A powerful heat wave is expected to grip much of the central and eastern United States this week, bringing dangerously high temperatures to several cities hosting FIFA World Cup knockout matches and raising concerns for both players and fans.
Forecasters say a strong heat dome will push temperatures and humidity to uncomfortable levels, with the heat index, or the "feels-like" temperature, expected to climb above 100 degrees Fahrenheit (38 degrees Celsius) in several areas. Open-air stadiums in cities including Boston, Philadelphia and Kansas City are expected to be among the hardest hit.
"This week is going to be extremely hot," said Geoff Cornish, assistant chief video meteorologist at AccuWeather. He described it as a major heat wave that is not seen every year.
Heat has been a growing concern throughout the tournament, which is being hosted by the United States, Canada and Mexico. Scientists say climate change, driven by the burning of fossil fuels, is making extreme heat more frequent and intense around the world.
According to the National Oceanic and Atmospheric Administration, extreme heat causes more deaths in the US each year than hurricanes, floods or tornadoes.
The National Weather Service has issued an extreme heat watch for Philadelphia and nearby areas from Wednesday afternoon through Saturday evening, with "feels-like" temperatures expected to reach as high as 110 degrees Fahrenheit (43.3 degrees Celsius).
In response, FIFA has adjusted the operating hours of its Fan Festival in Philadelphia. Saturday's fan event will end after the conclusion of the 1 p.m. match.
Kansas City is also under an extreme heat warning through Friday night, with the heat index forecast to range between 105 and 110 degrees Fahrenheit (40.6 to 43.3 degrees Celsius).
The New York City area could experience near-record heat, with temperatures feeling as high as 109 degrees Fahrenheit (42.8 degrees Celsius), while Boston and much of southern New England could see heat index values of up to 111 degrees Fahrenheit (43.9 degrees Celsius).
Weather officials warned that the extreme heat could be life-threatening for people without access to cooling or enough drinking water and could also put pressure on transportation, electricity and water systems.
Players will have several protections during matches, including medical support and mandatory three-minute cooling breaks in each half. However, health experts warn that even elite athletes remain at risk of heat-related illnesses.
Symptoms of heat exhaustion include severe tiredness, headaches, dizziness, nausea, muscle cramps and dehydration. In more serious cases, heat stroke can develop and requires immediate medical treatment.
Experts also say high temperatures can reduce the intensity of matches, as players often run less and conserve energy to cope with the heat.
Fans, volunteers and stadium workers may face even greater risks, especially in outdoor fan zones, parking areas and transport hubs where they could spend long periods in direct sunlight. Health experts also caution that drinking alcohol in extreme heat increases the risk of dehydration.
Several host cities and stadiums have expanded access to shaded areas, cooling stations and free drinking water. Medical teams will also be available at FIFA Fan Festivals and around stadiums to treat heat-related illnesses if needed.
Health experts advise people attending matches or outdoor events to drink plenty of water, stay in shaded or air-conditioned areas whenever possible, wear light, loose-fitting clothing and avoid strenuous activity during the hottest part of the day.
Maggie Aldousany, an associate clinical professor at the University of Miami, said people should begin drinking enough fluids well before attending matches or celebrations rather than waiting until they feel thirsty.
Experts also recommend limiting alcohol consumption and checking for signs of dehydration. Light yellow urine is generally considered a sign of good hydration.
Older adults, people taking certain medications and those not accustomed to hot weather are among those most at risk.
Cornish advised anyone who feels dizzy, overheated or notices they have stopped sweating to move to a cooler place, drink water and seek medical attention immediately if symptoms become severe.