The Rainbow Bridge, which connects the United States and Canada at Niagara Falls, was shut down for five hours Sunday evening amid fears that a hazardous material had been released.

Fire and emergency workers, customs officials from both sides of the border and FBI agents all flocked to the bridge, which closed down at about 5:45 p.m. local time.

Several U.S. customs officers on the bridge developed nose and throat irritations as they examined cash that was hidden in a vehicle, said FBI spokesman Paul Moskal.

Four people had to go through decontamination, showering down and putting on special suits, before they were treated at a medical centre in Niagara Falls, N.Y.

Dozens of other people were examined at the scene.

Barry Morrissey, spokesman for U.S. Customs and Border Protection, said none of the officers who were affected needed to be kept in hospital.

The FBI said the event didn't appear to be connected to terrorism.