Radiation is a damaging and often lethal status effect emitted from multiple radioactive sources. Unlike other status effects, radiation CANNOT be cured by drinking milk. The best way to cure yourself is to carry Antidotes, as they will cure all ills including radiation poisoning. You can also wear a full Hazmat Suit to completely protect yourself, though it will not cure you if you were already exposed. All levels of radiation last for a full minute, not counting continued exposure.
Radiation I is caused by proximity to Uranium Ore or exploding pieces of Antimatter (not referring to the explosives), and is completely survivable at full health and hunger (although you may not survive the blast with sufficient health).
Radiation III (caused by Nuclear Explosives and Radioactive Material) is deadly once contracted, proving fatal within about 10–20 seconds even at full health and hunger. Drinking a regeneration potion (or multiple healing potions) can mostly counterbalance the effect.
Radiation IV is caused by holding refined Uranium in your inventory (but not in chests), and is survivable with multiple Regeneration II or Healing II potions.