Siman 45

It is forbidden to enter a cemetery or within four amot of a corpse while tefillin are on one's head, for there is in this the concern of "mocking the poor." And if they are covered – it is permitted. And even within four amot of a cemetery – it is forbidden (Magen Avraham subsection 1). And when there is a partition for the cemetery – it is permitted up to the partition. And when one enters into a cemetery – it is forbidden even outside of four amot of the graves. For so it is implied from the wording of the Tur and the Shulchan Aruch, who wrote: It is forbidden to enter a cemetery or within four amot of a corpse. See there, and it is implied that in a cemetery it is forbidden even outside of four amot. And in Berakhot (18a) it is explicitly explained that outside of four amot it is permitted in a cemetery, see there. And it must be said that they wrote the wording of the Shas but their intention was only within four amot of the graves, and there is one who says that even though according to the essential law it is so, nevertheless they established a fence so that one should not stumble and enter within four amot, and they forbade even outside of four amot (Taz subsection 1), and it is difficult to say so, that they would add to the words of the Shas. And know that this which was explained, that with covered ones it is permitted, the straps also must be covered, and it is simple (ibid. subsection 2), and he should remove the strap from the finger and hide it under his garment.

The bathhouse in the outer room, where everyone stands dressed—one may don tefillin there and recite the blessing ab initio. And in the middle room, where some people stand dressed and some are naked—one may not don them ab initio. But if they were upon him—he does not need to remove them. And in the inner room where everyone stands naked, even if they were upon him—he must remove them. And from the wording of the Tur and the Shulchan Aruch it is implied that only regarding that of the head is there a concern, but regarding that of the hand which is covered—we do not care. And the prohibition of tefillin in a bathhouse applies even when there is no person there, because its filth is abundant. But in a mikveh when there is no person there—it is permitted, for the filth is not abundant (Magen Avraham, subsection 2). And any holy object is forbidden to be brought into the bathhouse unless they are well covered. And therefore the Rambam, of blessed memory, rebuked in a responsum those who embroidered verses upon their garments, for the reason that they go in them to the bathhouse (ibid.).