The affix itra comes after the verb pū 'to purify' with the sense of the agent or instrument when it is respectively used in connection with a sage or a deity.,
That is to say, when applied to a Rishi the force is that of an instrument, and when applied to a Devata the force is that of an agent. As, पवित्रोऽयमृषिः 'this sage is sacred' (through whose instrumentality one is sanctified); अग्निः पवित्रं स मा पुनातु 'the God Agni is sacred' (one that purifies), 'let him purify me'. So also, वायुः सोमः सूर्य इन्द्रः पवित्रं ते मा पुनन्तु ।,