As the FIFA World Cup begins across the United States, public health officials are monitoring for a range of potential health threats, from infectious diseases and food-borne illnesses to sexually transmitted infections. However, experts say the most immediate danger facing players, teams and fans may be far more familiar: extreme heat.

Health authorities in several host cities have identified high temperatures and poor air quality as the leading public health concerns during the tournament, particularly as large crowds gather outdoors during the summer months.

According to the U.S. Centers for Disease Control and Prevention, extreme heat is the nation’s deadliest weather-related hazard, contributing to approximately 2,000 deaths each year.

In Philadelphia, one of the tournament’s host cities, officials say heat-related illnesses have been a major focus of planning efforts from the outset.

“We’ve really said from the beginning of the planning that our biggest concerns here in Philadelphia from a public health perspective during FIFA events are really our bread-and-butter public health concerns,” said James Garrow, communications director for the Philadelphia Department of Public Health.

“Heat and air quality are the issues we’ve focused on because they affect everyone,” he added.

While some experts and members of the public have raised concerns about the potential spread of rare diseases such as Ebola or hantavirus, Garrow said those risks are considered significantly lower than the dangers posed by extreme weather conditions.

“There are folks who were worried about Ebola or hantavirus,” he said. “Those are obviously concerning, but we thought the chances of them happening here are much lower.”

Public health agencies across the country are continuing to monitor disease outbreaks and other health threats throughout the tournament. At the same time, officials are urging fans to stay hydrated, limit prolonged exposure to the sun and pay attention to heat advisories as temperatures rise.

With millions of spectators expected to attend matches and fan events, health experts say simple precautions could play a crucial role in preventing heat-related illnesses during the month-long competition.