How is tayammum valid for one who fears harm to himself despite the the availability of water, and if tayammum is for the one in a state of ritual impurity (junub) who had intercourse with his wife and finds the major ablution difficult?

If a person in a state of ritual impurity (junub) is unable to use water due to fear of illness, or its aggravation, or delayed recovery, even after taking measures that would facilitate ghusl without harm, such as heating water and staying in a warm place, then it is permissible for him to perform tayammum. Similarly, tayammum is permissible for someone not in a state of ritual impurity if using water would result in illness, or its aggravation, or delayed recovery, after taking the aforementioned measures.