If a person hears Kaddish, Kedusha or Barechu while reciting the Beracha of Asher Yasar (the Beracha recited after using the restroom), may he interrupt his Beracha to respond?
The Halacha in this case depends upon at which point in Asher Yasar one hears the Kaddish, Kedusha or Barechu. Once a person recited the words, "Baruch Ata Hashem" at the beginning of the Beracha, he may then not interrupt until he completes the phrase, "Hashem Elokenu Melech Ha’olam Asher Yasar." However, after one recites the words, "Asher Yasar," his status with regard to interruptions is the same as somebody reciting the Berachot before and after Shema. Meaning, he may interrupt his recitation for the first five "Amen" responses in Kaddish, to answer "Barechu," and to answer "Kadosh, Kadosh" and "Baruch Shem Kebod" in Kedusha. According to the Kaf Ha’haim (Rabbi Yaakob Haim Sofer, Baghdad-Israel, 1870-1939), one who hears Kaddish in this case may also respond with "Yeheh Shemeh Rabba Mebarach" and continue until "Da’amiran Be’alma." One may not, however, answer "Amen" to a Beracha while reciting Asher Yasar; as mentioned, he may – and in fact should – interrupt only for the first five "Amen" responses in Kaddish (including "Yeheh Shemeh Rabba"), to answer Barechu, and for the verses of "Kadosh" and "Baruch Shem" in Kedusha.
This is the ruling of Rabbi Abraham Danzig of Vilna (1748-1820), in his work Hayeh Adam (5:13), cited by Hacham David Yosef in his Halacha Berura (vol. 1, p. 132).
If a person hears Kaddish, Kedusha or Barechu immediately after using the restroom, but before he had a chance to wash his hands, should he respond?
Hacham David Yosef (Halacha Berura, vol. 1, p. 134; listen to audio recording for precise citation) writes that a person in this case should preferably first rub his hands on some material that cleans them. This could be a piece of furniture, a stone, a garment, a paper towel, or something of this nature. If one does not have the opportunity to even wipe his hands before he hears Kaddish, Kedusha or Barechu, then he may nevertheless respond, despite the fact that he has yet to wash or even wipe his hands.
Summary: If a person hears somebody reciting a Beracha while he recites Asher Yasar, he may not interrupt his Beracha to answer "Amen" to the other person’s Beracha. If, however, he hears Kaddish while reciting Asher Yasar, and he had already recited the words "Asher Yasar" in the Beracha, then he may (and should) answer the first five "Amen" responses in Kaddish. If he hears Kedusha, he may answer "Kadosh, Kadosh" and "Baruch Shem," and he may also answer to Barechu (again, assuming he had already recited the words "Asher Yasar"). One who hears Kaddish, Kedusha or Barechu after using the restroom but before washing his hands may respond, though he should preferably first wipe his hands on some surface.
