Odors coming from drains are a common problem in many homes. Some people only have the problem when they run water in a sink or a bathtub. The source of the odor in almost all of these instances is a buildup of biofilm on the sides of the pipes that connect the sink or fixture to the actual P-shaped trap under the fixture. This vertical tailpiece pipe can accumulate a seething bacteria-filled organic layer of slime over time. When water rushes past the slime, it can dislodge some of the molecules into the air and they waft up out of the sink and into your nose.
Biofilm on the drainpipe leaving a shower or any fixture can be a source of odors. Look at the horrible black goo inside the drain!
To eliminate biofilm as the source of the odor, take the chrome or brass strainer cover off the shower drain so you can see into the drain pipe. Use soap and water and a larger-diameter bottle brush to thoroughly clean the underside of the strainer, the bowl-shaped drain assembly under the strainer as well as the sides of the vertical drain pipe that extends downward into the p-trap. Rinse thoroughly with clean water until the entire drain is perfectly clean. If the odor still persists, it is time to move on.
Check for mold in the shower and areas near the shower. Active mold growth produces odor. That musty odor you smell when around mold is actually a puff of gas that ejects the mold spore from the mold organism. Imagine if hundreds, thousands or tens of thousands of small mold spores are being ejected each day in your shower. This could easily produce enough gas for you to smell. Remember, this mold could be behind the shower tile or walls and hard to discover. A recent leak that is still invisible could be the source of water that is fueling the mold growth.
A dead animal that somehow found its way into the wall space next to the shower or under the shower assembly could be the source of the odor. This is a remote possibility and the stench of a decaying animal is very distinctive and quite unlike the odor produced by sewer gas or biofilm from a drain assembly.
