Is it permissible for a person suffering from allergies, sinusitis, and asthma, who is harmed by frequent bathing (ghusl) and ablution (wudu'), to perform tayammum for prayer? And for how many days is he permitted to perform tayammum? And is it valid for him to perform wudu' without washing his head to avoid harm?

Allah Almighty has legislated Tayammum for the sick person if using water would harm them, or if they fear an increase in their illness, a delay in their recovery, or the onset of a new illness, based on His saying: {And if you are ill or on a journey... then perform Tayammum with clean earth}. The sick person is obligated to wash whatever parts of their body will not be harmed by washing, and to perform Tayammum for what they are unable to wash or would be harmed by washing. If you fear harm from washing certain limbs, it is permissible for you to forgo washing them and to perform Tayammum for them. This concession continues as long as the fear of harm persists. When the fear subsides, it becomes obligatory to wash the entire body and all limbs.