As far as tayammum (dry ablution) is concerned, Dr. Hammudah Abulati, the well-known Muslim scholar, states in his book, Islam in Focus:
Tayammum or resort to pure earth may substitute for the ablution and even the bath. This is allowed in any of the following cases:
1. When a person is sick and cannot use water.
2. When he has no access to water in sufficient quantity.
3. When the use of water is likely to do him harm or cause any disease.
In any of these instances it is permissible to make tayammum, which is performed as follows:
1. Strike both open hands (palms down) lightly on pure earth or sand or stone.
2. Shake the hands off and wipe the face with them once in the same way as done in the ablution.
3. Strike the hands again and wipe the right arm to the elbow with the left hand then the left arm to the elbow with the right hand.
Tayammum is a symbolic demonstration of the importance of the wudu’ (ablution), which is so vital for both worship and health. When Islam introduced this repeatable ablution, it brought along with it the best hygienic formula, which no other spiritual doctrine or medical prescription had anticipated.”