[Ichneumon]

Well, the FIRST thing you should do is to go get a hepa filter, some ducting, a ducting fan, and some wide (like dryer hoses) hose. You really need to get a good filtration air intake system put into a window and sealed BEFORE making your safe room.

A HEPA filter won't do jack against chemical weapons. In fact, a setup like you describe will bring airborne chemical weapons into the house faster than a totally unsealed house with the doors and windows closed and the the A/C off.

[Centurion2000]

A HEPA filter won't do jack against chemical weapons.

Against certain chemicals, no it won't, but then activated charcoal prefilters will get that. Against biologicals (more likely) it most certainly will. Also it will allow you to get fresh oxygen with reduced risk.