There’s another scary Goldilocks aspect too, which is what I thought your comment was going to be about when I started reading it. Stuff with a really short ha

There’s another scary Goldilocks aspect too, which is what I thought your comment was going to be about when I started reading it. Stuff with a really short half life is horribly radioactive, but not for long. Stuff with a half life of millions of years sticks around forever, but it’s not throwing off that much radiation. But stuff in the middle (a half life of perhaps decades to a thousand years) can be very dangerous and remain that way for a long time.