A summer World Cup in the United States was always likely to face extreme heat, but temperatures during this week's knockout matches could create hazardous conditions for both players and fans.

A powerful heat dome is expected to blanket much of the central and eastern United States, driving heat index values, which combine air temperature and humidity, into the triple digits in some areas. Several World Cup host cities, including Boston, Philadelphia and Kansas City, Missouri, all home to open-air stadiums, are forecast to experience well above-average temperatures.

"This week is going to be very, very hot," said Geoff Cornish, assistant chief video meteorologist at AccuWeather. "This is going to be a significant heat wave, the likes of which we don't see every single year."

Fears over extreme heat at the tournament, being staged across the United States, Canada and Mexico, have been building for months. Scientists say human-caused climate change, driven largely by the burning of fossil fuels such as oil, gas and coal, is making heat waves more frequent and intense. In the United States, extreme heat claims more lives each year than any other weather-related hazard, including hurricanes, floods and tornadoes, according to the National Oceanic and Atmospheric Administration.

Here's a preview of how hot it will get, how cities, stadiums and FIFA have prepared for the threat, and how to stay cool.

The National Weather Service issued an extreme heat watch for Philadelphia and surrounding areas starting Wednesday afternoon through Saturday evening, with dangerously high heat index values, or "feels-like" temperatures, reaching up to 43.3 degrees Celsius (110 degrees Fahrenheit) during the day. As a result, FIFA's Fan Festival in Philadelphia is shifting its hours this week, including Saturday's watch party, which will end at the conclusion of the 1:00 p.m. match.

"It's going to be sweltering and dangerous for anybody who's spending a long period of time outside unprepared for the heat," Cornish said.

An extreme heat warning is also in effect for Kansas City and other parts of west-central Missouri through Friday night, with heat index values as high as 40.6 to 43.3 degrees Celsius (105 to 110 degrees Fahrenheit) expected.

The New York City area could see near-record heat index values of up to 42.8 degrees Celsius (109 degrees Fahrenheit), with little overnight relief. An extreme heat watch will be in effect from Wednesday afternoon through Saturday evening.

"This level of heat can be deadly for those without adequate cooling and hydration," the National Weather Service said, adding that it could also affect transportation and electrical and water systems.

The heat index in Boston and much of southern New England could climb to 43.9 degrees Celsius (111 degrees Fahrenheit), with the extreme heat watch beginning Wednesday morning.

Players have medical staff, mandatory three-minute cooling breaks midway through each half and other heat safeguards. But even the most highly trained elite athletes are vulnerable to exertional heat illness.

This occurs when the body's temperature rises too high because of intense physical activity in hot conditions. Symptoms include extreme fatigue, impaired performance, headache, irritability, nausea, dizziness, cramping and dehydration. Exertional heat stroke requires immediate medical attention and is the third-leading cause of death among athletes.

Heat also lowers game intensity. Players adapt by reducing how much they sprint, the distance they cover and the risks they take. In a 2023 survey by World Athletics, the global governing body for track and field, 75% of responding athletes said climate change was negatively affecting their health and athletic performance.

For spectators, volunteers and workers, the greatest dangers may be outside the stadiums in fan zones, along transportation routes, in parking lots and during outdoor celebrations, where they are more likely to be exposed to the heat for hours. Many soccer fans also drink alcohol while watching the World Cup. Doing so in extreme heat increases the risk of dehydration.

Some cities and stadiums have increased access to shade, cooling areas and water for spectators and workers and said they will issue heat advisories to the public. Medical personnel also will be stationed at FIFA Fan Festivals and around stadiums during matches to treat heat-related illnesses.

Drinking plenty of fluids, seeking shade or air conditioning and staying out of direct sunlight can help protect against heat-related illness. If you have to be outside, wear light, loose-fitting clothing and limit strenuous activity during the hottest parts of the day. Cooling fans or towels and icy drinks, such as slushies, can also help.

Limit the amount of alcohol you drink and hydrate before temperatures climb, said Maggie Aldousany, associate clinical professor in the Department of Kinesiology and Sport Sciences at the University of Miami.

"It's really important to realize that it's not enough to wait until you get thirsty," she said. "Even if you're going to one of the games in person or celebrating in your hometown with a backyard barbecue, make sure you're thinking about hydration in the days leading up to it and not just waiting until the morning of the event."

Spectators can check whether they are adequately hydrated by looking at the color of their urine. A light yellow color is generally a good sign.

People taking certain medications, older adults and those who are not acclimated to the heat are among those at greatest risk.

"If somebody realizes that they're hot, but they're not sweating, or if they begin to feel a little bit dizzy, those are signs that they really need to take a break, get inside, find some cooling and drink plenty of water," Cornish said. "And if they really begin to experience significant symptoms, they need to seek medical attention right away."