"The initial release of vapor has dissipated and that's completely gone," said Larry Ragonese, press director of New Jersey Department of Environmental Protection.
The train carrying the chemical was on a bridge over Mantua Creek when some of its tanker cars derailed, causing one to leak. About 12,500 gallons of vinyl chloride was released into the air. An equal amount remains "self-refrigerated" in the car, which did not enter the water, officials said.
At room temperature, vinyl chloride is a flammable, colorless gas with a sweet odor. The major concern is it getting into the air, said Charles Haaz, a professor of environmental engineering at Drexel University.
"It really depends on the level of exposure," Haaz said. "The acute effects for most people who are exposed is respiratory and eye irritation.
"If people have pre-existing health conditions it makes them more vulnerable, but most people are expected to recover," he said.
Short-term exposure may cause eye and throat irritation, headache, shortness of breath, dizziness and sleepiness. Breathing high levels of the chemical can lead to loss of consciousness and death.
Another concern is the presence of vinyl chloride in the water supply. As a precaution, authorities were testing the water today to see if the chemical exceeded the allowable amount of .2 micrograms per liter, said Maya Van Rossum, the Delaware Riverkeeper.
The closest drinking water intake for Philadelphia and New Jersey, the Baxter intake, is 20 miles upstream from the derailment.
"That is a fair distance," Van Rossum said. "It seems like most of vinyl chloride goes up in the air."
She said the chemical doesn't bio-accumulate in wildlife.
Laura Copeland, a spokeswoman for the Philadelphia Water Department, said the agency's water quality models show "that we should not be impacted, but we're continuing to monitor it."
Contact Kathy Boccella at 856-779-3812, firstname.lastname@example.org or follow @kmboccella on Twitter.