The Gemara states that the Sages imposed the status of Tum’a (ritual impurity) upon all lands outside Eretz Yisrael. This enactment, referred to as "Tum’at Eretz Ha’amim," means that anyone who is in Hutz La’aretz (outside the Land of Israel) automatically assumes a Rabbinic status of Tum’a. This Halacha is codified by the Rambam (Rabbi Moshe Maimonides, Spain-Egypt, 1135-1204) and by the Shulhan Aruch.
The question, then, arises as to whether Kohanim in Eretz Yisrael are allowed to leave in Israel. As Kohanim are forbidden to become Tameh (ritually impure), perhaps it should be forbidden for a Kohen living in Israel to leave to Hutz La’aretz.
This issue is subject to a debate among the Halachic authorities. The Bah (Rav Yoel Sirkis, Poland, 1561-1640) and the Taz (Rav David Halevi Segal, Poland, 1586-1667) maintained that when the Shulhan Aruch codified this Halacha, he referred to ancient times, when we had access to the ashes of the Para Aduma and thus had the opportunity to become Tahor (ritually pure) in Eretz Yisrael. In those days, it was forbidden for a Kohen to leave Eretz Yisrael, as he would then become Tameh. Nowadays, however, people in Eretz Yisrael are also considered Tameh, and thus there would be no prohibition for a Kohen to leave Israel.
The Hatam Sofer (Rabbi Moshe Sofer of Pressburg, 1762-1839) disagreed with this ruling. He noted that the Shulhan Aruch was written as a practical halachic code, and does not include information that only applied in ancient times and will become once again applicable only in the Messianic era. If the Shulhan Aruch codified the law of "Tum’at Eretz Ha’amim," the Hatam Sofer argued, then we must conclude that this Halacha applies even nowadays. Many other authorities followed the Hatam Sofer’s position, and ruled that a Kohen may not leave Eretz Yisrael except for the purpose of a Misva. This would include leaving to get married, even for an elderly person who can no longer beget children, since it is a Misva to marry irrespective of the obligation to reproduce. This also includes leaving Eretz Yisrael to earn a livelihood if this is too difficult in Israel, to learn Torah, and if one has a court case outside Israel and would lose money if he does not show up in court. Likewise, a Kohen may leave for the purpose of Kibud Ab Va’em (honoring his parents) and Nihum Abelim (comforting a mourner).
When it comes, however, to leaving Israel simply for vacation and the like, rather than for a Misva, according to many authorities, this is forbidden for a Kohen. Rav Shemuel Pinhasi’s son, in Mamlechet Kohanim (p. 221), cites his father as ruling that a Kohen should not leave Israel for purposes other than a Misva. Interestingly enough, he also cites Rav Shlomo Zalman Auerbach (Israel, 1910-1995) as raising the question of whether there is a Misva for Kohanim in Hutz La’aretz to make efforts to move to Israel, in order to avoid "Tum’at Eretz Ha’amim."
We are not going to definitively rule that a Kohen may not leave Israel except for a Misva, or that Kohanim in the United States must move to Israel, but this discussion demonstrates the special status of Kohanim and the complexity of the laws that relate to this status, and reminds Kohanim of the need to consult with a competent Rabbinic authority for guidance.
Summary: According to some authorities, it is forbidden for a Kohen to leave Eretz Yisrael except for the purpose of a Misva, such as for marriage, to earn a livelihood, to honor parents, and so on.
