Toxic gas plume from Ohio/Pa.

[Crick]

They are doing a controlled burn of vinyl chloride. When vinyl chloride is burned it releases hydrogen chloride and phosgene gas.

Phosgene gas was responsible for the vast majority of the chemical weapons deaths in WWI.

It was also popular with the Nazis for their showers. When polychlorofluoroethanes (freon, et al) are raised to 1500F, they will also form phosgene.