The Janehu/Upanayanam ceremony is a very important tradition among the Brahmins. After the Namkaran (naming) ceremony, it is probably the next biggest occasion for the Brahmin boy. This ceremony marks the boy's entry into studies.
The Upanayanam ceremony takes place when the boy is around 7-8 years old. A sacred thread, called Yagnopavitam is placed around the left shouder of the boy. This thread is a composite of 3 threads, signifying Goddesses Gayatri (thought), Saraswati (word) and Savitri (deed) respectively. There is also a knot in the middle that signifies the unending Brahman.
A head priest whispers the Gayatri Mantra into the boy's ears, and he is formally initiated into education.
After this, the boy is supposed to tie the thread around his ear whenever he passes urine, so as to avoid getting it into contact with the excretory organs and thus getting contaminated. The thread is also changed once a year on the occasion of Shravani Purnima, the full moon day in the month of Shravan. This is also known as Avaani Avittam in Tamil, and coincides with the Raksha Bandhan or Rakhi festival of North India.
The below photo shows Brahmins taking part in the thread changing ceremony.
Further, we should take into account the fact that the Brahmins of olden days did not lead a very active lifestyle (i.e.) they were mostly conducting Pujas, where their primary role was to be seated and chant Mantras. In this scenario, it would have been difficult to keep their kidneys in top-shape. This is why this alternative system of Yagnopavitam (sacred thread) has been formulated.
This practice is not something that is to be done away with, as there are chances of this problem being carried on down the generations. This practice of Yagnopavitam can also be carried out in other castes - after all, keeping our urinary system healthy is not caste specific, right?