Bad odors emanating from household drains are typically caused by decomposing organic slime (biofilm), hairballs, food scraps, and grease trapped inside the pipe walls or P-trap, where anaerobic bacteria break down the waste and release foul sulfur gases. Another frequent cause is a dried-out P-trap in infrequently used guest bathrooms or floor drains, which loses its protective water barrier and allows sewer gas to vent directly into the room. Verification: Pour fresh water down all suspect drains and clean out the P-trap to eliminate internal biofilm buildup.
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