(CNN) -- A bridge failed in Paulsboro, New Jersey, on Friday, sending four train cars -- at least one leaking a toxic chemical -- crashing into a creek near the Delaware River.
Four of the cars involved in the crash were carrying the chemical -- a highly toxic and flammable substance called vinyl chloride -- which leaked from at least one tank car into Mantua Creek, New Jersey emergency and environmental officials said. A total of seven rail cars derailed.
Some 71 patients with respiratory issues and scratchy throats went to Underwood-Memorial Hospital, officials said. Sixty-eight were later discharged; three remained in stable condition, said hospital spokeswoman Molly Tritt.
Nearby schools placed students on lockdown, and authorities ordered evacuations in a very limited area around the accident.
At a news conference, a state environmental official indicated that much of the danger had passed, but he warned residents to stay alert for public safety announcements.
"It appears that all of the vinyl chloride that was in the particular car has dissipated," said Larry Hajna of the New Jersey Department of Environmental Protection. "There's no more release going on."
Booms have been placed in the creek as a precaution, he said, to contain about 180,000 pounds of chemicals that leaked.
The air surrounding the wreck is being monitored and is safe, the official said. Later, department spokesman Larry Ragonese offered CNN a more definitive assessment, saying, "There is no environmental emergency."
Video of the train wreck site from CNN affiliate WPVI showed two tanker cars dangling from the collapsed railroad bridge. Other cars were in the creek, partially sunken amid rising smoke.
Vinyl chloride is used to make plastic and vinyl products, including PVC pipes, according to the Environmental Protection Agency, which has labeled the chemical a Group A human carcinogen. "Short-term exposure to high levels of vinyl chloride in air has resulted in central nervous system effects, such as dizziness, drowsiness, and headaches," the EPA says on its website.
The remaining overturned railroad cars "are going to have to be righted," said Paulsboro spokesman John Burzichelli.
Mindful of the two cars dangling from the wrecked bridge, Burzichelli said that "there's probably going to be one car that's going to be a very delicate move."
"It's a time for caution," he said, warning residents to listen for announcements as the emergency is assessed.
Burzichelli described the bridge as "a very old structure" that underwent "a major rebuild" three years ago. Officials said some posts had failed before the work.
Three "major trains" go across the bridge each day, he said. Federal safety officials will look at maintenance histories for the track and bridge and conduct standard drug and alcohol testing of the crew.
Deborah Hersman, chairwoman of the National Transportation Safety Board, told reporters near the scene that the freight-only bridge is owned and operated by Conrail. It was too early to determine a cause of the incident, she said.
Other agencies will work to remove the wreckage and get the rest of the material out of the breached tank car. "We will be proceeding with caution," Hersman said.
The train consisted of two locomotives, 82 rail cars and a caboose and originated from Camden.
Conrail said it will work closely with local, federal and public health authorities in the wake of the crash. "We very much regret the impacts to the community and deeply appreciate the efforts of the first response teams and officials," it said in a written statement.
Immediately after the wreck, residents told WPVI that they smelled an odor in the area. Authorities warned residents to stay indoors as a precaution and to keep windows closed. The Coast Guard sent crews to the scene to begin cleanup efforts.
Paulsboro is southwest of Camden, about a 20-minute drive from Philadelphia.
CNN's Phil Gast contributed to this report.