1) In most cases, duct tape and plastic sheeting alone will not create a sealed, airtight environment out of the room that you’ve chosen as your sanctuary. If a biological or chemical agent is released in your city or town via an airborne delivery system, it’s doubtful that anything less than a sealed environment will keep you safe from harm.
2) If you do manage to seal yourself in, you will find yourself in an airtight environment. This is problematic because, well, we breathe air. Your friendly government has recommended ensuring a ten foot square area for every person in the "safe room." A 10x10x10, airtight room will provide about enough breathable air for one person to survive without asphyxiation for 5 hours. Yes, that’s right, I said five hours. Not to be too pessimistic, but it’s likely that haz-mat teams will require a little more than five hours to eliminate the harmful agent(s) from the environment.
3) Which, of course, raises the next question: why are we being asked to stockpile several days to several weeks’ worth of canned food and bottled water and up to thirty days worth of any required perscription medication? There’s a simple, if grim, answer. In the event of the use of a chemical or biological agent against the civilian population of a given area, the authorities will ask the population to repose to their "safe rooms" and await further instructions. This will serve to isolate those who have been exposed to the agent and will give them the illusion of safety and self-sufficiency. The contaminated population is thus centralized and controlled to a degree that authorities can concentrate on containment and (later) cleanup efforts. The truly macabre piece of all of this is that you’re unwittingly assisting in the cleanup efforts by gathering all of the bodies in your household into a single room and providing the authorities with the supplies (plastic sheeting, duct tape) necessary to create makeshift body-bags.