You'd need a lot of data to really say for certain.

A manifest of what was being transported, at least hour-by-hour accounting of what was reacting/burning, and wind data.

I think it was something like 5 of the cars carrying hazardous material, so like 500K liters potentially? 1km^3 =1000000000000L so if it spread perfectly through 1km^3 that's 0.5ppm which isn't all that bad. I'd be more concerned about the groundwater in the immediate area personally.