WFRP3rd, Liber Mutatis S. 17:
Minor Exposure:
-- Prolonged exposure to skaven, beastmen or Chaos cult paraphernalia or locations
-- First hand witness to daemonic activities or rites
-- Contact with a traveller of the Chaos Wastes
-- Brief exposure to a small Warpstone chunk
Moderate Exposure:
-- Prolonged exposure to a Chaos Warrior, daemonic cult or tainted paraphernalia or location
-- Contact with a cursed weapon, profane artefact or daemonic entity
-- Exposure to a large amount of warpstone
Major Exposure:
-- Wounded by a cursed weapon, profane artefact or daemonic entity
-- Exposure to the savage Chaos Wastes and its environs
-- Consumption of warpstone powder
-- Direct exposure to a large amount of warpstone, or using warpstone to fuel spell