Only weaker still. They just sit there and barely glow for a very long period of time. Little miniature flashlight bulbs or matches are a good analogy to fallout particles. One or two of them in a room with you will not harm you. But surely you can imagine the situation where if you had thousands and thousands the light would either be blinding or the heat so intense that you would be incinerated.
Fallout is just the same way.
A few pieces inside a shelter with you will not harm you, but if you go outside where there are millions of the little beasts lying around then you have had it.