About

Tayammum (Arabic: تيمم‎) is the Islamic act of dry ablution using sand or dust, which may be performed in place of ritual washing (wudu or ghusl) if no clean water is readily available. "O you who believe! When you intend to offer As-Salat (the prayer), wash your faces and your hands (forearms) up to the elbows, rub (by passing wet hands over) your heads, and (wash) your feet up to ankles . If you are in a state of Janaba (i.e. after a sexual discharge), purify yourselves (bathe your whole body). 

But if you are ill or on a journey, or any of you comes after answering the call of nature, or you have been in contact with women (i.e. sexual intercourse), and you find no water, then perform tayammum with clean earth and rub therewith your faces and hands. Allah does not want to place you in difficulty, but He wants to purify you, and to complete His Favour to you that you may be thankful." Qur'an, Sura 5 (Al-Mai'da), ayat 6

ayammum may be substituted for wudu or ghusl when access to water is restricted or impractical, namely When sufficient amounts of water for ritual washing are not available, including when using the available water for wudu or ghusl would leave insufficient water for drinking, When obtaining water is hazardous or prohibitively expensive, When using water poses a health risk and When the water available is impure.