The main purpose of this article is to help define radiation damage to humans.
Biological damage for various radiation levels was determined about 50 years ago by the US government's Civil Defense Agency, now known as FEMA. I trust human radiation damage exposure levels listed in this older manual than those in the July 1990 FEMA manual which replaced it.
You probably don't want to know how the US government obtained the above exposure damage data. Even at 500 millisieverts it's not known if damage to DNA is taking place. Most experts today state that any elevated radiation level is bad for you and increases the risk of cancer proportionately.