I saw an exemption for wearing a tallit katan if the tempature is extremely high. I think this was cited by the Tzitz Eliezer, but I can't find it.
Are there other exceptions?
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Rav Yitzchak Abadi (Ohr Yitzchak vol 1:8) writes that if it is uncformatable wearing tzizts (obviously coming from a true discomfort) then one would be exempt. Such examples he gives is working out,or it is hot outside. It should be pointed out that mitzvos of tzizts is a very beloved mitzvah and should be taken seriously.