states, 79.9 F (26.6 C) in Central states, and 76.3 F (24.6 C) in Northern regions.

For unacclimatized individuals and those with poor physical fitness, the health risk jumps at lower thresholds.

Gulf Coast states — particularly Southern Texas, Louisiana, Mississippi, Alabama and Florida — tend to experience the highest WBGT values. High-risk conditions begin in May in South Texas and South Florida, before expanding northward and peaking in July and August. Studies suggest the number of extreme humid heat events has more than doubled in most parts of the U.S. since 1980, and that globally, climate change added roughly three weeks' worth of dangerous humid heat in 2024 alone.

An individual's overall health and fitness, hydration levels and amount of exertion need to be considered, and systemic factors like socioeconomic status also come into play, Murfree said. For instance, lower-income families are more likely to live in hotter urban environments and lack access to air conditioning, meaning they may already be experiencing heat stress at home.

While high humidity and heat make it so sweat can't evaporate easily, the body has other means to cool down. It also sends more blood to the skin, arms and legs, allowing heat to be released through the skin. But physical exertion in extreme heat makes this more difficult.

"We only have a limited amount of blood in our body, and we're simultaneously trying to use it to fuel the reactions in our muscles to be able to facilitate exercise, as well as send it to our skin to help us cool down," said Grant Lynch, a research fellow in the Heat and Health Research Centre at the University of Sydney.

When the body can no longer get rid of extra heat, symptoms of heat exhaustion ‪—‬ including dizziness, nausea, headache and muscle cramps ‪—‬ may set in. If the body continues to overheat, this can progress to heat stroke, a serious and potentially fatal medical emergency where a person's core body temperature rises to 104 F (40 C) or above. Heat stroke can cause central nervous system dysfunction and delirium, convulsions or coma.

In the U.S., exertional heat stroke is a leading cause of preventable death during sport and exercise and is a particular concern for youth athletes. Children, including teens, produce more heat relative to their body weight than adults do while sweating less. Additionally, they may feel less able to advocate for themselves on the playing field if they start to feel ill, Murfree said.

Reducing health risk in hot weather

The body can acclimate to extreme heat, to some extent. At the University of Sydney’s thermal ergonomics laboratory, researchers use a climate chamber to test the body’s capacity to adapt by increasing its blood plasma volume and lowering heart rate and resting temperature.

"We would have them come in for anywhere from five to 14 consecutive days of exposure," Lynch said. "They would be cycling or running for between 90 minutes to 2 hours every day, and the conditions would be [95 F to 104 F] 35 C to 40 C in the room for that entire duration."

The benefits achieved through this kind of heat acclimation aren't permanent. Research suggests that, for every two days spent in a cool environment, one day of heat acclimation is lost. For people exercising just a few times a week in hot weather, "it's pretty unlikely you are physiologically adapting as much as you think you are," Lynch said.

While WBGT is considered the "gold standard" for heat safety monitoring, the special device required to measure it may not be accessible to every community sports organization or individual person exercising. The danger, Lynch said, is that many people focus on ambient temperature alone to determine risk, when multiple factors are involved and can compound.

Sports Medicine Australia, the country's national body for sports medicine and sports science, developed a tool to calculate risk and suggest safety precautions. It combines data automatically extracted from weather stations — including ambient temperature, humidity, wind speed and solar radiation — with sport-specific information, such as typical uniforms and duration of activity.

Generally speaking, the sports body recommends staying hydrated before, during and after physical activity; wearing lightweight and breathable clothing; taking frequent rest breaks using ice towels and misting fans when possible; and delaying or cancelling games when the risk level gets too high. If someone begins to feel overheated, immediate action is important to prevent further heat strain.

"The most important thing to do, without a doubt, is to stop or reduce the intensity of the activity you're performing, seek shade, go into an area where you are not exposed to direct sunlight, and then douse the skin with water," Lynch said.

Heavy sweating, painful muscle cramps, nausea, dizziness, and cool, pale, clammy skin are all signs that someone may be suffering from heat-related illness. Heat stroke symptoms include a very high temperature; hot, dry skin or profuse sweating; confusion; and loss of consciousness. These symptoms should prompt immediate cooling with a cold water bath and emergency medical care.

While rescheduling events to cooler parts of the day is an option, heatwaves are already increasing in most regions and that trajectory is expected to continue as average global temperatures rise. So, some researchers and sports organizations are exploring infrastructure changes that could help: stadiums with shaded and green spaces, schoolyards with grass instead of concrete, and artificial turf with a built-in irrigation system for use in school sports fields.

Murfree said she would like to see heat safety training introduced in schools in areas prone to extreme heat in the U.S., similar to existing 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."

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