The Gemara in Masechet Berachot (22) has a lengthy discussion regarding the question of whether one must immerse in a Mikveh after becoming Tameh (ritually impure) through a semenal emission, before he can pray or study Torah. Numerous different views are presented, each requiring different conditions to be met for immersion to be mandated. The Gemara ultimately concludes its discussion by citing Rav Nahman’s ruling that Halacha follows the view of Rabbi Yehuda Ben Betera, who held that "words of Torah are not susceptible to ritual impurity." Rav Nahman observed that the general custom was to allow Torah study, prayer and the recitation of Shema after a semenal emission without requiring immersion in a Mikveh.
The commentators address the question of how Rabbi Yehuda Ben Betera could rule against the enactment issued by Ezra and his Bet Din forbidding Torah learning and prayer after a semenal emission before immersing in a Mikveh. There is a fundamental rule that a Bet Din cannot repeal an enactment made by an earlier Bet Din. How, then, could Rabbi Yehuda Ben Betera claim that Halacha does not require following Ezra’s enactment concerning prayer and study in a state of ritual impurity?
The commentaries explain that whenever the Sages issued an enactment, they did so with the implicit condition that it is feasible for most Jews to abide by it. There was always an inherent stipulation that if the measure proved too difficult for most Jews to observe, then it could be voided. Rabbi Yehuda Ben Betera observed that it was too difficult for Jews to immerse each time they became Tameh before praying or studying Torah, and, as such, Ezra’s enactment was not binding. Rav Nahman confirmed this observation by noting that the prevalent practice was not to require immersion. If Ezra had known that this measure would prove too difficult, he would never have issued such an enactment, and it is therefore not binding.
The Shulhan Aruch (Orah Haim 88; listen to audio recording for precise citation) records Ezra’s enactment, writing that Ezra decreed that men who experience a semenal emission may not learn Torah, recite Shema or pray before immersing. He then writes that this decree was repealed because it proved too difficult, adding that the prevalent custom is indeed not to require immersion.
It should be noted, however, that the Gemara nevertheless encourages immersing before praying or learning after experiencing an emission, and writes that one who acts stringently in this regard is rewarded with longevity. Therefore, those who are able to abide by this measure are certainly encouraged to do so. Furthermore, one can fulfill this measure by having nine "Kabin" of water, or approximately 3.5 gallons, poured on him; this is equivalent to immersion in terms of rendering a person Tahor (ritually pure). Therefore, standing under a shower for a few minutes will have the same effect as immersion, and this is certainly a practical option for those who wish to observe Ezra’s enactment. One who showers should have this intention in mind, that the water poured on him should have the effect of rendering him Tahor in fulfillment of Ezra’s decree.
Summary: There is no Halachic requirement to immerse after experiencing a semenal emission before learning Torah or praying, but this is a worthwhile practice. One can observe this practice by standing under a running shower for several minutes, with the intention that it should have the effect of rendering him Tahor (ritually pure).
