What are the limits of harm that invalidate an act of worship?

The hardship that necessitates a concession is that which reaches the extent of danger to life, limbs, or their functions. So, if using water poses a danger to one's life, a limb, or its function, then tayammum (dry ablution) becomes obligatory. Cracking and chapping of the feet is not considered a hardship that necessitates a concession because it does not lead to the destruction of life, nor does it lead to the destruction of the feet or their function. Enduring a necessity that is considered valid by Islamic law and not availing oneself of the concession does not invalidate the worship.