While trimming, the barber’s hand might accidentally touch the Poonal or the water used to wet the hair or the cloth that wraps the person, when anyone of those come into contact with the sacred thread, it is considered unclean. A sacred thread must always be clean.
One must perform all austerities with a clean yagnyopavitam, it should be changed when it becomes impure. Based on these, it is said the poonal must be changed after having a haircut.