On tier 1 and 2 there is a randomly chosen military location with low radiation level which is dangerous only in term of long exposure. The north of the map (tier 3 and 4) are whole covered in one radiation zones with different borders and with different levels of radiation each restart so you never know what to expect.
How to deal with an invisible enemy, few words about radiation.
How can I protect myself from radiation?
My character is sick.
How can I measure the radiation level?
My character got irradiated, how can I get rid of the sickness?
How to survive in irradiated zones?