As mentioned, sealing off windows, doors, fireplaces, AC vents, toilets, faucets, and drains is a really simple meathod to keep any chemical or biological agents from entering a home or room. By the same token, this is how the government recommended surviving fallout.
In any event - Im not in any rush to go buy plastic and duct tape, but the advice, at least in terms of effectiveness, isnt roadkill.