Iron poisoning. This can easily lead to death. Be careful.

Iron overdose

Iron overload is a genetic disorder that is inherited, but it can also develop from multiple blood transfusions or chronic iron poisoning.

Dangerous in what sense? If you mean toxic, the answer is generally no, though iron poisoning is possible if you ingest too much. Fine iron powder can be flammable if dispersed in air. A solid iron rod can be an effective blunt weapon.

Yes, it is ok to use iron out in a pool for "cleaning only". If you intend to swim in the pool iron out can cause skin and eye irritations and poisoning if swallowed. The makers of Iron Out said it is not recommended for swimming pools.