Why is radon in houses still dangerous?

1. I've been wondering about this a lot. It's about the health risks of radon in houses, since radon has a half-life of 3.8 days, and most radon houses(at least where I live(sweden)) were built it the sixties, how come it's still such a big problem? Shouldn't the radon have decayed by now? I know the "parent" element to radon is radium and that it's half-life is 1620 years so is that the problem? The amount of radium?





Thanks in advance