Crowd surging – the deadly phenomenon that claimed more than 150 lives in South Korea – is explained by simple physics, an expert told Insider.

At least 154 people died in Seoul on Saturday when a Halloween street party caused a crowd so dense it crushed people to death.

The crush took place in the busy Itaewon neighbourhood of Seoul, a popular nightlife spot that attracted tens of thousands of people on Saturday, The Guardian reported.

There was no single event planned, per Reuters. But crowds from bustling bars and nightclubs poured into a narrow, sloped alley connecting a subway station to a main street.

Sometime after 10 pm, the street became full beyond capacity. Social-media accounts of the night, compiled by Reuters, said that people near the top of the alley lost their footing and fell into crowds below, starting a fatal crush.

The tragedy prompted national mourning in South Korea and questions of whether more could have been done to prevent it.

Medhi Moussaïd, a research scientist at the Max Plank Institute in Berlin who studies crowd dynamics, spoke to Insider about when crowding turns deadly

"Most people don't realize the danger," he said, arguing that people should be better informed as cities become denser and big crowds more common.

Crowds acting like waves

Crowd surging is driven by a simple principle. If a group of people becomes dense enough – more than six or seven people per square yard – a crowd starts acting like a fluid.

At this point, the people inside largely lose the power to control their own movement.

If someone is shoved, they will push their neighbor, who will fall on their neighbor, and so on and so forth.

"Then this movement is transmitted," Moussaïd said. It is a little like a ripple through water, as these movements spread, they grow bigger.

The pressure from the wave can be too intense to bear for people in the crowd, especially if they are pushed into an obstacle. As seen in Seoul, it can be fatal.

"Those waves are pretty dangerous because people can be compressed against the walls and also against one another. And whenever two waves cross, people can feel the pressure from both sides," said Moussaïd.

What to do if you get caught in a crush

In the overwhelming majority of cases, crowded events will be safe. But Moussaïd listed some things that could help if things ever turn dangerous.

The main thing is awareness: if you feel too crowded, you're probably right. Move away quickly to a less dense spot. This can protect you and also relieve the pressure on others.

"If just a small part of people start doing that, it reduces the density and solves the problem," he said.

Once the crowd reaches that critical threshold, however, the pushing wave can build very quickly. Then it is a case of survival, said Moussaïd.

"If you feel the pushing wave, don't try to resist. Go with it and keep your balance."

Do your best to stay standing. If one person falls over, it will create a wave of people toppling. Those at the bottom of the pile are then likely to be crushed by the weight of the bodies above them.

Hold your arms up against your ribcage like a boxer to make it easier to breathe. The pressure from the wave can cause people to faint and fall.

Don't struggle against the flow of the crowd. If you push back, the pressure in the system will grow, which will make the situation worse for the next couple of seconds to minutes, Moussaïd said.

Information is key

This is not the first time crowd surging has killed. Previous examples include the Love Parade in Germany in 2010 where 18 people died and Travis Scott's Astroworld Festival in Houston last year where eight people died.

With events like these, proper planning can reduce the risk by ensuring that too many people don't gather at once.

But Moussaïd said the event in Seoul was different because it was a spontaneous gathering in the streets. It would have been very difficult to prepare for.

According to Reuters, authorities had expected a crowd of about 100,000 but did not think the area required more planning than a normal Halloween weekend.

"Many people gather for Halloween every year," said an unnamed woman who identified herself to Reuters as living nearby.

"But there were just so many last night, incomparably more than before COVID," she said.

As the world population grows and more and more people are packed into urban areas, this could happen more often, said Moussaïd.

"An easy fix would be letting people know that crowds can be dangerous."

This article was originally published by Business Insider.

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