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.