DEAR TIM: We have a horrible odor coming from the shower drain in our acrylic shower. Five different plumbers have not been able to solve the problem. The first and fourth plumbers said nothing could be done, the second and third plumbers thought the problem was caused by a rocking toilet so they replaced the toilet's wax ring. The fifth plumber said he thought the plumbing drain lines were installed improperly and that the odor was caused each time the toilet flushed. The problem is, the odor just started and all was fine several months ago. I have poured every imaginable cleaning solution into the shower drain and the odor persists. What could be the problem? Shari W., Rancho Palos Verdes, CA

DEAR SHARI: You might be eligible for a place in the record books for having called in that many plumbers for one problem. It is amazing that you have not made any progress on solving the problem. That tells me the source of the odor might not be plumbing related. But to determine this, I am afraid you might have to perform a little exploratory surgery on a wall that is adjacent to the shower.

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.

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.

If this shower shares a common wall with a closet or some other wall surface that can be sacrificed, it is time to cut into the wall(s) to see behind and under the acrylic shower assembly. The first holes can be down low towards the floor so that you can see under the shower pan. Purchase an inexpensive makeup compact mirror and attach it to a stick so you can see around any corners or obstructions under the shower pan. Use a flashlight with the mirror to look up the wall cavities as much as possible to see signs of mold growth.

Odors from drains are voices telling you something is wrong. Think of them as low-powered smoke detectors. This is especially true if the source of the odor is some form of mold. It is not uncommon for a water leak to remain invisible for weeks or months and cause mold to bloom in a hidden location.

Add to this the power of the average human's sense of smell. You would be shocked at how few odor molecules it takes to trigger a response. Natural gas is a great example of this. In its pure state, natural gas is odorless. Gas companies add mercaptan chemicals to the gas so we can smell it if there is a dangerous gas leak. Believe it or not, you can easily smell mercaptans at a concentration level of one-half one part mercaptan to 1,000,000 parts of air!

Investigate the source of all odors. Enlist the help of friends if possible, especially those who might have a better sense of smell. Small holes drilled into walls can be very helpful to help track down the source of an odor. Insert a small clear plastic tube into the hole and determine if the odor is stronger or weaker in that particular location.