It is absolutely nonsense. People are subjected to stronger, more direct magnetic fields all the time in MRI’s, and MRI’s are substantially safer than most other imaging modalities in medicine (besides ultrasound). The amount of radiation from non-atmospheric sources vastly outweighs the cosmic (non-UV) radiation humans are subjected to, to the point that it’s not really even worth considering outside of maybe astronauts or people who take long-haul high altitude flights extremely frequently.

The amount of ferrous material in blood is negligible at best, and there’s an estimated 3 to 4 grams of iron in the entire human body. The pressure from your heart pumping and the relatively high percentage of blood’s mass that is not iron (about 5kg) means that the effect of the iron if it was responsive to magnetic fields is slim to none.