A Halloween festival in Seoul, South Korea, resulted in a deadly crush on October 29. That Saturday night, tens of thousands of people converged on Itaewon—a district of the city that is known for its nightlife—until the crowd was so compact in many areas that people were unable to move. The tight conditions led to a dangerous surge of people through the sloping streets of Itaewon—especially one narrow alleyway. More than 150 people died, and more than 170 were injured.

The accident in Seoul is what’s known as a “crowd crush.” Such events are sometimes called a “stampede,” but that term is misleading: Most deaths in a crowd crush are caused by asphyxiation resulting from the pressure on the lungs rather than trampling. (When people do get trampled, that often happens after they pass out from lack of oxygen.) Because of the crowd’s density and motion, people in a crowd crush generally find themselves unable to control their movements as they are swept along by the many individuals around them.

Scientific American spoke with Ali Asgary, an expert in disaster and emergency management at Ontario’s York University, and Alison Hutton, an expert in the health and safety of mass gatherings at Australia’s University of Newcastle, about the causes of Seoul’s crowd crush, ideal crowd management strategies and what people can do to stay safe during mass gatherings.

What went wrong in Seoul to cause such a deadly crowd crush?

“In a situation like this, it is hard to say exactly what went wrong,” Asgary says. “There are typically investigations planned and conducted after events like this to go and point out what went wrong in detail.”

Still, prior to any full-scale investigation, Asgary notes a few key factors that likely led the Seoul gathering to turn deadly. The crowd’s tremendous size was an obvious issue—an estimated 100,000 people converged on Itaewon—but so was its motion. “In this case, we are talking about a crowd that is not standing or staying in one place,” Asgary says. “This is a dynamic crowd, moving in the space in different directions, especially in the area that was subject to this incident. [People] were moving in opposite directions in a narrow street, which means the likelihood of danger is already high.”

Asgary says the time of day likely contributed to the crush. “This happened at night, when visibility is low,” and people are less alert, he notes. In addition, unconfirmed reports say there was talk of celebrities—or an “attraction”—in the crowd, which drew people in, Asgary says. Rumors about a potential threat can have a similarly powerful effect on crowd behavior, he adds.

Furthermore, Hutton points to the geography of this district of Seoul. “If you look at the terrain, apparently the streets are uneven and sloping, so of course that would have added” to people’s lack of control, she says. “And once you’re in the crowd, you can’t see where you’re going to go.”

Still, all these factors could have been overcome with proper crowd management, according to Asgary and Hutton—but this particular event had minimal oversight, they say. “I think there was no consideration that this was [one of] the first [times] that young people would be out en masse since COVID restrictions were lifted in South Korea,” Hutton says. “So I think it’s probably just the lack of forward planning and a lack of insight into the number of people who would be in that district on that night” that allowed the event to turn deadly.

What can event organizers do to prevent crowd crushes?

“I think signposting is really important—about entries, exits, etcetera—and making sure there’s enough room for egress,” Hutton says.

Proper lighting is another important factor, she notes—and so is communication. “Obviously, once you’re in such a crowded space, it’s really hard for people to communicate,” she says. “You know, you can only really communicate with people who are next to you.... But if we’re going to forward plan, it needs to start at the [beginning of each event]. I think some messaging about how it’s going to be a big night and to be mindful of each other can be helpful.”

Hutton says that designated crowd managers, who can keep tabs on how many people are coming and going and redirect the crowd when needed, can also be essential. Better management would have certainly helped control the crowd in the Seoul alley where many people died, she notes. The fact that “you could get into that alleyway from a number of different ways, that was an issue, because some people were coming in from one direction, and others [were] coming from another, which causes that crush in the middle,” Hutton says. “Making that a one-way-only street would have helped with the flow.... And keeping tabs on how many people were coming and going would have helped determine how many people to expect in that alleyway at that time.”

“We are social creatures,” Asgary adds. “We do want to be together. We actually enjoy the crowd. There’s nothing wrong about that. However, we also should know that there are risks when the crowd forms, and these risks are coming from different angles and sources.” He says it’s the responsibility of everyone—governments, event organizers and the public—to consider those risks and prepare to respond.

Is there anything a person can do if they find themselves in a crowd crush?

“In a moving crowd, the crowd becomes like a fluid,” Asgary says. “It is no longer you as a person who is controlling yourself. You are controlled by the whole—like a drop of water in an ocean.”

Because of that lack of control, Asgary says, a person’s main goal should be avoiding dangerous crowds in the first place. What makes a crowd dangerous? Asgary says that five or six people per square meter is where things start to get problematic—though he admits that this can be difficult to gauge intuitively. He recommends looking behind you to see if you’d be able to exit the crowd should the need arise.

Asgary also urges people to look at a crowd’s motion before entering. “If it’s a moving crowd, make sure it moves—and that it’s not moving and stopping, moving and stopping. That’s a sign of possible problems in the crowd.”

It’s also helpful to be aware of your own risk factors, he says. Children and lower-height people are more likely to be crushed in a crowd. And as most crowd crush deaths are the result of asphyxiation, people with respiratory ailments should be especially cautious.

If you do find yourself in a crowd crush, your options are limited, Asgary says. It is futile—and dangerous—to try to fight the motion of the crowd, he explains. “For example, don’t try to get down to pick up anything. This happens often, especially in moving crowds. As soon as you stop, it not only harms yourself, it is creating this crowd domino effect” of people halting and likely falling over on one another, he says.

Instead it’s best to follow along with the crowd’s motion, Asgary says. If you do want to try to work your way free, he says, you can try moving forward and diagonally toward the crowd’s edges—but he notes this is only likely to work in limited, lower-density situations. Calmly following the crowd, without pushing, is generally your best option.

Additionally, you should try to protect your lungs. “The majority of people who are harmed in a crush cannot get oxygen to their body because of the pressure to their organs from the crowd. So many people suggest you need to protect your chest, to make sure that your chest is not under pressure,” Asgary says. “Some say to try to bend your arms together in a way that protects your chest” while attempting to avoid pushing on your own chest, he explains.

Hutton has a more pessimistic stance on people’s ability to protect themselves. “You can put your arms up, but then, of course, you’re going to crush your arms—and crush your lungs with your arms,” she says. “I think once you’re in there, you’re in there, and there’s not much you can do.” For that reason, Hutton emphasizes the need for effective crowd management that can prevent crowd crushes before they start.

As for the disaster in Seoul, “I think it’s a horrible, tragic situation, and my heart goes out to the families losing their loved ones,” Hutton says.