Check the "Alpha Disaster Contingency" website. I can provide you with a URL after I get to the office. They discuss the creathion of a safe room with a powered filtration system. I am not convinced of the need for that.

Nerve agent is a mist. If deployed in the open, it will simply evaporate in time. Unfortunately, there is no meaningful alert or warning system that would tell us to take shelter in an emergency. Sheltering in a safe room would not be useful in protecting us against a biological attack. IMHO, if I lived in Israel or another middle eastern country, I would establish a positive pressure safe room. A scud attack would be a reason to shelter.

I can envision no similar situation in the US.

Regards