EAST PALESTINE, OHIO • 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.