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Almost all of the danger from fallout is caused by radiation from visible fallout particles of heavy, sand- like or flaky material. The air does not become radioactive due to the radiation continuously given off by fallout particles.
The visible fallout particles rapidly "fall out" of slow moving air. The air that a KAP pumps through a shelter moves at a low speed and could carry into the shelter only a very small fraction of the fallout particles that cause the radiation hazard outside. This fraction, usually not dangerous, can be further reduced if occupants take the simple precautions described in these instructions.
The visible fallout particles rapidly "fall out" of slow moving air. The air that a KAP pumps through a shelter moves at a low speed and could carry into the shelter only a very small fraction of the fallout particles that cause the radiation hazard outside. This fraction, usually not dangerous, can be further reduced if occupants take the simple precautions described in these instructions.
Nuclear War Survival Skills
http://www.oism.org/nwss/s73p937.htm
KAP it's talking about is the Kearny Air Pump, http://en.wikipedia.org/wiki/Kearny_Air_Pump
Also, a bit more about the exact purpose of the underground bunker.
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Fallout protection is almost exclusively concerned with protection from radiation. Radiation from fallout is encountered in the forms of alpha, beta, and gamma radiation, and as ordinary clothing affords protection from alpha and beta radiation,[13] most fallout protection measures deal with reducing exposure to gamma radiation.[14] For the purposes of radiation shielding, many materials have a characteristic halving thickness: the thickness of a layer of a material sufficient to reduce gamma radiation exposure by 50%. Halving thicknesses of common materials include: 1 cm (0.4 inch) of lead, 6 cm (2.4 inches) of concrete, 9 cm (3.6 inches) of packed earth or 150 m (500 ft) of air. When multiple thicknesses are built, the shielding multiplies. A practical fallout shield is ten halving-thicknesses of a given material, such as 90 cm (36 inches) of packed earth, which reduces gamma ray exposure by approximately 1024 times (210).[15][16] A shelter built with these materials for the purposes of fallout protection is known as a fallout shelter.
The danger of radiation from fallout also decreases with time, as radioactivity decays exponentially with time, such that for each factor of seven increase in time, the radiation is reduced by a factor of ten. For example, after 7 hours, the average dose rate is reduced by a factor of ten; after 49 hours, it is reduced by a further factor of ten (to 1/100th); after two weeks the radiation from the fallout will have reduced by a factor of 1000 compared the initial level; and after 14 weeks the average dose rate will have reduced to 1/10,000th of the initial level.[16]
The danger of radiation from fallout also decreases with time, as radioactivity decays exponentially with time, such that for each factor of seven increase in time, the radiation is reduced by a factor of ten. For example, after 7 hours, the average dose rate is reduced by a factor of ten; after 49 hours, it is reduced by a further factor of ten (to 1/100th); after two weeks the radiation from the fallout will have reduced by a factor of 1000 compared the initial level; and after 14 weeks the average dose rate will have reduced to 1/10,000th of the initial level.[16]
