The New Yorker had a fascinating article by John Seabrook about crowd control, crowd deaths, and crowd psychology. What I found most chilling was the description of what actually physically happens when you are in a dense crowd.
In fact, a crowd is most dangerous when density is greatest. The transition from fraternal smooshing to suffocating pressure—a “crowd crush”—often occurs almost imperceptibly; one doesn’t realize what’s happening until it’s too late to escape. Something interrupts the flow of pedestrians[….] At a certain point, you feel pressure on all sides of your body, and realize that you can’t raise your arms. You are pulled off your feet, and welded into a block of people. The crowd force squeezes the air out of your lungs, and you struggle to take another breath.
John Fruin, a retired research engineer with the Port Authority of New York and New Jersey, is one of the founders of crowd studies in the U.S. In a 1993 paper, “The Causes and Prevention of Crowd Disasters,” he wrote, “At occupancies of about 7 persons per square meter the crowd becomes almost a fluid mass. Shock waves can be propagated through the mass sufficient to lift people off of their feet and propel them distances of 3 m (10 ft) or more. People may be literally lifted out of their shoes, and have clothing torn off. Intense crowd pressures, exacerbated by anxiety, make it difficult to breathe.” Some people die standing up; others die in the pileup that follows a “crowd collapse,” when someone goes down, and more people fall over him. “Compressional asphyxia” is usually given as the cause of death in these circumstances.
This happens more frequently than you might expect: rock concerts, sporting events, mass religious gatherings, "doorbuster" sales. As is mentioned later in the article, the saddest thing about crowd deaths is that the people who are right around you can't do anything. They're being pushed themselves by people ten feet away, who have no idea that you are being suffocated. Humans are not a social enough (or, traditionally, numerous enough) species to have evolved a way to communicate across long distances in a crowd. (Ants, for instance, can communicate within a swarm using pheromones.) With humans, you're limited to your lungs and hands…which are usually already pinned by the time you realize you're in trouble.
Some tips for how to be safe in a pressing crowd, wherever it might be :
- Know where exits are in relation to a position in a crowd.
- Move away from a dense crowd when you have limited mobility.
- Try to keep your composure, Remain Alert!
- Do NOT panic.
- In a crowd crush surge, try to escape by moving away from the direction of the crushing.
- Do not stop to pick up personal items in dense and moving crowds.
- Use nonverbal communication to gain attention when noise levels are too high.
- If you fall and cannot get up, […] cover your head and curl up, bringing arms and legs close to your body. (copyright 2001, Crowd Management Strategies, from You and the Festival Crowd booklet)
Sense of Science 
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