A stampede implies that people had space to run, which was not the case in Itaewon, he said. The more people that are in the crowd, the greater the force of the crowd crush is.
“The whole crowd falls over as one, and if you’re in a confined space, people then can’t get up again,” Still said.
In a Twitter thread on Saturday, one person who said they were in the crowd described people “falling like dominoes and screaming.”
“I really felt like I would be crushed to death,” they said in another tweet. “And I breathed through a hole and cried and thought I am dying.” The person continued, writing that they were near the top of the crowd, crying, “Please save me!” and people nearby pulled them up.
During a surge, the pressure from above and below people in the crowd makes it hard to breathe because their 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, Still said.
People can also injure their limbs and lose consciousness when they are struggling to breathe and escape the 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.
Crowd surges can be triggered by many tight situations, for example when people push others or if someone trips, Still said. But the events are not usually caused by people in distress or pushing to get out of a crowd. Those reactions typically come after the crowd starts collapsing, Still said.
“People don’t die because they panicked,” he said. “They panic because they’re dying. So what happens is, as bodies fall over, as people fall on top of each other, people struggle to get up and you end up with arms and legs getting twisted together.”
At Astroworld, most of the fans who died were near one another in the venue’s south quadrant. The venue had metal barriers surrounding it, which would have compressed people if a crowd had surged near them, allowing no way to regulate the flow of people.
Although the crush in Itaewon happened on a street, the crowd was so densely packed that movement was extremely restricted and there was no way for people to exit vertically, said Norman Badler, a professor at the University of Pennsylvania who has researched crowd compression.
Over the past year, crowds have been gathering more frequently since pandemic restrictions have been widely relaxed, another factor in recent crowd surges. More people are probably attending events such as the Halloween celebrations in Itaewon, Still said, because they’ve been restricted for so long.
He added that the increase in mass gatherings that are now allowed underscores the need for crowd-management training, which tapered off when the pandemic hit because large events were uncommon.
Martyn Amos, a professor at Northumbria University in England who studies crowds, said those large events need proper planning and people who are trained to manage crowds.
“The general point is that these incidents will keep happening so long as we don’t put in place proper crowd-management processes that anticipate, detect and prevent dangerously high crowd densities,” Amos said in a statement to The Washington Post.