DEAR TIM: We recently have begun to notice a sewer odor from only one of the three bathrooms in our house. It might be the toilet, but it's hard to tell. The smell is not constant and some times it is very strong. Everything in the house drains fine, including the toilet with the odor. Where can the odor be coming from? I have put a bacterial additive in the toilet but this does not seem to help. Is this serious? What can I do to fix the problem? Karen B., Watseka, IL

DEAR KAREN: The odor can be coming from numerous places. Often the source of the problem is simple, but in rare occasions the problem can be a serious plumbing flaw that is hidden behind a wall. I have found more often than not that the problem can be solved in just a few moments with a quart of water.

Plumbing fixtures or floor drains that get little use can loose this effective water seal. The water in the trap simply evaporates and/or it can be sucked from the trap by a clogged vent pipe or a poorly designed or installed pluming system. You can be fooled when you look into a drain and see the reflection of water. Even though you see water, sewer gas can be passing over this small amount of water left at the very bottom of a trap. Tubs and showers in guest bathrooms frequently are the culprit. Often people will use a toilet or sink in a bathroom but the tub and shower go unused for months at a time.

Pour a quart of water in the sink and tub drains. This is plenty of water to fill the trap and provide the full water seal. If this is the problem, the odor should go away in a very short amount of time. If the odor is still present, then it is time to look at other possibilities.

The inside surface of bathroom sink, tub and shower drains can also be a huge source of odors. Kitchen sinks frequently have similar odor problems. Almost all sinks have a piece of pipe called a tailpiece that extends from the bottom of the sink into the top of the p trap. This pipe is constantly exposed to the air in your bathroom. All of the bacteria, dirt, grime, mold, etc. pass through this pipe on its way to the sewer or septic system. But often some is left behind. Over time a thick layer of slime starts to collect on the inside surface of this vertical pipe. Mold and bacteria can begin to grow and some produce noxious odors.

Because of the fixed metal parts at the bottom of sink and tub drains, it is virtually impossible to completely clean these pipes while they are in place. Often you can take apart the drain assembly and clean the inside of this pipe in a jiffy. Pay attention to the rubber washers and gaskets as you take them apart. If the drain is old, these parts often need to be replaced with new ones to make sure the drain is leak free once you re-assemble it.

Bathroom sinks can also develop odors in the overflow part of the actual sink. The overflow is a little hole at the top of the sink that connects to the bottom of the sink. It is also susceptible to slime buildup. If you are on a city sewer system you can try to pour a 50/50 mix of chlorine bleach and water into this hole. A turkey baster works well for this purpose. Never mix chlorine bleach with ammonia or other cleaning liquids. Toxic gases can form. If you are on a septic system do not use chlorine bleach. It can harm the bacteria in your septic tank. Try to use a small bottle brush to clean this passageway.

If after all of this the odor persists, the problem may be a serious plumbing problem. The toilet seal may be broken. A hidden vent pipe may have cracked. A plumbing professional has the tools, equipment and know-how to often quickly diagnose problems like this. Call a pro if you can't solve the odor problem.