that is wonderful. Let's talk about crowd crushes.

Have you ever been standing in a crowd before? Say you were waiting for Weird Al to come

out from backstage after a show. And then suddenly he does, and all of a sudden it seems

like all the people around you multiply exponentially. And as far as you can see, you're surrounded

by people. And all of them start to press forward. And you, against your own will, are

propelled forward. And you're pushed into the person in front of you, and you propel

them forward. And you stop and think, "Uh oh!"

Well if you thought that, you were thinking very clearly. Because you were in a very dangerous

situation: something called a high density crowd. A high density crowd is one where there's

6 or more people per square meter. It can get a lot more packed than that, but that's

the lowest level threshold. The reason that's the threshold is because, when you have 6

people per square meter, individuals in the crowd start to lose their ability to move

on their own accord. Things get more and more packed, and the crowd tends to behave a lot

like a fluid.

So there's a couple of ways that you can actually die in a high density crowd. The first one

is called a crowd crush. When you first start to get to about 6 people per square meter,

the individuals lose their ability to move around. The next step is that you lose your

ability to move your arms from your sides. And as people pack in further and further,

the pressure from all sides keeps your lungs from inflating and deflating, which means

you lose your ability to breathe.

What's amazing (and horrific) is that people suffocate in crowds because they're squeezed

so tightly by the people pressing against them. That's a crowd crush.

Another way you can die in a crowd is what's called progressive crowd collapse. So, say

you have a bunch of people crowded together in a high density crowd. And one of them falls

down. That creates a hole in this crowd, and the people who were formerly leaning against

this person who just fell down start to fall down. And so on and so forth. A domino effect

is created. People start to pile up and the ones on the bottom are literally pressed to

death by the humans who have piled up on top them.

So why don't people just get up and go, get out of the crowd? Well, the short answer is,

they can't.

One mark for humanity is that crowd researchers have shown that when individuals are given

information - say, something like, "Oh, someone ahead is being crushed to death" - they respond

positively by, say, backing up and alleviating the pressure on the crowd in front.

The problem is humans aren't ants. We don't transmit information through crowds like ants

do. And so people end up dying in crowd crushes and progressive crowd collapses because the

people in the back are pushing forward.

Another common misconception are mass panics and stampedes. There's this idea that people

stampede over one another and that's how deaths occur in crowd crushes. This is really off

point. In fact, if you have enough room to rush out over your fellow humans to get from

Point A to Point B, there's probably enough room for those fellow humans to get out of

the way. So, stampedes really don't cost any lives whatsoever. That's not the problem with

crowd crushes.

And the same with mass panics. Very rarely do entire crowds panic and move in a panicked

way. In fact, you can suffocate in a crowd crush in a very calm crowd that's just entered

a bottleneck in a narrow corridor, and trying to get out of an exit. People just quietly

die pinned up against their fellow human beings who are leaving the place.

So, crowd researchers are using computers now to predict high density crowds, and stop

them before they turn fatal. They're using video feeds fed into artificial intelligence,

which analyze crowds for things like increases in body heat, or people swaying from side

to side, which is a telltale sign that they're no longer moving on their own volition.

What do you do if you find yourself in a crowd? Well, get out. That's the best thing you can

do. But, as you're entering a crowd, crowd researchers suggest that you pay attention,

stop talking, and listen ahead for people calling for help, or saying "Move back!" or

any other indication that there's a crowd crush going on. In that case, get back as

far as you can.

If you find yourself in a crowd, and it starts to surge forward, follow the crowd movement,

but move to the side as much as possible. Just stay out of crowds. How about that?

Have you ever been in a crowd crush or a crowded situation where you thought, "Uh oh?" Let

us know in the comments section below. And while you're down there, go ahead and subscribe

to BrainStuff. And head on over to HowStuffWorks.com for even more cool stuff like this.