If your shower drain emits a musty aroma, you almost certainly have a mold problem. Mold releases nasty fumes that cause a sour, thick smell. Shower drains are extremely susceptible to mold due to the amount of water and dirt that flow through the connected pipes.
A clogged drain can pave the way for mold and mildew. Mold spores need two things to grow: a steady supply of moisture and an ample amount of nutrients. Dirt particles can provide the necessary nutrients and a clog supplies the water. When mold spores collect within the clogged drain, they begin to devour the nutrients and germinate. The smell then wafts through the drain and into the bathroom.
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In some cases, you may notice the musty odor even after ruling out the possibility of a clogged drain. This occurs when mold grows deeper within the plumbing system. The musty odor then emerges through open vents such as your shower drain. This problem may result from a clog somewhere within the pipes, or simply from mildew growing along the inner wall of a pipe.
Check for and remove clogs. Unscrew the drain cover and use a plunger or plumbing snake to to pull out any debris. Pour 3/4 cup of dry baking soda into the drain, followed by 1/2 cup of vinegar. Baking soda and vinegar both have strong antifungal properties and can destroy clogs. After 30 minutes, pour a pot a scalding hot water into the drain. This should remove clogs and kill even mold spores residing deep within the pipes.
If you also notice the musty odor in other parts of your home, or if your drain-cleaning efforts fail to do the trick, call a mold remediator. In some instances, you may have a leak within the plumbing system, or a mold colony growing within the walls. If you ignore the problem, it will only get worse and can endanger your health. A mold expert can tell you exactly what kind of problem you have.