If someone has wounds on more than half of his body, or is suffering from small-pox, then it is not obligatory on him to bath. Instead, tayammum could be made.
If tayammum was made in a field and there was water nearby but he did not know about it, then both the tayammum and the salaat are proper. When he comes to know about the water, there is no need to repeat the tayammum and salaat.
While on a journey, if someone else has water, then one should judge for oneself and see what ones heart says: “If you ask for the water the person will give you.” In such a case it will not be permissible to make tayammum without asking for the water. And if the heart says: “If you ask for the water, that person will not give you.” In such a case it will be permissible to make tayammum even without asking for water. But if after performing the salaat, he asked for the water and the person gave it, then the salaat will have to be repeated.
If zam-zam water is kept in cans, tayammum is not permissible. It will be obligatory to open the cans and make wudhu and ghusl with it.
A person has got water with him, but the journey is so bad that no water is obtainable. He therefore has the fear of death or illness on account of thirst. wudhu should not be made. Tayammum will be permissible.