What to do in a crowd crush.

The crowd surge in Seoul that left at least 151 people dead and dozens injured is among the deadliest episodes of its kind in recent years.

Anytime large groups of people gather in close quarters, there is a risk of what experts call a “crowd crush,” which is when a group is packed so close together that no one can move. The most common injury and death during a crowd crush is what is known as compressive asphyxia, which occurs when people are pushed against one another so tightly that their airways become constricted.

Though officials have not yet said what may have caused the surge or how it happened so quickly, Norman Badler, an emeritus professor at the University of Pennsylvania, said that many of the victims may have been trampled and suffocated under the weight of others.

Professor Badler, who has created computer graphics simulations of high density crowd surges, said that the ability to escape or move becomes more difficult when people have been trampled because it can limit movement and block escape routes.

He also said that victims can suffocate even while standing because of the excessive crowd density surrounding them.

The Centers for Disease Control and Prevention advises those stuck in a crowd crush to keep their hands in front of their chest “like a boxer” and to maintain firm footing. The agency also said not to resist the force of the crowd, but instead try to move diagonally to the edge of the crowd when there is a lull in movement. If you fall down, the C.D.C. advises to curl into a ball to try to protect yourself.