Technically, Saran Wrap & Duct Tape could feasibly limit the spread of NBC (Nuclear, Biological, Chemical) agents if used to seal ventilation entries, door and window joints, etc. Saran Wrap is too porous, however, to contain the more lethal varieties of Chemical weapons - now if you add charcoal filters to the mix you have a better chance of survival. The HS 'FAQ' on the subject is more like the 60's campaign telling people to curl into a ball under a desk/table in the event of a Nuclear strike by the Soviets - only good for psychological reassurance [for those who believe it], not good for actual prevention/survival.
NBC training in the military teaches you that the best bet is to find an unventilated room that can easily be sealed - even then, the chances of surviving an NBC attack without prior preparation/alertness is slim to none (especially considering the difficulty in determining whether an agent has been released in the case of B/C attacks - some can take long periods of time to make themselves known, and have an exposed life measured in months rather than seconds). The current Biological/Chemical compounds that are weaponized are pernicious and the military responds by responding to even "best guess" intel by going to a higher chemical alert level before the detection of actual contamination. (I will tell you, it sucks to be stuck in NBC gear in the middle east @ 120 degrees in the shade)