Hundreds of passengers fleeing from a Shanghai train when a Westerner fainted on-board is because of panic rather than heartlessness.

CCTV shows the man slumping in his seat and leaning heavily on the shoulder of a stranger, who pulls away quickly.

Her reflex response triggered a chain reaction from the rest of the passengers in the carriage, who all clear out onto the platform.

Two people were hurt in the mad rush to exit the train, leaving the unconscious man lying alone on the floor of the carriage.

But while the passengers have been condemned by shocked viewers across the globe, renowned psychologist Bob Montgomery said the reaction was normal.

"People pick up the cues from those around them that something is unusual and potentially threatening, and they panic," Dr Montgomery told ninemsn.

"It's actual a well-researched phenomenon — people don't know what to do, and they look at what the others around them are doing."

The short video filmed on August 9 shows just how quickly mass hysteria can take off, with hundreds of passengers fleeing the train in seconds.

"Something happened," one passenger can be heard shouting to others.

As the Westerner slides onto the floor, the rest of the commuters can be seen rushing onto the platform, with two people hurt in the panic.

The fainting man soon regained consciousness and got back to his feet by himself, climbing out of the now-empty train at the next station.

Bloggers have pinned the passengers' rash reactions on terrorism taking place in public places in China.

"People have been strained and [are a bundle of] nerves nowadays as terrorist attacks have been widespread all over the world," blogger Li Zhiren wrote.

"In the closed space of a subway train, a stranger's abrupt collapse … can trigger every frightened person to flee from the scene."

A mass knife attack from Xinjiang separatists at a Kunming train station in March was cited by the South China Morning Post as one such terrorist incident.

Dr Montgomery said that if one person had reacted properly to the fainting man, the mass hysteria would have probably not taken place.

"You'll find it's usually someone with relevant training that goes against the crowd," he said.

"You have a real responsibility as a citizen if someone else is in trouble, act. "It's better the firies or the police get three calls instead of no calls."

Dr Montgomery said the most common reaction to an emergency is not to react at all.

"If everyone's standing on a wharf and a fisherman is washed into the ocean, most people would watch him drown," he said.

"It's called bystander non-intervention."

NSW Ambulance advises you should call triple-0 if they witness a person fainting. In addition to sending paramedics, triple-0 operators are able to advise on the best course of action for specific fainting instances.