"We don't really know" is simply false. This isn't a mystery chemical; we've been working with the stuff since 1835, we've spilled it into the environment numerous times, and we are decently familiar with it enough that even given experts with vested interests will downplay the risks, I have no reason to believe panic is in order. It's not safe to swim in, I wouldn't spread it on toast, but it's a well-known HAZMAT story with a well-understood mitigation.
While NPR reported there were concerns from citizens, they also went looking for people who would provide causes for concern in their reporting and found none: not only from the rail company (where you wouldn't expect to find them), but from anywhere. The largest concerns expressed were from an OSU professor that said to wash surfaces and look out for low-lying areas. Plenty of chemical experts would have plenty to gain contradicting the party line if the party line were untrue; the fact we don't see them really should indicate in this case that there's (no pun intended) no fire here.
What is your goal in this exchange?
> It has never been burned at anywhere near this quantity
I'm curious your source for that assertion.
https://www.csb.gov/formosa-plastics-vinyl-chloride-explosio...
This is where you're wrong. Experts don't make mistakes in situations like this. They consistently [0] downplay [1] every [2] environmental [3] disaster [4] and withhold information from neutral parties until it loses the public's attention. When are we going to stop acting like this isn't standard operating procedure? When are we going to assume that this is default behavior of people responsible for disasters?
[0] - https://www.cbsnews.com/news/a-toxic-cover-up/
[1] - https://www.history.com/news/chernobyl-disaster-coverup
[2] - https://www.reuters.com/article/us-oil-rig-leak-idUSTRE6430A...
[3] - https://www.nbcnews.com/storyline/flint-water-crisis/six-mor...
[4] - https://www.cnbc.com/2022/01/05/california-finds-pge-equipme...