Purity: Fear of Illness or Thirst
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 287:
If someone fears that by performing wuḍūʾ he will fall ill, or that if he uses water for performing wuḍūʾ he will remain thirsty, he does not have any obligation to perform wuḍūʾ. If one does not know that water is harmful for him and he performs wuḍūʾ and the water really was harmful for him, his wuḍūʾ is void.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)