Brachot 25 - Reciting the Shema in a filthy alley
Rav Huna said in the name of Rabbi Yochanan, "If one was walking in a filthy alley (where excrement or urine is found), and the time for Shema was passing, he may place his hand over his mouth, and then say the Shema. Rav Chisda answered him, "My God! Even if Rabbi Yochanan himself told me this, I would not listen to him." Rather he should pause. If he did not, the verse from Ezekiel applies to him, "I (God) too gave you decrees that were not good" - since he denied the goodness contained in Shema. If he did stop, however, his reward is in the verse " And with this matter (or speech) shall you prolong your days ."
If one's hand is resting in the airspace of a latrine, can he say the Shema then? Rav Huna says that he is permitted, relying on the verse in the psalms, " All that breathe shall praise God " - that is, only the position of his nose and mouth matters. However Rav Chisda forbids it, based on a different verse, "All my limbs shall exclaim, ' God, who is like You! '"
Art: David The Younger Teniers - The Five Senses Series- Smelling