They can be burned safely in an incinerator. Ad hoc burning can produce very toxic chemicals, though, including dioxins, hydrogen cyanide, vinyl chloride and furans. It could release lead. It depends on the type of plastic (PVC? Polyethylene? Polypropylene?). This is why it’s troubling to see people burning bags of trash and tossing in plastic bags and soda bottles as if they magically disappear.

This is a good rundown: https://www.directplastics.co.uk/about_plastics/post/whats-i...

MIT says... https://engineering.mit.edu/engage/ask-an-engineer/can-we-sa...

“When plastic is burned, it releases dangerous chemicals such as hydrochloric acid, sulfur dioxide, dioxins, furans and heavy metals, as well as particulate.”