#410 Undershirt Tzitzis
There is a product on the market which is an undershirt with tzitzis on it. It’s worn by some people in hot weather, or while exercising or jogging. In order for such an undershirt to be kosher for tzitzis it must be ascertained first and foremost that most of the length on both sides of the undershirt are open at the seams, not just a small section at the bottom; otherwise it cannot be considered as having four corners and requiring tzitzis. The holes for the arms cannot be regarded as part of the opening. The mitzvah min hamuvchar (preferred manner of doing the mitzvah) is to wear a woolen tallis katan, which obviously, the undershirt tzitzis is not.
There are some additional concerns regarding this garment. Among them:
It’s important to note that one may not make a brachah on tzitzis if the garment is exempt from tzitzis, and wearing such a garment on Shabbos might be considered carrying. The hechsher on the product usually pertains only to the tzitzis and is not an indication of the kashrus of the garment as far as the mitzvah is concerned. Nevertheless, the consensus among poskim is that if the seams on the two sides are sufficiently open, an undershirt with kosher tzitzis on it, while not recommended, can still be considered kosher if the garment isn't made from synthetic material—but should be worn only when perspiring heavily or under other extenuating circumstances. Otherwise, regular tzitzis—and preferably a woolen tallis katan—should be worn over a regular undershirt.