Thanks for the analogy, that really helps to put it in perspective. I was trying to work out the number of molecules per metre that would leave you with, but either my sense of scales is off kilter or I've got it wrong.

From what I can find, there are approximately 2.5e25 molecules per m^3^ at 1atm. Given an 11km cube has a volume of 1.3e12 m^3^, that gives around 2e13 molecules per m^3^ per m^3^ released. That sounds high, have I got the figures wrong somewhere?