A Kohen is prohibited by Torah law from becoming Tameh (ritually impure) by coming in contact with a corpse. Even though most Kohanim today are already Tameh, nevertheless the Rambam and other authorities rule that they are not permitted to "add" more Tumah. However, The Ra’avad disagreed and held that there are no restrictions on a Kohen who is already Tameh.
A Kohen is not only prohibited to touch a corpse, but also from being under the same roof with one, as well. This is called Tumat Ohel (Impurity under a Tent). Therefore, a Kohen must be careful not to enter a building in which there may be a corpse, such as a hospital or funeral home.
Rabbenu Tam holds that any metal item, such a fork, knife or even a paper clip, that were under the same roof as a corpse, becomes Tameh to the same degree as the corpse itself. Accordingly, even if the corpse was removed from the building, any remaining metal items would prevent the Kohen from entering. This opinion would make it exceedingly difficult for a Kohen to enter any building. He could never be sure whether the metal items inside were ever under the same roof as a corpse.
This position presents such a difficulty to Kohanim, that Rabbenu Tam’s student, Rav Haim HaKohen, who was a Kohen, asked his master, "Ayeh Makom Menuhati?" (According to your opinion, where can I rest?). He, as well as Rambam and Ramban argue with Rabbenu Tam, and hold that the metal objects in the room do not have the same status as the corpse.
The Rama (YD 369) cites both opinions and concludes that the custom is to be lenient. Hacham Ovadia (Hazon Ovadia, Hilchot Avelut, Vol. 2) ruled that Sepharadim are also lenient in this Halacha, even though Maran (YD 372) brings the stringent opinion of the Yera’im (Rabbi Eliezer of Metz). He reasons that there is a "Safek Sefekah" (Double Doubt) to justify being lenient. First, perhaps the Halacha is in accordance with the Rav Haim HaCohen and the other Rishonim who disagree with Rabbenu Tam. Second, even if the Halacha is like Rabbenu Tam, there is still the opinion of the Ra’avad, who holds that there is no restriction at all for Kohanim, who are already Tameh, to be exposed to corpses. Therefore, a Kohen may enter a building without concern whether the metal objects inside where in contact with a corpse.
A Kohen may enter a building without concern whether the metal objects inside where in contact with a corpse.