When a small problem turns into a fatal catastrophe

When too many people are squeezed into too small a space the situation can turn dangerous very quickly, experts say. That, they say, may be key to understanding the deaths of more than 700 pilgrims on the outskirts of Mecca.

‘‘It’s largely a physical phenomenon, not a psychological one,’’ says Dirk Helbing, a professor of computational social science at ETH Zurich, who has studied crowds and disasters.

When the density is too high, movements of a body ‘‘transfer forces to other bodies. These forces can add up and create uncontrollable movements in the crowd’’, he said.

‘‘As a result . . . people might fall on the ground and be trampled by others’’ or die of suffocation as others fell on top of them, he said.

And it can happen fast. Even a small incident like two people starting a fight or trying to walk against the crowd can quickly snarl a free-flowing crowd in large-scale congestion, he said. As more and more people pour in, the density builds up, setting the stage for lethal turbulence.

So ‘‘a small problem turns into a big problem that is not controllable anymore’’, Helbing said. A large crowd can ‘‘get out of control very quickly’’.

Even for those who stay on their feet, the pressure of the surrounding bodies builds up ‘‘and people can’t breathe’’, said Keith Still, a professor of crowd science at Manchester Metropolitan University in England. ‘‘People don’t die because they panic. They panic because they are dying.’’

Still, who has worked on haj crowd management with security officials in the past but had no direct knowledge of this year’s situation, said Thursday’s disaster appeared to result from too many people jammed into a space too small to hold them.

‘‘Every system has a finite limit, the number of people who can go through it,’’ Still said. ‘‘When you get above that number the risks increase exponentially.’’ At the haj, he said, ‘‘it just looks like the system has gone beyond its safe capacity’’.

The Saudi Interior Ministry has said the crush appeared to result from two waves of immigrants meeting at an intersection. King Salman has pledged a speedy investigation to improve crowd management. One effective strategy for crowd safety, Still said, was a hold-and-release approach. People were stopped temporarily from following a route and then let go in pulses. ‘‘That creates space,’’ Still said.

Helbing said the haj was ‘‘one of the most difficult mass events to organise’’, in part because of pilgrims who weren’t registered for the event and so didn’t adhere to assigned camps or official schedules, as well as the many origins and languages of participants.

In any case, he said, even a good track record was no guarantee of future safety.

‘‘When such an event has been safe for a number of years, that’s not a reason to relax and take things easy,’’ he said. ‘‘There is always a kind of a critical threshold.

AP