each tanker had at least 200,000 pounds of chemicals
Five tank cars were loaded with Vinyl Chloride Mononer (VCM). Only one was compromised & it had a capacity of 25,740 gallons. Conjecturally, that’s in the realm of 200,000lbs of VCM.
The decision to vent & burn the VCM loads was based on the risk of proximal fires overheating the tanks leading to a possible tank blevey capable of throwing half a railcar a quarter mile or more.
VCM combustion yields about 3% Hydrogen Chloride (HCl), 6% CO2, <1% CO, and 0.004% Phosgene, plus about 6% carbon soot by weight.
Of those hazards, the HCl is worst, but still less dangerous than the potential of having five railcars explode and toss multi-ton hunks of steel in God knows what direction, in conjunction with concussive blasts that blow out windows halfway across town, and incinerate all the VCM in a succession of violently destructive fireballs.
Either way all the VCM burns.
One way is planned, and somewhat controlled.
The other is a crapshoot.
I favor the planned strategy.