The Talmud teaches that it is forbidden to touch the parchment of a sefer Torah with bare hands. Tosafot (Shabbat 14a, s.v. ha-ochez) asserts that this is true not only of a sefer Torah, but of all holy writings. Tosafot elsewhere (Chagigah 24b, s.v. d’tnan), however, implies otherwise, that the rule applies only to a Sefer Torah.

R. Akiva Eiger (Responsa, I, 58) considers Tosafot in Shabbat to be normative, and notes that the Rama (O.C. 146) recommends stringency in this area. Thus, he objects to the practice of placing one’s hand on top of a mezuzah, when the mezuzah is without any covering. He notes that the idea of placing a hand on the mezuzah is without a Talmudic source (the Rama, 285, cites the custom from the Maharil) and thus certainly does not take precedence over a genuine halakhah. Thus, he recommends, if the mezuzah is uncovered, one should extend his sleeve over his hand.