Large crowds surge toward exits, onto playing fields, or press up against a stage with such force that people are literally squeezed to death. It has happened at a music festival in Houston, a football stadium in England, a hajj pilgrimage in Saudi Arabia, a nightclub in Chicago, and countless other gatherings.

More than 140 people were killed and 150 more were injured when a mob pushed forward during Halloween celebrations in Seoul, the capital of South Korea, acting like a vice on the tiny street they were on.

Also read: 5 worst crowd rush incidents in history

Due to the COVID-19 epidemic, the probability of such tragic incidents decreased while venues were closed and people stayed at home, but it has since increased.

To be sure, most events where large crowds gather happen without injury or death, with fans coming and going without incident. But those that went horribly wrong shared some common traits. Here is a look at why that happens:

The majority of victims who die in a crowd surge are suffocated, contrary to what is implied by movies that depict throngs frantically trying to flee.

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Forces that are so powerful they can bend steel are not visible. Thus, even something as basic as breathing becomes impossible. People die while still standing up, and those who fall also pass away as a result of the pressure from the bodies above them making breathing impossible.

“As people struggle to get up, arms and legs get twisted together. Blood supply starts to be reduced to the brain,” G. Keith Still, a visiting professor of crowd science at the University of Suffolk in England, told NPR after the Astroworld crowd surge in Houston last November. “It takes 30 seconds before you lose consciousness, and around about six minutes, you’re into compressive or restrictive asphyxia. That’s a generally the attributed cause of death — not crushing, but suffocation.”

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Survivors describe being forced further under what feels like an avalanche of flesh as others, in a desperate attempt to escape, scramble over them while they struggle for breath. of being squeezed between closed doors and impermeable fences.

“Survivors described being gradually compressed, unable to move, their heads ‘locked between arms and shoulders … faces gasping in panic,’” according to a report after a human crush in 1989 at the Hillsborough football stadium in Sheffield, England, led to the death of nearly 100 Liverpool fans. “They were aware that people were dying and they were helpless to save themselves.”

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2003 saw a crowd surge into a Chicago nightclub after security personnel used pepper spray to break up a dispute. The ensuing mass surge claimed the lives of twenty-one persons. And last month, 131 people died in Indonesia when tear gas was thrown into a stadium that was only partially locked, causing a stampede at the exits.

In 1988, a heavy downpour in Nepal caused soccer fans to race for sealed stadium exits, which resulted in 93 deaths. Some news sources claimed that in the most recent occurrence in South Korea, the crush happened as a result of many people rushing to a bar upon learning that an unnamed celebrity was present.

Also read: 5 worst football field stampedes in history

But Still, the British professor who has testified as an expert witness in court cases involving crowds, pointed to a variation of the age-old example of someone shouting “Fire” in a crowded movie theater. He told the AP last year that what lights the fuse of such a rush for safety in the U.S., more than in any other country, is the sound of someone shouting: “He has a gun!”

Stadiums are getting crowded again. As the pandemic spread and the games continued, teams came up with some imaginative ways to make things appear to be relatively normal. A sports version of the laugh track from a comedy performance was played while cardboard fans were installed in some of the seats and crowd noise was streamed in.

But now the people are back and the threats of crowd surges and stampedes follow.

“As soon as you add people into the mix, there will always be a risk,” Steve Allen of Crowd Safety, a U.K.-based consultancy engaged in major events around the world, told the AP in 2021.