The order was lifted around 11 a.m., but vinyl chloride remains a threat, as it has since the derailment.
Officials were surprised by the elevated gas readings because hours earlier, they had achieved their first goal in the cleanup: to remove most of the vinyl chloride in the tank car that was the source of the leak and to close a breach in its skin of metal and insulation.
Though the levels always were within acceptable limits for longtime exposure, officials said they were exercising extra caution.
Lisa Faulk, who lives about a half-mile outside the 12-block-square evacuation zone, said she wanted to be relocated as well.
Her 15-year-old son, David Beckles, has asthma and allergies, and his eyes have been watering since Friday, she said. Faulk said she had experienced chest pains and headaches.
"So that means the air just stops there?" Faulk said of the perimeter of the no-go area.
Her fears escalated with the shelter-in-place order.
"That means [the situation] is worse," Faulk said. "They make it seem like we're in jail, but they don't want to move us anywhere."
Koreen Warrington, who was evacuated with her three children to a Woodbury hotel, also was upset.
She had packed for three days, as instructed when she left home Friday night, she said. Now the evacuees have been told they will not be allowed to return until Saturday.
"It's stressing," Warrington said. "All my clothes are still there. I have my kids."
While Conrail was paying for the hotel, she said it was not helping with food and gas.
"We're very, very frustrated," she said.
Conrail, which owns and maintains the bridge where the freight train derailed, did not respond to a request for comment. The corporation has provided little detailed information except to say it was offering assistance to affected residents.
A limited understanding of the danger and effects of vinyl chloride gas has added to the problem in Paulsboro, a refinery town where the air is often heavy with smell of oil.
The federal Occupational Safety and Health Administration has set an exposure limit of one part per million of vinyl chloride in industrial settings - the maximum workers may be exposed to without adverse effects, averaged over an eight-hour workday or 40-hour week. The limit for short-term exposure - 15 minutes - is five parts per million.
Coast Guard Capt. Kathy Moore, whose agency heads the unified response command, said the shelter-in-place order was prompted by readings of one part per million.
High-level exposure to vinyl chloride can cause headaches, dizziness, drowsiness, and loss of consciousness. Extremely high levels of exposure can be fatal.
With most of the vinyl chloride removed from the breached tank car, the next major task will be removing the car and two others from the bridge and creek. The two additional cars, which are intact, also are loaded with the chemical.
Officials have said little about their plan, only that they are assessing the situation. It is not clear if they will use a giant barge crane that they had ordered sent from New York Harbor.
Charles Haas, the Betz professor of environmental engineering at Drexel University, said vinyl chloride is shipped under pressure as a liquid. When the pressure is lessened, the substance becomes a gas and rapidly vaporizes.
As long as the seals remain intact on the other tank cars, Haas said, he sees no immediate threats. But, he said, "if it gets much warmer than this, then you might want to worry about potential overpressurization."
He said each rail car likely has a valve enabling a gradual release of gas to avoid "a catastrophic rupture."
Vinyl chloride is used to make PVC, which is used to make products including pipes, automotive parts, packaging products, construction materials, and furniture.
Contact Joseph Gambardello at 856-779-3844 or firstname.lastname@example.org.
Inquirer staff writer Sandy Bauers contributed to this article.