Days after crews released and burned toxic chemicals transported by a wrecked train in Ohio, residents remain concerned about the toxic substances that could be lingering in their evacuated neighborhoods.

About 50 cars, including 10 carrying hazardous materials, derailed in a fiery crash Friday in East Palestine, according to rail operator Norfolk Southern and the National Transportation Safety Board. Vinyl chloride was slowly released into the air Monday from five of those cars before crews ignited it to get rid of the highly flammable, toxic chemicals in a controlled environment, creating a dark plume of smoke.

Residents in the immediate area there and nearby in Pennsylvania were evacuated beforehand because of health risks from the fumes and can’t yet return Wednesday, as the impact of burning vinyl chloride is a concern.

WHAT IS VINYL CHLORIDE?

The gas is used to make the polyvinyl chloride hard plastic resin in plastic products. It is found in products such as credit cards, furniture and car parts, but is most notably used in PVC plastic piping, a common material for plumbing.

IS IT DANGEROUS?

Vinyl chloride is associated with increased risk of liver cancer and other cancers, according to the federal government’s National Cancer Institute.

The effect was studied in PVC pipe makers, who breathed in vinyl chloride and developed rare liver cancers, said Ruth Lunn, who studies carcinogens at the National Institute of Environmental Health Sciences.

“If you worked longer, you had a higher risk, and if your exposure levels were high, you had a higher risk,” Lunn said.

Vinyl chloride is dozens of times less toxic per molecule than the U.S.-banned insecticide DDT but more dangerous per part than ammonia and natural gas, according to federal regulations that dictate acceptable levels in the air.

WHAT HAPPENS WHEN IT BURNS?

Officials warned the controlled burn would send phosgene and hydrogen chloride into the air. Phosgene is a highly toxic, colorless gas with a strong odor that can cause vomiting and breathing trouble and was used as a weapon in World War I.

Phosgene is considered safe at 0.1 parts per million during an eight-hour exposure, or 0.2 ppm for a 15-minute exposure. The eight-hour exposure threshold would have to be even lower when measuring inside people’s homes, where residents often spend more than eight consecutive hours.

Hydrogen chloride is a colorless to yellowish gas with a strong odor and its primarily effect on humans is skin, eye, nose and throat irritation. It is considered safe at 5 ppm for an eight-hour exposure.

Neil Donahue, a professor chemistry at Carnegie Mellon University in nearby Pittsburgh, said he worries that the burning could have formed dioxins, which are created from burning chlorinated carbon materials.

“Vinyl chloride is bad, dioxins are worse as carcinogens and that comes from burning,” Donahue said.

Dioxins are a group of persistent environmental pollutants that last in the ground and body for years and have been one of the major environmental problems and controversies in the United States.

Dr. Lynn Goldman, dean of George Washington University’s School of Public Health, agrees this is a possible risk, but is more concerned about uncombusted vinyl chloride vapors that could be lurking in the immediate vicinity.

“Until there has been a thorough assessment, the soot as well as any other materials should in my opinion be treated as contaminated by vinyl chloride and/or dioxins or other contaminants until proven otherwise,” she said.

WHAT IS BEING MONITORED?

James Justice of the U.S. Environmental Protection Agency said a network of air station monitors inside and outside the evacuation zone was collecting samples and that none of their readings found anything to be concerned about. “We want to make sure that’s not going to change,” he said.

Justice said the agency is still working with experts to determine safe levels for various gases before reopening the evacuation zone. The incident response team did not specify what substances they are monitoring.

The gases that experts suspect are in the area are heavier than air, which means they could be sitting in low-lying areas if not completely dissipated.

National Guard members wearing protective gear are taking readings inside homes, basements and businesses, Major General John Harris Jr. said.

The EPA also sampled nearby rivers to determine whether there has been any water contamination and is awaiting results.

WHEN WILL THE RISK BE OVER?

Whatever chemicals are in the air, gases largely dissipate fairly rapidly when out in the open, said George Gray, a public health professor at George Washington University. “Sunlight can change that, the movement of air can change that, temperature can change that,” Gray said.

Residents are concerned about long-term effects of low-grade exposure.

“There’s all that smoke and all those chemicals in there,” said Mason Shields, who lives in East Palestine and visited an aid center outside the evacuation zone. “I’m wondering if it’s even going to be safe for people to return within the next week or month or however long.”

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Borenstein reported from Kensington, Maryland. Associated Press videojournalist Patrick Orsagos contributed from East Palestine, Ohio.

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