The prohibition of Tum’at Kohanim forbids Kohanim from not only touching a human corpse, but also being under the same roof as a corpse. Therefore, if a funeral is being held in a funeral parlor or synagogue, and the deceased is present, Kohanim may not be in the room, even in the back row far away from the deceased. The status of Tum’at Met descends upon everybody under the same roof as a human corpse, and therefore Kohanim must avoid such situations, unless they are immediate relatives of the deceased.
Many Kohanim are unaware of the fact that this Halacha applies not only to roofs, but to any sort of covering. For example, it often happens when people leave a funeral parlor on a rainy day that they all have umbrellas open, thus creating a "tent" over the crowd. Kohanim must ensure not to be under a "tent" of umbrellas together with the deceased. By the same token, they must ensure not to be under an overhang together with the deceased. Occasionally, the procession passes by the front of buildings with large overhangs. Anybody who is under an overhang at the same time as a human corpse becomes Tameh, and therefore Kohanim must see to it that they walk at a distance from the procession to avoid such a situation. Large trees can also pose this problem, especially during the summertime, when trees are laden with leaves and thus their branches are thick. Kohanim may not be under the branches of a tree at the same time as the deceased. It is therefore crucial for Kohanim attending a funeral procession to stand off to the side at a distance from the procession.
Summary: It is forbidden for Kohanim to not only touch a dead body, but also to be under the same roof or other covering as a dead body. Thus, for example, Kohanim may not be under an umbrella, overhang or branches of a tree at the same time as a human corpse. Therefore, during a funeral process, Kohanim must ensure to stay to the side at a distance from the procession.
