> The amount that came from ten cars, spread into the atmosphere, is virtually nothing.

Erm, on what basis do you make that claim?

Ten train cars of dioxin spread over a square mile is a lot. Spread over 100 square miles? Probably still a lot, like the place in missouri that dumped it on all their roads to keep the dust down. That's probably that same order of "10 cars over 10x10 square miles", of a sufficiently nasty thing.

10 cars = safe is a number that needs context and validation and it will depend on the particular chemical.

Where are you getting dioxin from? There was no dioxin involved in this incident.

Dioxin is an example of why saying "10 train cars is fine" is an oversimplification, 10 train cars of dioxin is a problem even if it's spread out over a couple square miles.

Maybe 10 train cars of formaldehyde is fine, maybe not, but, this is something that should be assessed and not dismissed out of hand, even during discourse. Random internet commentators probably don't really know if it's fine. And despite the people on-site saying measurements are fine... animals are getting sick, which is the classic warning sign with chemical spills that things are not fine.

Although there is indeed dioxin formed during the combustion apparently. Bear in mind too that this isn't a controlled incinerator, there could be various things produced at different temps etc.

It wasn’t formaldehyde either.

Vinyl chloride will form some dioxin upon combustion.

When left to break down in the environment, after several days vinyl chloride will break down into a combination of formaldehyde and hydrochloric acid. The latter of which won't be harmful, but the former will be.

Vapor phase formaldehyde breaks down in the air within hours.[1]

It will also be destroyed in the soil by any number of mechanisms, with a half-life of about 14 days. [2]

[1] https://www.atsdr.cdc.gov/toxfaqs/tfacts111.pdf

[2] https://webwiser.nlm.nih.gov/substance?substanceId=1&identif...

I do not think combustion of vinyl chloride at atmospheric pressure is going to create much if any dioxin

Ten train cars worth of formaldehyde averaged over the entire atmosphere would be nothing to worry about, I agree.

Let's say that two train cars worth sank into the ground before breaking down. Now there's two train cars worth of formaldehyde in the groundwater in a localized area.

It's not obvious to me that this is a safe situation. There are plenty of plausible scenarios that could be reality, that would be a very unsafe situation for local residents.

Thus is should be treated as though it is that worst-case bad scenario unless shown otherwise, rather than wand-waving "oh it's probably fine". These are people's lives that are at stake.