I've done physics, I completely understand what nuclear radiation is and why it is harmful. What I can't seem to figure out are the rules surrounding radioactive emitters/items on flights. Surely it's not just any radioactive items, because everything is at least slightly radioactive.
You get about 12 times the sieverts per hour at 35,000 feet than you generally do at sea level (3 μSv/hr in the air, 0.25 μSv/hr at sea level). And even then it's essentially harmless.
What if I had a strong radiation source, but encased in thick lead shielding? Or a strong emitter of only alpha particles? Do they measure it in Sieverts per hour, or Becquerels? Or Gray? All of it makes a difference.
Something that measures high in Gray or Becquerels could be damn near close to undetectable in Sieverts per hour - arguably the only relevant unit in terms of radiation effects on human health.
There's not enough info available to nerds like me, so insight would be greatly appreciated.