I think that it's also important to realize that there are different levels of the poop hitting the air circulation device. You should be planning for everything, and have levels to your system.
This is just a off the top of my head example.
The SHTF for one week or less. Things like the Rodney King riots, mass panic, power outage, etc...
The SHTF, and stuff wont be back to normal for a few months, with a period of rebuilding after. Natural disasters, shipping crisis, etc... Don't expect any help from the government until the hard part is over.
All the way up to TEOTWAWKI. Things are not going back to normal. Not for years, or maybe generations.
I'm just saying, it's a very wide envelope. You need to consider it all.