Unbelievably, for five dollars, you can buy a microcurie of radioactive Americium. Americium is a man-made chemical. Like uranium, it is fissionable, so enough of it could be used to make an atomic bomb. But the amount of americium in all the smoke detectors in the world is probably much less than that needed for a nuclear device.
Americium is an alpha emitter. Alpha particles are easily stopped by the skin, so americium is fairly safe unless inhaled or ingested. Then it becomes a problem.
Smoke particles also stop alpha particles, hence the reason for using americium in smoke detectors.