I have some questions regarding wearing tzitzit.
1. Is one allowed to wear his tzitzit directly on his skin, or is it considered disrespectful because of the sweat.
2. Does one HAVE to wear an undershirt.
3.Are there any leniencies when it comes to this issue.
There is no formal prohibition on wearing tzitzis directly on one’s body.
Some are particular to wear the tzitzis over an undershirt, so that the garment will not become sweaty and sullied (which is disrespectful for the tzitzis; see also sources), and not be in direct contact with the body. However, the general consensus and custom is that the garment will not become an “undershirt” (just like a shirt worn on the body doesn’t become an undershirt), and that there is no formal problem in wearing tzitzis on one’s body.
See Kaf Hachaim 8:3, who writes, based on kabbalistic principles, that the tzitzis garment should not be worn on the flesh. This is also stated by Rav Chaim Kanievsky (Techeiles Ve’argaman 5), and by Teshuvos Vehanhagos (1:25), for fear that the garment would be considered an “undershirt,” which might be exempt from the obligation of tzitzis.