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Why does my drain smell bad?

Why Does My Drain Smell?


Have you ever walked into your bathroom or kitchen and been greeted by an utterly unpleasant smell coming from one of your drains? 

Odors in the environment are very common and can be caused by anything ranging from regular household occurrences to nature to industrial activity.[1] Not all odors are toxic, many are simply a nuisance, but some smells in your drain can indicate more sinister problems and be a true cause for concern. 

While such stinky drains are certainly unwanted, the good news is that the majority of the time they are entirely non-toxic. Moreover, there are many low-cost and effective ways to treat smelly drains. 

Whether it smells like mold or rotten eggs, there are multiple causes of drain odor. But not to fear! SimpleLab and the Tap Score team are here to guide you in your quest to identify the stinky culprits and rid them from your drains. 

In this piece, we take a look at:

  • Common sources of drain odors
  • Health effects of stinky drains
  • How to get your drains to stop smelling foul 

What Causes Drains to Smell Bad?  

Gunk and debris buildup: 

Causes: When substances like hair, food, grease, and other debris get caught in a drain–they can block and clog your pipes. Such blockages form an ideal breeding ground for a wide array of bacteria and other microbes which enjoy the moisture, warm temperature (and food) in your drain. The bacteria grow and grow, feeding off the buildup and emitting foul odors as a byproduct.  

Health effects: While it’s very uncommon to be physically exposed to such microbes in your drain, these bacteria or mold can potentially cause harm. The most likely cause of health effects in this scenario is if such microbes emit spores or if a disease is somehow transported from the drain into your drinking water. Such issues are more common in less-developed infrastructure, but can also happen in any home and, if unaddressed, may lead to chronic health issues.[2]

Buildup and clogs in drains may also lead to stagnant water pooling. This may attract pests–like mosquitos and other insects–that may carry diseases. 

How to fix:  While there are chemical over-the-counter options for unclogging buildup, we suggest trying a natural remedy first (in an effort to limit the amount of chemicals entering the water supply!). For a DIY solution, we suggest a combination of baking soda, vinegar, hot water, and a drain brush to clear out the clog and eliminate the bacteria.[3]

Mold and mildew: 

Causes: Mold is a form of fungus that spreads through the production of spores. Often an insidious threat that can be out of sight, mold and mildew can grow in the tiniest of leaks in pipes underneath your sink or inside the walls. Once spores grow into mold, some release microbial volatile organic compounds (MVOC), which diffuse into the air. As they disperse in the air, these compounds fill nearby areas with an unpleasant odor. Depending on the type of mold or mildew growing, you may smell a musty, rotten, or “wet sock” smell. 

Health effects: Some people may feel no effects from inhaling mold fumes, while others who are sensitive may suffer from “nasal stuffiness, throat irritation, coughing or wheezing, eye irritation, or sometimes skin irritation”.[4] For people with pre-existing conditions, mold inhalation can worsen their symptoms. For example, people with lung disease may develop lung infections if exposed to mold, and people with asthma may experience heightened wheezing and coughing. 

How to fix:  Similar to our recommended natural remedy for gunk and othe buildup, we suggest starting off with a mixture of baking soda, vinegar, and hot water to rinse the pipes.

Empty drain traps:

Causes: There are likely a variety of different types of waste and drain traps in your home, but they all serve the same essential purpose: to prevent sewer gasses from re-entering. 

These traps are typically found below or within a fixture and are shaped so that a small amount of water is always retained within. Some of the most common shapes for drain traps are U-bend and S-bend. These shapes trap a small amount of water which acts as a shield to block unpleasant odors associated with the waste pipes your system is connected to from making their way back up. The most common types of sewer gasses include hydrogen sulfide, ammonia, and methane. 

Health effects: Depending on the type of gas making its way back into your home, exposure can lead to the following at elevated concentrations: headache, dizziness, memory loss, and even asphyxiation. However, it is highly unlikely that these symptoms will occur at the levels caused by an empty drain trap. 

How to fix: While sewer smells coming from your drains may send alarm bells ringing, there is a really easy fix if an empty drain trap is the problem. Simply pour water down every drain you can find. While commonly used sinks, toilets, and showers are less likely to have this issue–unused ones may have empty traps which allow rancid smells back into your home. After airing out the smelly areas of your home (ie. by ventilating the space), the smells should not return.

Water contamination:

Causes: If the issue is not related to a fixture or buildup, it is possible that the cause is coming from your water itself. One of the most common water-related odor issues is a rotten-egg smell.The culprit is almost always sulfur–usually in the form of hydrogen sulfide gas (H2S) produced by sulfate-reducing bacteria. The average person can detect H2S-related odor at levels around 0.5 parts per million (PPM) and higher. 

Health effects: It is unlikely that H2S concentrations would reach a toxic level in drinking water. Generally, hydrogen sulfide levels are less than 10 PPM, but have been reported as high as 75 PPM.[5] Even at such levels you aren’t necessarily going to experience health effects.

At very high concentrations, hydrogen sulfide is flammable and may be toxic, leading to: nausea, illness, and (in extreme cases) death. However, it is very unlikely that H2S concentrations would reach anywhere near these levels at your tap. 

How to fix: Like all water-related issues, step one is always to test your water. Once you can identify the specific cause of the problem, you can take clear, efficient, and actionable steps to remedying the problem.

Essential Home Water Test
54 Analytes Tested
$ 179.00
H2S Gas Strips (2)
1 Analyte Tested
$ 5.99
Sulfate-reducing Bacteria
1 Analyte Tested
$ 29.00


To remove low levels of hydrogen sulfide that DO NOT include bacterial problems, the best solution is an activated carbon filter.

For higher concentrations( up to ~5 to 7 PPM),  hydrogen sulfide can be removed using an oxidizing filter.

If your water has concentrations exceeding 7 PPM, hydrogen sulfide can be removed by injecting an oxidizing chemical (i.e. household bleach or potassium permanganate) followed by filtration.

Hydrogen sulfide gas resulting from bacterial problems is most common in private well water. To help tackle bacterial contamination in well water, take a look at this guide.

Plumbing issues: 

Causes: If mistakes are made during plumbing installation (ie. traps or vents are installed incorrectly), smells may accumulate due to a variety of reasons. Additionally, leaks and rotting drain tubes may also lead to odors.

How to fix: Unfortunately, if the smell is due to a substantial plumbing issue like incorrectly installed or missing traps, it may require you to call a plumber professional to address the issue. 

More Resources:

The Agency for Toxic Substances and Disease Registry (ATSDR) has created a searchable database of common odors by characteristic, such as sweet, strong, rotten, nutty, etc.[6] You can begin the search to identify your drain odor by typing words that describe the smell into this database. 

There are many causes of drain odor, some of which are easily identified and fixed by a householder, and some of which are trickier. If you’re unsure, it’s best to hire a professional or feel free to email the Tap Score team at

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