Officials said shelter-in-place order came at about 6 a.m. when the level of vinyl chloride spiked.
U.S. Coast Guard Capt. Kathy Moore said the agencies involved in cleaning up the cars are unsure why the chemical level rose Monday.
She said officials in towns near industrial Paulsboro, just across the Delaware River from Philadelphia International Airport, have been put on notice in case the chemical level rises in their communities, too.
She said that sheltering in place was a better move than evacuating Paulsboro because on a day without much wind, the gas stays close to the ground but does not penetrate sealed homes. "We just don’t want people walking around outside," Moore said.
By 9 a.m., she said the level had dropped. But it was not clear when the sheltering order would be lifted. The order meant that everything in town was closed Monday morning.
Work on the cleanup was also halted because of the chemical levels, which were above 1 part per million in the air.
Seven cars on a freight train derailed early Friday as the train passed over a bridge that buckled in 2009. One car carrying vinyl chloride, a gas used to make PVC plastic, ruptured. Dozens of nearby residents were sickened — though none had life-threatening conditions. More than 100 residents were evacuated.
The presence of the chemical, which in high amounts, is linked to problems from breathing trouble and dizziness to death, has complicated the cleanup and investigation of what went wrong.
National Transportation Safety Board members have been conducting interviews and reviewing records, but staying away from the site.
Late Sunday, crews began removing the vinyl chloride, which had naturally solidified, from the ruptured tanker. Moore said that work stopped at about 2 a.m. Monday because workers were having trouble reaching the remaining chemical at the bottom of the tanker car.
She said it’s not clear whether the chemical removal is connected to the higher levels found in the air hours later.
The NTSB has said that signal problems were reported at the bridge the day before the derailment.
Some politicians have said the accident points to a bigger problem of heavy trains passing over aging infrastructure.
The low bridge that partially collapsed last week was originally built in 1873.