The gemara concludes that there were 3 gezeiros made on ארץ העמים, all lands outside of E.Y. Originally the יוסי בן יועזר ויוסי בין יוחנן made a gezeira that any Teruma that comes into contact with the land of eretz ha'amim is safeik tamie, the Rabbonon of 80 years prior to churban widened the gezeira that even teruma that is in air space of eretz ha'amim is safeik tamei, and when the Sanhedrin went into exile and were in Usha they decreed that the land itself is m'tamei vadai even to burn teruma, and the air remains safeik.
How are Kohanim lenient nowadays to live outside of E.Y. and not be concerned with tu'mah of eretz ha'amim?
The Shach (hil. aveilus) writes that since nowadays there is just as much tu'mah in E.Y., there is no advantage to E.Y. over eretz ha'amim. However, R. Akiva Eiger cites the Rikash who says that the reason people are lenient nowadays is either because they need to for parnasa or since we are all assumed to be tamei meisim anyway, it doesn't matter. The second approach seems to suggest that for a tu'mah d'rabonon such as eretz ha'amim we can rely on the Ra'avad who says that once someone is already tamei, there is not issur to make themselves tamei again. On the other hand, Pischei Teshuva (5) citing Shevus Ya'acov seems to uphold the gezeira of eretz ha'amim even nowadays.