[Ichneumon]

Any method air can use to enter the room from the outside is a possible source of contamination, including floor vents too, and should be all sealed. That includes doors, windows, etc, and sheeting them with plastic and duct taping any place air can come into the room.

Also be aware that hardwood floors leak air like a sieve. There's a quarter inch or so of air space underneath them, and the cracks between the wood slats allow the floor to "breath". If your best saferoom has a traditional hardwood floor, it's best to just carpet the room wall-to-wall with heavy mil plastic sheeting and seal the edges to the surrounding walls with duct tape.

Electrical outlets, light switches, and ceiling-mounted light fixtures are also often overlooked sources of "air leaks".

[Travis McGee]

Forget sealing all those cracks! It's much easier to jury rig an "Apollo 13" style forced air filter, and over-pressure your house. Bring in good air intentionally, and send it out all the cracks.