679. If a person has an eye disease and water is harmful to him he should perform tayammum.
680. If a person performs tayammum on account of certainty or fear about water being harmful to him but realises before Namaz that it is not harmful, his tayammum is void. And if he realises this after having prayed he should offer the prayers again with Wudhu or Ghusl.
681. If a person was sure that water was not harmful to him, and he did Ghusl or Wudhu, but later realised that water was harmful to him, his wudhu and Ghusl will be void.
682. If a person fears that if he uses water for Ghusl or Wudhu, he will be involved in hardship because of thirst, he should perform tayammum. Tayammum is permissible in the following three cases:
1. If he fears that by using up the water for Ghusl or Wudhu he will suffer an acute thirst, which may result in his illness or death, or it may cause intolerable hardship.
2. If he fears that his dependents whose protection is his responsibility, may become ill or die due to thirst.
3. If he fears that others, human beings or animals, may die or suffer some illness or become unbearably restless and distressed due to lack of water.
Apart from these three conditions mentioned, it is not permissible to perform tayammum when water is available.
683. If besides the Pak water which a person has for Wudhu or Ghusl he also has najis water enough for drinking, he should keep the Pak water for drinking and pray with tayammum. When water is required for other people attached to him, he would keep Pak water for Wudhu and Ghusl and let them quench their thirst with najis water, regardless of whether they know about the najasat or not, or whether they care about it or not. If water is required for an animal or a minor child, it should be given najis water to drink and Pak water be used for Wudhu or Ghusl.