A "typical" nuclear fallout of dust would be slightly contained with any "typical" dust mask.
The need of a fully sealing gas mask with a clean shave, no facial hair at all, the ability to change the filters in a "clean" environment, the amount of needed filters, living in a fully sealed house, bunker, vehicle or other living quarters, the direction of the wind, strength of the wind, duration of wind "removing" the fallout to another location, your own movement and in which direction, amount of people needed to be protected, length of time to be protected, de-contamination possibilities and facilities and needed chemicals, disposal of such contaminated clothing, filters and chemicals, type of fallout as in rainy/wet (sticks everywhere and not wind carried), dry/dust (blows away in time if you have wind), air burst/air-ground burst/ground burst/air-water burst or water burst of a nuclear weapon, and the size of the weapon determines the amount/type/yield of fallout...........
There are too many other factors in a fallout situation than just the mask. The effects of the ""relatively small" (no pun intended) fallout of what is still being felt in Europe after Chernobyl reactor failure are still so, that deformed children, men with breasts and even the wild pigs not being edible or only in limited amounts since they dig for their food and do not eat topsoil food like deer, cows or sheep are common. Do some reading on the subject from the FOIA available documents and military lectures or technical manuals to learn more.
The needed monies, facilities and training is too high for us normal folks, even the military with its great potential had and has still its limits...
EMP is a greater problem than fallout even if fallout lasts longer, the results in the grid from EMP lasts a long time too and covers a greater space at a faster rate...live free, Gary