The radioactivity actual comes from a minute particle of matter (created by a nuclear explosion) that is so tiny that you can't see it, but that tiny particle of matter attaches itself to another piece of dust or sand that is blown away from the site of explosion and sometimes carried great distances by air streams before it "falls out" of the air. It is from this that we get the name 'fallout'.

Energy radiates (it is called radiation) off of the piece of fallout in just the sameway radiation as light radiates off of a lit match head. The only difference about this radiation is that like X-ray, it is not visible to the human eye. (Coincidently, like the radiation on the match head, it will burn itself out but on average it takes it a couple of weeks to do so, or to become so low in energy that it is insignificant).

The amount of radiation, that one receives from a single grain of fallout, is insignificant, and much less energy than the amount from the single light sources in the example that I am about to give, but because there can be millions of pieces of fallout lying about on the ground outside - their effect can be fatal.

Now, for my example. Suppose you, along with tens of thousands of other individuals, attended some event inside a large domed stadium. And suppose EVERY light in the place went out. There were no emergency lights anywhere, no windows, no light working its way in through the doorway ramps. Nothing. It is pitch dark. And then some bright minded soul gets an idea and takes out his Bic and flips it. Suddenly, everyone in the place can see that pin point of light, although nothing else. But everyone else also happens to have a Bic and they all take it out and flick it - and NOW the stadium is so bright that you could read a newspaper in there.

The quantity of fallout, lying about, outside a shelter can be so immense (you could probably even feel the dust by running your finger over the hood of a car) that the energy it is giving out (though not visible) is many times greater than that from EVERYONE having flicked their Bic inside the domed stadium.

However, the real situation with fallout is that it falls out - outside, and with a properly designed shelter there is very little that is in the air that flows through on the inside. It is sort of like only one or a half dozen people in the domed stadium had Bics. That little bit of light from the Bics was not going to help you - and that little bit of radiation from the fallout inside a shelter is not going to harm you.

Now, we didn't always know this. And some knowledgeable people, while absolutely agreeing that one is many, many, times better off inside a shelter, do not completely agree - because they feel that even breathing a particle or two into your lungs is detrimental - still MOST experts agree that it is not a matter to concern yourself about.

Many decades ago, when I first started designing shelters, we all worried about having one micron filters. But now, while I have the most elaborate shelter that I know of, I do not worry about such things. Ninety degree turns in the doorways and air intakes, with the air intakes being four feet above the ground where the fallout lands - and there should not be much come into the shelter. If you are still concerned, then hang a wet bedsheet over the air pathway of the air coming into the shelter. That will trap the particles and you won't be breathing them. There are not likely to be so many in the sheet that you need to worry about it either - but that you can tell with a radiation detector.

As to filtering air for chemicals and biologicals - that is a whole different matter. My contention is that they are not a threat in my particular locale. The defense against plagues and such later will require a much different strategy. While I could go into all these subjects, my experience has been that most people who are interested in them are just talkers. They have never actually built even a fallout shelter. In fact, after decades of experience in talking to tens of thousands of people about shelters I now know that most people just talk, talk, talk and never build anything. I hope that you are not one of those people and that you will actually build a shelter - and not worry about such elaborations as one micron filters. Ninety-nine percent of your protection will come from the most simple first steps.