[nickjj]

I know nothing about this field but I remember when California had really bad wildfires, when it was happening (in 2020 or 2021? It all blends together) there were charts showing air quality levels across the US. I'm in NY and I visibly saw different colored clouds when the "smoke clouds" made their way over here. I have no idea if the same thing applies to this train's chemicals but that is a case where there was a coast to coast impact on something that travels in the air.

[bombcar]

It's a cube law thing - you can work out how much "air" there is for everything to spread into, and how much "stuff" there is but basically, the further away you get the rapidly it goes down in hazard.

The western wildfires were absolutely mind-glowingly immense in ways we don't really deal with normally.

Five tank cars is 30,000 gallons * 5 = 150,000 gallons, roughly 600 cubic meters.

Safe exposure is 0.1 part per million, so 600 cubic meters * 10 million circle (assume the stuff never gets more than a meter off the ground, spherical cows and all that) = 45 kilometer radius circle. Obviously it's much more concentrated near the epicenter but let's assume it dispersed equally.

So 200 kilometers should be well outside the danger zone UNLESS a cloud of it doesn't disperse and instead heads in your direction.

[YPCrumble]

This is a great answer, thank you. However this assumed the atmosphere is 45 km high and it seems very unlikely that these gases are light enough to go that high. If you assume the gases only go 1km in the air wouldn’t that assume a radius roughly 40x your initial estimate, or at least 1800 km to be within the 1 ppm threshold.

[bombcar]

That's why I did a little bit of a sleight of hand - worked out how many cubic meters it would need to be to disperse, and then converted that to square meters (so assumed a depth of one meter).

If I had done a sphere (half a sphere to be precise) you'd get a half-dome only 1.5 km across.

[nickjj]

I wish more concrete data were shared. Having half the US affected is a big deal.