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What Is That Rotten Egg Smell A Plumbing Mystery

You know that distinctive smell that assaults your nostrils the moment you walk into a bathroom. The smell of rotten eggs. The smell of sulphur.

Why is that sewer smell in your home? Ewww. Nasty.

What is going on?

Is something wrong with the plumbing?

Is that smell from the sewer line?

What can you do about it? Call a plumber and hope it’s not a major, costly repair at your home?

The pungent smell of rotten eggs or sewer gas in your home is alarming from a sensory perspective but it is not catastrophic. It is something you can investigate and solve on your own or, if you prefer, you can call a plumber for help pinpointing the origin of the rotten egg smell and what to do about it.

In this post we take a look at the annoying rotten egg smell — what it is and what’s causing it.

What Is That Foul Smell?

The rotten egg smell you encounter is mostly found in your home’s bathroom(s), near toilets, sinks, showers, and tubs, or in laundry or “mud/utility” rooms. It is hydrogen sulfide, a gas that comes from decaying organic matter, commonly known as sewage.

There are a few ways hydrogen sulfide is created:


  • Groundwater. Hydrogen sulfide can be created naturally in groundwater from the decay of organic matter and/or from the chemical reactions with minerals found in soil and rocks that contain sulfur.
  • Sulfur Bacteria. This can be found in groundwater, wells, and water distribution systems. When sulfur bacteria come into contact with sulfate compounds, they have the ability to change the sulfate compounds into hydrogen sulfide gas.
  • Water Heaters. These can sometimes be a breeding ground for hydrogen sulfide gas for two reasons: First, warm water can promote the bacterial growth and, second, the electrons emanating from the sacrificial anode help sulfate in the tank convert into hydrogen sulfide gas.
  • Sewage/Pollution. In rare residential instances, the smell may come from sewer waste or pollution that has gotten into the groundwater.


In a residential home, hydrogen sulfide exists in low levels but can become pronounced if a plumbing problem exists.


When your home’s plumbing is working properly, the naturally-occurring hydrogen sulfide is directed up and out through a vent system, exiting through the roof and leaving no noticeable smell.


When you smell this gas it’s usually the result of a small problem like a dried-out water seal in a main floor drain.


Other times the gas is a sign of a bigger problem like a broken vent stack. Diagnosing the problem can be simple or complicated, depending on the cause. Here you will want to call a plumber or plumbing contractor for inspection and possible repair.


Health Risks


So, how dangerous is hydrogen sulfide?


With minimal exposure over short periods of time, it’s more annoying than dangerous.


However, prolonged exposure can be a problem for some homeowners and residents. According to the International Association of Certified Home Inspectors (interNACHI), sewer gas accumulation in a home creates the following risks:




  • Hydrogen sulfide can be overbearing to some people even at extremely low concentrations. The gas’s odor is a safeguard, however, because it alerts homeowners to a leak or problem long before they’re in any serious danger.
  • It is important to note that at roughly 100 ppm, the olfactory nerve becomes paralyzed, removing a person’s sense of smell and, subsequently, awareness of any danger.
  • Another “warning smell” comes from ammonia, which will sear the nostrils and progressively irritate the mucous membranes and respiratory tract. This gas, unlike hydrogen sulfide, is sufficiently irritating that residents are likely to vacate before its concentration rises to toxic levels.


Hydrogen Sulfide Poisoning


  • Hydrogen sulfide can be smelled at 0.47 parts per billion by half of human adults, but the gas only will begin to cause eye irritation at 10 parts per million (ppm) and eye damage at 50 ppm. Note: That’s a lot of exposure.
  • Other low-level symptoms include nervousness, dizziness, nausea, headache and drowsiness.
  • Exposure to much higher concentrations can lead to pulmonary edema, and still higher levels (800 to 1,000 ppm) will cause almost immediate loss of consciousness and death.




  • When large concentrations of sewer gases diffuse into household air, they gradually displace oxygen and suffocate occupants. The effects of oxygen deficiency include headache, nausea, dizziness and unconsciousness.


Fire or Explosion


  • Methane and hydrogen sulfide are explosive components of large amounts of sewer gas, rarely seen in residential settings. Vapors from improperly disposed fuel can further increase the risk of fire or explosion.


Sources of Hydrogen Sulfide


Clogged Pipes/Traps and Clogged/Cracked Vent Pipes are two notable sources of hydrogen sulfide.


Clogged Pipes and Traps


A major source of sewer gas can be plumbing fixtures whose traps have gone dry or have lost enough water that the seal within has broken. This often happens when a bath or toilet isn’t used on a regular basis. Surprisingly, water can rapidly evaporate from toilets and traps below tubs, floor drains, and just about any fixture within a few months.


Leaking connections at either end of a plumbing component’s trap piece enable the water to drip out of the catch. Damage on the catch piece itself can also lead to a dry trap, implying you must get rid of the damaged trap and replace it with a new one.


Clogged or Cracked Vent Pipes


Sewer gas problems can also be caused by plumbing vent pipes that are clogged or cracked.


This can happen in old homes where a cast iron vent pipe gets clogged by years of rust scale that falls off the inside of the pipe and clogs a 90 degree bend in the pipe.


Tennis balls, leaves, and all sorts of other debris can clog plumbing vent pipes. When a vent pipe is clogged, the replacement air needed by the system will get sucked into the pipes through a fixture inside the house. When a large amount of water is placed into the drain pipes by a toilet or a powerful washing machine pump, it can readily suck the water out of a nearby bath tub trap or even a kitchen sink trap. Once this happens, sewer gas immediately enters the room through the dry fixture trap.


Defective vent pipes are far more elusive to detect. They can leak vast amounts of sewer gas and you might not be able to locate the source easily without a special machine that professional plumbers use. Obviously, here, you will want to call a plumber for inspection and, most likely, repair.


# Biological Slime


Lastly, biological slime can grow in sink, bath tub and bath drains, inducing an odor that can be much like sewer gases.


The slime is caused by:


  • Soap
  • Hair shampoo
  • Skin oils
  • Hair sticking to the sides of the drainpipe


Germs types in the slime, triggering the stink to flare each time you run water down the drainpipe. You should get rid of any kind of stoppers or drainpipe covers, run hot water down the tubes, and afterwards scrub it with soap and a big container brush to remove the slime and do away with the aroma.

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Benjamin Franklin Plumbing® offers customers on-time plumbing services, replacement and repairs done right the first time with courtesy, convenience, cleanliness, competence and character.

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Ben Franklin Plumbing Southeast
261 Mendel Parkway East
Montgomery, AL 36117
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