You should not be able to detect any odors from your sinks or bathtub. If you do, there could be several reasons, ranging from gunk stuck in your drain to faulty plumbing. Let’s take a look at the most common causes of drain odors.

1. A clogged drain. Over time, hair, soap, dirt, and debris can build in a drain or drain pipe. Even though they may not totally block the drain, they can become the breeding ground for bacteria and mold and odors. If your drain is slow, this could be the reason. Lift the stopper and inspect the drain to see if you can see any debris or buildup. Use a snake to remove the buildup and follow with a thorough cleaning of bleach. Caution: Never use bleach in combination with a drain cleaner—it can create gases and explosions.
2. There is debris stuck in the P-trap. Under the sink is a P-trap, which is designed to hold water and seal gases from coming up through the sink. If debris is stuck in the trap, you need to remove it and flush clean water through the drain to reseal it. If your sink doesn’t have a P-trap, gases could be coming back up the drain. In this case, installing a P-trap should take care of the problem.
3. The overflow drain is trapping water or debris. Water will enter an overflow drain when the water level in the sink reaches it. Sometimes, that water can become stagnant or debris can enter the overflow drain. Both of these could create odors. If the problem is stagnant water, the introduction of hot water will help move it along and replace the stagnant water. If there is debris in the overflow drain, use a foaming pipe snake to clean it out.

By identifying the source of the odor, you can remove it and eliminate any unwanted odors, whether they are the result of septic cases or build up. If you cannot eliminate the problem using the above methods, there might be clogs or problems further in your plumbing or septic system that require the services of an experienced plumber.