The Physics and Biology behind crowd surge turning deadly

Recently we saw what happened in Seoul. Crowd surging – the deadly phenomenon that claimed more than 150 lives in South Korea. At least 154 people died in Seoul recently when a Halloween street party caused a crowd so dense it crushed people to death. Suddenly 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  said that people near the top of the alley lost their footing and fell into crowds below, starting a fatal crush.

 These things happen frequently in India too. I too faced such a situation once. When I was in school, I, along with my family members - my dad, mom, and sister attended a world Telugu conference. I was very young then. Lakhs of people came to see it like us. Suddenly in the evening everybody came out at once. People pushed us from all sides. We held  one another's  hands but easily lost our grip and I  thought we would either get lost or get killed on that day. But luckily my dad's presence of mind saved us. He pushed all of us to a side where there were less  people. And we escaped a tragedy. 

But why do such things happen? What is the science behind such tragedies?

This can be explained using crowd dynamics. Crowds act 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 neighbour, who will fall on their neighbour, and so on and so forth.

Then this movement is transmitted. 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. It can be fatal. The reality is most people who die in a crowd surge are suffocated.

Crowd surge

What can’t be seen are forces so strong that they can bend steel. That means something as simple as drawing breath becomes impossible. People die standing up and those who fall die because the bodies on top of them exert such pressure that breathing becomes impossible. As people struggle to get up, arms and legs get twisted together. Blood supply starts to be reduced to the brain. 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. Survivors described being gradually compressed, unable to move, their heads ‘locked between arms and shoulders … faces gasping in panic. They are aware that people are dying and they are helpless to save themselves!

Survivors tell stories of gasping for breath, being pushed deeper under what feels like an avalanche of flesh as others, desperate to escape, climb over them. Of being pinned against doors that won’t open and fences that won’t give. During a surge, the pressure from above and below people in the crowd makes it hard to breathe because people’s lungs need space to expand. It takes about six minutes to go into compressive or restrictive asphyxia, the probable cause of death for people killed in a crowd crush.People can also injure their limbs and lose consciousness when they are struggling to breathe and escape a crowd. It takes about 30 seconds of compression to restrict blood flow to the brain and for people in a crowd crush to get lightheaded. 

Those waves are very 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.The whole crowd falls over as one, and if you’re in a confined space, people then can’t get up again.

How can you avoid this type of situation? Follow what my dad did. This is also what experts say: 

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

 So 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.

Once the crowd reaches that critical threshold, however, the pushing wave can build very quickly. A “domino effect" takes hold. Then it is a case of survival. If you feel the pushing wave, don't try to resist. Go with it and keep your balance.At that level of density, it’s no surprise that the first few people begin to faint, because they’re too tight and they can no longer breathe. And if this continues then all the people in that zone will no longer have enough oxygen, even after they faint, and will die one after the other.

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.

And if you can, try to avoid visiting crowded places.