Nasty odors rising from household drains are typically caused by dried-out P-traps, decomposing organic slime (biofilm) trapped inside the pipe walls, or a blocked roof vent stack that siphons water out of drain traps. When wastewater coats the interior of a P-trap, anaerobic bacteria break down the organic residue, releasing foul-smelling methane and hydrogen sulfide gases directly up through the open drain basket into your room. Verification: Ensure all P-traps contain standing water and clean out overflow channels regularly.
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