What specific environmental or operational condition within a high-density crowd is most likely to initiate a dangerous 'compression wave' that could lead to crowd crush incidents?

The specific environmental or operational condition most likely to initiate a dangerous 'compression wave' within a high-density crowd is a sudden, localized obstruction or significant reduction in the available space, coupled with continued, forceful pressure from the crowd behind. A high-density crowd is a gathering where people occupy space at approximately four to five people per square meter or more, a threshold where independent movement becomes severely restricted and the crowd behaves more like a fluid or solid mass. When such a dense crowd encounters an unexpected halt or a bottleneck—for example, a narrow doorway, a sudden barrier, or individuals falling down—the forward momentum of the people behind pushes against those who have stopped. This action creates a severe localized pressure point. A 'compression wave' is the physical propagation of this intense pressure through the crowd, like a ripple, causing individuals to be violently squeezed and jostled as the force moves through the human mass. These waves can generate immense, uncontrollable forces, strong enough to lift people off their feet and cause compressive asphyxia, which is the primary cause of death in crowd crush incidents, where people are fatally suffocated by the pressure of other bodies.