the prep depends on the agent...is it gas or liquid? is it persistent? lots of other variables....and if it is a nerve agent you are more than likely going to be dead before you can do anything about it...you would pretty much have to have chemical detection all around,keep your chemical suit,boots,gloves and mask with you at all times and be able to get into your gear before you were overwhelmed...
I don't think you're going to be able to have an effective filtering system to actually get bio/chem/radiation out of the air without spending a LOT of money.