The passage (Devarim 16:16) obligates kol zekhurkhah – every Jewish male – to make a pilgrimage to Jerusalem three times a year. Our Gemara interprets kol zekhurkhah to mean that every Jewish male who can travel to Jerusalem with others is obligated to do so. Three people are mentioned who cannot travel with others – a mekametz (a gatherer), a metzaref nehoshet (someone who purifies copper) and a bursi (a tanner) – and are therefore free of this obligation.
In the Tosefta, two approaches are offered in defining mekametz. One suggestion is that the mekametz is a bursi katan – a non-commercial tanner, that is, someone who has a private business tanning skins at home in small quantities. Someone who has such a business does all aspects of the job himself. The other suggestion is that the mekametz is someone who collects dog droppings. Until the last century, softening skins in preparation for tanning them was done by adding dog droppings to the water where the skins were being soaked. The fermenting that takes place and the enzymes that are released play a central role in this process.
Although copper is occasionally found in pure form, most deposits that contain the metal have it mixed with various sulfides. The purification process separates out the copper from the sulfur, and the residue that is created often has a very strong smell attached to it. The metzaref nehoshet who works regularly with these metals will likely find that his clothing – and his body – absorb these odors. The bursi and mekametz have a similar problem.
The Ge’onim explain that the reason these people are not obligated in the mitzva of traveling to Jerusalem is not due to our concern with the comfort of those around them who may be offended by their smell. Rather our concern is for these people, themselves, and their own honor. Were someone to travel to Jerusalem and find that everyone deliberately keeps their distance from them, this would be a great embarrassment to them, and they are, therefore freed from this obligation.